Carbon Ion Radiobiology

HyperSHArc: Single-Isocenter Stereotactic Radiosurgery of Multiple Brain Metastases Using Proton, Helium, and Carbon Ion Arc Therapy

Purpose

This work presents a proof-of-concept study of HyperSHArc, spot-scanning hadron arc (SHArc) therapy for single-isocenter stereotactic radiosurgery of multiple brain metastases (MBMs). HyperSHArc plans using proton, helium, and carbon ions were compared with state-of-the-art volumetric modulated photon arc therapy.

Methods and Materials

Treatment design and optimization procedures were devised using commercial and in-house treatment planning systems. Planning and delivery methods considered dedicated energy, spot, and multiarc selection strategies. Proton, helium, and carbon HyperSHArc plans were generated for patients with MBM exhibiting 3 to 11 intracranial lesions with gross tumor volumes (GTVs) between 0.03 and 19.8 cc, at prescribed doses between 19 and 21Gy in a single-fraction. Planning target volumes (PTVs) considered a 1-mm isotropic margin around the GTV, and robust optimization with 2.5%/1 mm criteria for range and position uncertainty was applied. Photon hyper-arc volumetric modulated arc therapy (HA-VMAT) plans were optimized for the PTVs using the HyperArc® single-isocenter stereotactic radiosurgery platform (Varian, Palo Alto, CA, USA).

Results

HyperSHArc plans were comparable between particle species, achieving highly conformal target doses and satisfying clinical coverage criteria. Particle arc plans reduced V2Gy and V4Gy in the healthy brain compared with HA-VMAT, while intermediate doses (V8Gy-V16Gy) were similar or reduced depending on the number of lesions. Particularly for the case with 11 targets, a considerable reduction in V12Gy was observed that could be relevant for reducing the risk of treatment-induced radionecrosis. HyperSHArc using carbon ions boosted dose-averaged linear energy transfer inside the target relevant to overcoming radioresistance factors (>100 keV/μm).

Conclusions

We present the first particle arc therapy strategies for MBM. Results demonstrate that with HyperSHArc, dose conformity comparable or superior to HA-VMAT is achievable while reducing the low-dose bath and increasing mean dose-averaged linear energy transfer in the GTV. Our findings suggest that HyperSHArc using light and heavy ions could be an effective and efficient means of treating MBM. Further development of HyperSHArc optimization and delivery is justified.

Carbon-ion irradiation together with autophagy inhibition and immune checkpoint inhibitors protect against pancreatic cancer development in mouse model

BACKGROUND

Pancreatic cancer remains fatal because of resistance to chemo-, radio-, and immunotherapies. Carbon-ion radiotherapy (CIRT) has been beneficial for patients with pancreatic cancer. The purpose of this study was to identify the mechanism by which CIRT exerts its anticancer activity, particularly in combination with immunotherapy.

METHODS

We implanted murine pancreatic cancer cells treated with CIRT and autophagy inhibitor HCQ (CIRT+HCQ) into syngeneic mice, followed by the application of a regulatory T (Treg) cell blockade using immune-checkpoint inhibitors. We compared CIRT+HCQ-treated tumors with those implanted without any treatment. Further, we also implanted CIRT+HCQ-treated pancreatic tumors into CD8+ T cell-depleted mice. To characterize immunological alterations, we conducted immunohistology and flow cytometry of implanted tumors.

RESULTS

CIRT+HCQ-treated tumors exhibited reduced growth, higher numbers of CD8+ T cells, and lower numbers of Treg cells compared with control tumors. CD8+ T cell depletion restored growth in CIRT+HCQ-treated tumors. A Treg blockade resulted in greater tumor growth remission and elevated levels of intratumor CD8+ T cells in mice bearing CIRT+HCQ-treated tumors but not in mice bearing control tumors.

CONCLUSIONS

Treg cell-targeted therapy exerted an anticancer effect in mice bearing CIRT+HCQ-treated tumors but not in those bearing untreated pancreatic tumors by activating cancer-specific CD8+ T cells.

Nanomedicine-Enhanced Radiotherapy for Glioblastoma: Advances in Targeted Therapy and Adaptive Treatment Strategies

Glioblastoma multiforme remains one of the most aggressive and treatment-resistant brain tumors that necessitate innovative therapeutic approaches. Nanomedicine has emerged as a promising strategy to enhance radiation therapy by improving drug delivery, radiosensitization, and real-time treatment monitoring. Stimuli-responsive nanoparticles can overcome limitations of the blood-brain barrier, modulate tumor microenvironment, and facilitate targeted therapeutic interventions. The integration of nanotechnology with proton and X-ray radiotherapy offers improved dose precision, enhanced radiosensitization, and adaptive treatment strategies. Furthermore, Artificial Intelligence-driven nanoparticle designs are optimizing therapeutic outcomes by tailoring formulations to tumor-specific characteristics. While promising, clinical translation remains a challenge that requires rigorous validation to ensure safety and efficacy. This review highlights advancements in nanomedicine-enhanced radiotherapy and future directions for glioblastoma multiforme treatment.

Pancreatic Cancer: Pathogenesis and Clinical Studies

Pancreatic cancer (PC) is a highly lethal malignancy, with pancreatic ductal adenocarcinoma (PDAC) being the most common and aggressive subtype, characterized by late diagnosis, aggressive progression, and resistance to conventional therapies. Despite advances in understanding its pathogenesis, including the identification of common genetic mutations (e.g., KRAS, TP53, CDKN2A, SMAD4) and dysregulated signaling pathways (e.g., KRAS-MAPK, PI3K-AKT, and TGF-β pathways), effective therapeutic strategies remain limited. Current treatment modalities including chemotherapy, targeted therapy, immunotherapy, radiotherapy, and emerging therapies such as antibody-drug conjugates (ADCs), chimeric antigen receptor T (CAR-T) cells, oncolytic viruses (OVs), cancer vaccines, and bispecific antibodies (BsAbs), face significant challenges. This review comprehensively summarizes these treatment approaches, emphasizing their mechanisms, limitations, and potential solutions, to overcome these bottlenecks. By integrating recent advancements and outlining critical challenges, this review aims to provide insights into future directions and guide the development of more effective treatment strategies for PC, with a specific focus on PDAC. Our work underscores the urgency of addressing the unmet needs in PDAC therapy and highlights promising areas for innovation in this field.

Impact of porous lung substitute on linear energy transfer (LET) assessed via Monte Carlo simulation and CR-39 measurement with a carbon-ion beam

BACKGROUND

Carbon-ion beam radiotherapy offers substantial physical and biological advantages due to its distinct Bragg peak (BP) depth dose distribution and higher linear energy transfer (LET) in the peak region that enhances its efficacy in tumor eradication compared to x-ray beams. Porous structures, such as those found in lung and lung-equivalent tissues, unfortunately, introduce significant uncertainties in both dose and LET distributions, which current treatment planning systems (TPS) inadequately address.

PURPOSE

This study aims to investigate the effects of porous lung-equivalent structures on LET distribution using Monte Carlo (MC) simulations and CR-39 measurements. It seeks to understand how porous structures influence LET spectra and dose-averaged LET (LETd) in carbon-ion beams.

METHODS

A Gammex LN300 phantom and a binary voxel virtual phantom composed of water and air were used to represent lung-equivalent tissues for measurements and MC simulations. LET spectra measured with CR-39 at different depths within the LN300 slabs were compared with MC-calculated LETd distributions. The impact of porous structures on dose and LETd distributions was evaluated using various beam configurations, including single-beam and multi-beam setups. Additionally, a convolution method with modulation power (Pmod) was proposed to improve LETd prediction in porous media.

RESULTS

The study demonstrated that porous structures broaden both the dose and LETd distributions, especially around the BP region. Multiple beam angles helped mitigate dose degradation but did not resolve discrepancies in the LETd distributions. Compared with calculation results based on CT images, intensity-modulated particle therapy (IMPT) using a distal LETd patching method in porous structure increased the median LETd in the target from 67.2 to 69.6 keV/µm, and the minimum LETd from 51.5 to 58.0 keV/µm, respectively. Moreover, to improve the prediction of LETd in porous structures, analytical convolution-based predictions showed good agreement with the MC simulations, with mean LETd deviations of -1.9% ± 1.6% in the plateau, -3.1% ± 4.9% in the BP, and -1.1% ± 7.7% in the tail region.

CONCLUSIONS

Porous lung-equivalent structures significantly affect LETd distributions in carbon-ion therapy, as confirmed by both CR-39 measurements and MC simulations. IMPT with LETd optimization may be more impacted by porous structures in terms of median and minimum LETd values within the target. The Gaussian convolution function shows promise for enhancing LETd calculation accuracy, but further validation in anatomically complex models is needed to assess its clinical feasibility.

FLASH Bragg-Peak Irradiation With a Therapeutic Carbon Ion Beam: First In Vivo Results

PURPOSE

In recent years, ultra-high dose rate (UHDR) irradiation has emerged as a promising innovative approach to cancer treatment. Characteristic feature of this regimen, commonly referred to as FLASH effect, demonstrated primarily for electrons, photons, or protons, is the improved normal tissue sparing, whereas the tumor control is similar to the one of the conventional dose-rate (CDR) treatments. The FLASH mechanism is, however, unknown. One major question is whether this effect is maintained when using densely ionizing (high-LET) heavy nuclei.

METHODS MATERIALS

Here, we report the effects of 20 Gy UHDR heavy ion irradiation in clinically relevant conditions, ie, at high-LET in the spread-out Bragg peak of a 12C beam using an osteosarcoma mouse model.

RESULTS

We show that UHDR irradiation was less toxic in the normal tissue compared with CDR while maintaining tumor control. The immune activation was also comparable in UHDR and CDR groups. Both UHDR and CDR exposures steered the metagenome toward a balanced state.

CONCLUSIONS

These results suggest that the UHDR irradiations can improve the safety and effectiveness of heavy ion therapy, and provide a crucial benchmark for current mechanistic FLASH models. However, additional experiments are needed to validate these findings across other animal and tumor models.

Impact of nuclear fragmentation and irradiation scenarios on the dose-averaged LET, the RBE, and their relationship for H, He, C, O, and Ne ions

BACKGROUND

Projectile and target fragmentation are nuclear phenomena that can influence the computation of the linear energy transfer (LET) and the relative biological effectiveness (RBE) in external radiotherapy with accelerated ions. Correlations between these two quantities are routinely established during radiobiological experiments to interpret the results and to develop and calibrate RBE models.

PURPOSE

This study systematically evaluates the impact of secondary fragments and irradiation scenarios on the dose-averaged LET, the RBE, and their correlation in the case of exposures to clinically relevant ion beams.

METHODS

57 600 microdosimetric lineal energy spectra and corresponding LET distributions were simulated with the Monte Carlo code PHITS across different scenarios, including track segment calculations, pristine, and spread-out Bragg peaks of 1H, 4He, 12C, 16O, and 20Ne ions within water phantoms. The LET distributions were analyzed to calculate the dose-average LET, both including or excluding the contribution of secondary ions of an element different from the primary beam. Similarly, the lineal energy distributions were processed in conjunction with the Mayo Clinic Florida microdosimetric kinetic model to compute the RBE for two theoretical cell lines (α/β = 2 and 10 Gy in the case of 6 MV x-rays). Correlations between the RBE and dose-averaged LET were established by analyzing the simulation results within water phantoms and then compared to the corresponding trends from the track segment calculations.

RESULTS

Excluding secondary fragments had a pronounced impact on the dose-averaged LET and the RBE, particularly in the entrance regions of proton beams and close to the distal edge of heavier ions. The correlations between the RBE and the dose-averaged LET were not universal, but highly dependent on the irradiation scenario. For proton beams only, the dose-averaged LET of hydrogen ions served as a practical first-order descriptor of the RBE. Track segment simulations, commonly used for calibrating and benchmarking RBE models, provided a reasonable approximation for low-energy beams but failed to fully capture the complexity of polyenergetic radiation fields.

CONCLUSIONS

Secondary fragments can substantially affect the dose-averaged LET and the RBE, even in proton beams. The dose-averaged LET, including or not the contributions from secondary fragments, was generally unable to adequately capture RBE variations across different scenarios. A more comprehensive approach, considering microdosimetric distributions, is necessary to accurately describe RBE variations in ion therapy.

Tumour hypoxia in driving genomic instability and tumour evolution

Intratumour hypoxia is a feature of all heterogenous solid tumours. Increased levels or subregions of tumour hypoxia are associated with an adverse clinical prognosis, particularly when this co-occurs with genomic instability. Experimental evidence points to the acquisition of DNA and chromosomal alterations in proliferating hypoxic cells secondary to inhibition of DNA repair pathways such as homologous recombination, base excision repair and mismatch repair. Cell adaptation and selection in repair-deficient cells give rise to a model whereby novel single-nucleotide mutations, structural variants and copy number alterations coexist with altered mitotic control to drive chromosomal instability and aneuploidy. Whole-genome sequencing studies support the concept that hypoxia is a critical microenvironmental cofactor alongside the driver mutations in MYC, BCL2, TP53 and PTEN in determining clonal and subclonal evolution in multiple tumour types. We propose that the hypoxic tumour microenvironment selects for unstable tumour clones which survive, propagate and metastasize under reduced immune surveillance. These aggressive features of hypoxic tumour cells underpin resistance to local and systemic therapies and unfavourable outcomes for patients with cancer. Possible ways to counter the effects of hypoxia to block tumour evolution and improve treatment outcomes are described.

Carbon minibeam radiation therapy results in tumor growth delay in an osteosarcoma murine model

Despite remarkable advances, radiation therapy (RT) remains inefficient for some bulky tumors, radioresistant tumors, and certain pediatric tumors. Minibeam radiation therapy (MBRT) has emerged as a promising approach, reducing normal tissue toxicity while enhancing immune responses. Preclinical studies using X-rays and proton MBRT have demonstrated enhanced therapeutic index for aggressive tumor models. Combining MBRT's advantages of spatial dose fractionation with the physical and biological benefits of carbon ions could be a step further toward unleashing the full potential of MBRT. This study aims to perform the first in vivo study of local and systemic responses of a subcutaneous mouse osteosarcoma (metastatic) model to carbon MBRT (C-MBRT) versus conventional carbon ion therapy (CT). Irradiations were conducted at the GSI Helmholtz Centre in Germany using 180 MeV/u 12C ions beam. All irradiated animals received an average dose (20 Gy) and displayed a significant and similar tumor growth delay in addition to a decreased metastasis score compared to the non-irradiated group. In the C-MBRT group, 70% of the tumor volume received the valley dose, which is a very low dose of 1.5 Gy. The remaining 30% of the tumor received the peak dose of 105 Gy, resulting in an average dose of 20 Gy. These results suggest that C-MBRT triggered distinct mechanisms from CT and encourage further investigations to confirm the potential of C-MBRT for efficient treatment of radioresistant tumors.

Radioresistant, Rare, Recurrent, and Radioinduced: 4Rs of Hadrontherapy for Patients Selections

Purpose

To describe the role of hadrontherapy (HT) in treating radioresistant, rare, recurrent, and radio-induced tumors, which can be defined, in assonance with the 4Rs of radiobiology, the "4Rs" of HT indications.

Materials and Methods

This is a narrative review written by a multidisciplinary team consisting of radiation oncologists, radiobiologists, and physicists on the current literature on HT, particularly carbon ion radiation therapy. To refine HT indications within the context of the "4Rs" framework, we evaluated tumor histologies across different clinical indication settings and emphasized the radiobiological mechanisms contributing to the effectiveness of HT.

Results

For rare, radioresistant, recurrent, and radio-induced tumors, HT has proven to be effective and safe, achieving high rates of local response with mild toxicity. The current review shows that the biological parameters can assist clinicians in identifying appropriate cases for HT treatment.

Conclusion

Biological characteristics of the tumor support the administration of HT in radioresistant, rare, recurrent, and radio-induced tumors and should be considered during multidisciplinary discussions.

Irradiation-responsive PRDM10-DT modulates the angiogenic response in human NSCLC cells in an SP1-dependent manner via the miR-663a/TGF-β1 axis

BACKGROUND

Photon radiation has been shown to stimulate the secretion of radioresistant factors from tumor cells, ultimately promoting tumor angiogenesis and metastasis. On the other hand, heavy-ion radiotherapy has been demonstrated to control tumor angiogenesis and metastasis levels. The molecular mechanisms responsible for the different angiogenic responses to photon and heavy-ion irradiation are not fully understood. This study aims to explore the irradiation-responsive genes related to tumor angiogenesis and reveal the regulatory effect.

METHODS

In order to clarify the potential regulatory mechanisms of tumor angiogenesis after X-ray or carbon ion (C-ion) irradiation, we performed RNA-sequencing (RNA-seq), as well as bioinformatics, public database analysis, Western blotting, immunohistochemistry, and immunofluorescence.

RESULTS

In this study, we identified the long intergenic noncoding RNA PRDM10 divergent transcript (PRDM10-DT), which was responsive to X-rays but not carbon ions. Mechanistically, PRDM10-DT triggers tumor angiogenesis by upregulating the TGF-β1/VEGF signaling pathway through its competitive binding to miR-663a. Additionally, the transcription factor SP1 facilitated the transcription of PRDM10-DT by binding to its promoter region. It's notable that the DNA-binding activity of SP1 was enhanced by reactive oxygen species (ROS). The knockdown of either PRDM10-DT or SP1 effectively inhibited NSCLC angiogenesis and metastasis.

CONCLUSION

These results illustrate the proangiogenic function of the PRDM10-DT/miR-663a/TGF-β1 axis and reveal the regulatory role of ROS and SP1 in the upstream response to radiation, with differential ROS production mediating the differential angiogenesis levels after X-ray and C-ion irradiation. Our findings suggest the potential of PRDM10-DT as a nucleic acid biomarker after radiotherapy and that targeting this gene could be a therapeutic strategy to counteract angiogenesis in NSCLC radiotherapy.

The role of apoptosis, immunogenic cell death, and macrophage polarization in carbon ion radiotherapy for keloids: Targeting the TGF-β1/SMADs signaling pathway

Keloids, characterized by excessive extracellular matrix (ECM) deposition and aberrant fibrous tissue proliferation, present significant therapeutic challenges due to their recalcitrant and recurrent nature. This study explores the efficacy of Carbon Ion Radiotherapy (CIRT) as a novel therapeutic approach for keloids, focusing on its impact on fibroblast proliferation, apoptosis induction, immunogenic cell death (ICD), macrophage polarization, and the TGF-β/SMAD signaling pathway. Utilizing a murine model of keloid formed by subcutaneous injection of zeocin in C57BL/6 mice, we demonstrated that CIRT effectively reduces collagenous fiber synthesis and collagen production in keloid tissues. Further, CIRT was shown to inhibit keloid fibroblast proliferation and to induce apoptosis, as evidenced by increased expression of apoptosis-related proteins and confirmed through flow cytometry and TUNEL assay. Notably, CIRT induced mitochondrial stress, leading to enhanced immunogenicity of cell death, characterized by increased expression of ICD markers and secretion of interferon-γ. Additionally, CIRT promoted a shift from M2 to M1 macrophage polarization, potentially reducing TGF-β release and mitigating ECM deposition. Our findings suggest that CIRT mediates its therapeutic effects through the inhibition of the TGF-β/SMAD signaling pathway, thereby attenuating ECM formation and offering a promising avenue for keloid treatment. This study underscores the potential of CIRT as an innovative strategy for managing keloids, highlighting its multifaceted impact on key cellular processes involved in keloid pathogenesis.

Investigating the Local Effectiveness of Carbon Ion Radiotherapy for Unresectable Female Genital Tract Melanomas: A Preliminary Real-World Study

Background/Objectives: Primary gynecological melanomas are rare malignancies with lower survival rates compared to cutaneous melanomas. Both preclinical and clinical data support the evidence that mucosal melanomas are photon-radioresistant but responsive to carbon ion radiotherapy (CIRT). The aim of this study is to assess, in a real-world cohort, the effectiveness and tolerability of radical CIRT in patients with inoperable gynecological mucosal melanoma. Methods: This is a real-world study aimed to assess the effectiveness and the safety of CIRT in this setting. We defined as the primary endpoints the objective response rate (ORR) and the clinical benefit (CB). The secondary endpoints included the actuarial local control rate (LC) assessed after 1 year and 2 years and the toxicity scored according to CTCAE v.5. Actuarial outcomes were analyzed using the Kaplan-Meier method, while potential predictors were investigated through the Log-rank test. Results: Between 2017 and 2023, eleven Caucasian patients underwent pelvic CIRT (total dose 68.8 GyRBE) for mucosal malignant melanoma of the vulva or the vagina. With a median follow-up of 18 months, we observed an ORR of 82% and a CB of 100%. LC at 1 and 2 years was 100% and 86%, respectively, and among the factors analyzed for their potential impact on LC, age < 60 years seems to be a potential predictor (p = 0.014). The treatment was well tolerated, with only one case of acute grade 3 erythema and, in the late phase, one case of grade 3 erythema and grade 3 urethral toxicity. Conclusions: CIRT was effective and safe for gynecological melanomas. Larger collaborative cohort studies and longer follow-ups are needed to take a step forward in comprehending the correct management of this disease.

An empirical model of carbon-ion relative biological effectiveness based on the linear correlation between radiosensitivity to photons and carbon ions

Objective.To develop an empirical model to predict carbon ion (C-ion) relative biological effectiveness (RBE).Approach.We used published cell survival data comprising 360 cell line/energy combinations to characterize the linear energy transfer (LET) dependence of cell radiosensitivity parameters describing the dose required to achieve a given survival level, e.g. 5% (D5%), which are linearly correlated between photon and C-ion radiations. Based on the LET response of the metrics D5%and D37%, we constructed a model containing four free parameters that predicts cells' linear quadratic model (LQM) survival curve parameters for C-ions,αCandβC, from the reference LQM parameters for photons,αXandβX, for a given C-ion LET value. We fit our model's free parameters to the training dataset and assessed its accuracy via leave-one out cross-validation. We further compared our model to the local effect model (LEM) and the microdosimetric kinetic model (MKM) by comparing its predictions against published predictions made with those models for clinically relevant LET values in the range of 23-107 keVμm-1.Main Results.Our model predicted C-ion RBE within ±7%-15% depending on cell line and dose which was comparable to LEM and MKM for the same conditions.Significance.Our model offers comparable accuracy to the LEM or MKM but requires fewer input parameters and is less computationally expensive and whose implementation is so simple we provide it coded into a spreadsheet. Thus, our model can serve as a pragmatic alternative to these mechanistic models in cases where cell-specific input parameters cannot be obtained, the models cannot be implemented, or for which their computational efficiency is paramount.

Vaginal Mucosal Melanoma Cell Activation in Response to Photon or Carbon Ion Irradiation

Purpose

Primary gynecological melanomas are uncommon with lower survival rates compared to cutaneous melanomas. Although melanocytes have been identified in a variety of mucosal membranes, little is known about their interactions or roles inside the mucosa layer. Melanin is a common pigment in nature and is endowed with several peculiar chemical, paramagnetic, and semiconductive characteristics. One of its latest explored functions is its interaction with ionizing radiation as a protective mechanism as well as its implication in the metastatic cascade of tumor cells.

Materials and Methods

In this work, we analyzed in vitro the effects of different doses of photon and carbon ion irradiation on dendrite formation, pigmentation, migration, and invasion abilities of human mucosal melanoma cells of the vagina. We evaluated the morphology and melanin production of HMV-II cells exposed to photon and carbon ion beams with single doses between 0.5 and 10 Gy.

Results

Our results showed that irradiation induces dendrite formation or elongation and pigmentation in HMV-II cells in a dose-type-dependent and radiation-type-dependent way but also a decrease in cell motility.

Conclusion

The present study describes for the first time an induction of dendritic formation, melanin production, and alterations in migration and invasion abilities by low-linear energy transfer and high-linear energy transfer radiation in human mucosal melanoma cells, suggesting a radioprotective response to further possible exposures increasing the radioresistance of these cells.

Pilot study to assess the early cardiac safety of carbon ion radiotherapy for intra- and para-cardiac tumours

PURPOSE

Modern photon radiotherapy effectively spares cardiac structures more than previous volumetric approaches. Still, it is related to non-negligible cardiac toxicity due to the low-dose bath of surrounding normal tissues. However, the dosimetric advantages of particle radiotherapy make it a promising treatment for para- and intra-cardiac tumours. In the current short report, we evaluate the cardiac safety profile of carbon ion radiotherapy (CIRT) for radioresistant intra- and para-cardiac malignancies in a real-world setting.

METHODS

We retrospectively analysed serum biomarkers (TnI, CRP and NT-proBNP), echocardiographic, and both 12-lead and 24-hour Holter electrocardiogram (ECG) data of consecutive patients with radioresistant intra- and para-cardiac tumours irradiated with CIRT between June 2019 and September 2022. In the CIRT planning optimization process, to minimize the delivered doses, we contoured and gave a high priority to the cardiac substructures. Weekly re-evaluative 4D computed tomography scans were carried out throughout the treatment.

RESULTS

A total of 16 patients with intra- and para-cardiac localizations of radioresistant tumours were treated up to a total dose of 70.4 Gy relative biological effectiveness (RBE) and a mean heart dose of 2.41 Gy(RBE). We did not record any significant variation of the analysed serum biomarkers after CIRT nor significant changes of echocardiographic features, biventricular strain, or 12-lead and 24-hour Holter ECG parameters during 6 months of follow-up.

CONCLUSION

Our pilot study suggests that carbon ion radiotherapy is a promising radiation technique capable of sparing off-target side effects at the cardiac level. A larger cohort, long-term follow-up and further prospective studies are needed to confirm these findings.

Clinical Outcomes of Carbon Ion Radiation Therapy for Malignant Peripheral Nerve Sheath Tumors

Purpose

To investigate the outcome and toxicity of patients affected by malignant peripheral nerve sheath tumors (MPNSTs) treated with high-dose carbon ion radiation therapy (CIRT).

Methods and Materials

We retrospectively analyzed the outcome of 23 patients with MPNSTs treated between July 2013 and December 2020. Out of these, 13 patients (56.5%) had incompletely resected tumors, 8 patients (34.7%) experienced recurrence after surgery, and 2 patients (8.7%) had unresectable tumors. Before CIRT treatment, 4 patients underwent a second surgery after the first local recurrence (LR), and 1 patient underwent a third surgery for the second local relapse of the disease. Six (26%) patients received neoadjuvant chemotherapy. The most frequent tumor site was the brachial plexus (n = 9; 39.1%). In 5 patients (21.7%), neurofibromatosis type 1 disorder was found, while 4 patients (17, 4%) had radiation-induced MPNSTs. The median CIRT prescribed total dose was 69.8 Gy (relative biological effectiveness; range, 54-76.8) delivered in a median of 16 fractions (range, 15-22). Eleven patients (47.82%) were treated according to a sequential boost protocol with a median prescribed dose to clinical target volume LR of 45 Gy (relative biological effectiveness; range, 41.4-54).

Results

After a median follow-up time of 23 months (range, 3-100 months), the overall survival rates at 1 and 2 years were 82.38% and 61.51%, respectively. The 1-year and 2-year local relapse-free survival rates were 65.07% and 48.80%, respectively, and the 1-year and 2-year progression-free survival rates were 56.37% and 40.99%, respectively. No patients showed acute or late grade 4 toxicity or any treatment-related deaths. Ten patients (43.48%) reported acute toxicities of grade ≥ 2, which included dermatitis in 6 patients, mucositis in 2 patients, and peripheral neuropathy in 4 patients. Eight patients (34.78%) reported late toxicities of grade ≥ 2, mainly due to loco-regional neuropathy.

Conclusions

High-dose CIRT shows favorable local effects with acceptable toxicities in patients with gross residual and LR after surgery or unresectable malignant peripheral nerve sheath tumors. Advanced treatment modalities such as particle therapy should be considered for MPNSTs.

Determination of RBE of 450 MeV/nucleon carbon ions using the micronucleus test and survival of mice after irradiation in different regions of the Bragg curve

PURPOSE

Determination of the value of relative biological effectiveness (RBE) of heavy charged ions in vivo is an important task for their optimal use in particle radiotherapy. The aim of this study was to determine the RBE value of a beam of carbon ions with an energy of 450 MeV/nucleon in different regions of the Bragg curve in irradiation of mice at low, medium, and high doses in comparison with X-ray radiation.

MATERIALS AND METHODS

SHK mice (n = 330) were irradiated in three regions of the Bragg curve in the dose range of 0-1.5 Gy for cytogenetic damage detection and at a dose of 6.5 Gy for determination of 30-day survival. For irradiation of mice in the Bragg peak, two widths of a spread-out Bragg peak (SOBP) were used: 10 mm (LET ∼100 keV/µm) and 30 mm (LET ∼39 keV/µm).

RESULTS

The RBE value was 0.8-0.9 before the Bragg peak (LET ∼15 keV/µm) and 0.8 after the peak (LET ∼5 keV/µm), and did not depend on the determination method, despite the differences in LET values. The RBE value determined by the micronucleus test was 1.1-1.7 for the 10-mm-wide SOBP and 1.0-1.3 for the 30-mm-wide SOBP, with the highest RBE value obtained in the low-dose region upon irradiation of mice in the 10-mm-wide Bragg peak. The RBE values in the high-dose region determined by the 30-day survival test lay in the range from 1.4 to 2.6 depending on the width of the Bragg peak and the chosen criterion for calculating the value. The RBE values in the 10-mm-wide Bragg peak (LET ∼100 keV/µm) were higher than those in the 30-mm-wide Bragg peak (LET ∼39 keV/µm) at all used criteria.

CONCLUSIONS

The present findings suggest that there is the complex relationship between LET and organism response to accelerated charged particle radiation, and the contribution of specific factors and mechanisms must be further considered.

Proton ARC based LATTICE radiation therapy: feasibility study, energy layer optimization and LET optimization

Objective.LATTICE, a spatially fractionated radiation therapy (SFRT) modality, is a 3D generalization of GRID and delivers highly modulated peak-valley spatial dose distribution to tumor targets, characterized by peak-to-valley dose ratio (PVDR). Proton LATTICE is highly desirable, because of the potential synergy of the benefit from protons compared to photons, and the benefit from LATTICE compared to GRID. Proton LATTICE using standard proton RT via intensity modulated proton therapy (IMPT) (with a few beam angles) can be problematic with poor target dose coverage and high dose spill to organs-at-risk (OAR). This work will develop novel proton LATTICE method via proton ARC (with many beam angles) to overcome these challenges in target coverage and OAR sparing, with optimized delivery efficiency via energy layer optimization and optimized biological dose distribution via linear energy transfer (LET) optimization, to enable the clinical use of proton LATTICE.Approach.ARC based proton LATTICE is formulated and solved with energy layer optimization, during which plan quality and delivery efficiency are jointly optimized. In particular, the number of energy jumps (NEJ) is explicitly modelled and minimized during plan optimization for improving delivery efficiency, while target dose conformality and OAR dose objectives are optimized. The plan deliverability is ensured by considering the minimum-monitor-unit (MMU) constraint, and the plan robustness is accounted for using robust optimization. The biological dose is optimized via LET optimization. The optimization solution algorithm utilizes iterative convex relaxation method to handle the dose-volume constraint and the MMU constraint, with spot-weight optimization subproblems solved by proximal descent method.Main results.ARC based proton LATTCE substantially improved plan quality from IMPT based proton LATTICE, such as (1) improved conformity index (CI) from 0.47 to 0.81 for the valley target dose and from 0.62 to 0.97 for the peak target dose, (2) reduced esophagus dose from 0.68 Gy to 0.44 Gy (a 12% reduction with respect to 2 Gy valley prescription dose) and (3) improved PVDR from 4.15 to 4.28 in the lung case. Moreover, energy layer optimization improved plan delivery efficiency for ARC based proton LATTICE, such as (1) reduced NEJ from 71 to 56 and (2) reduction of energy layer switching time by 65% and plan delivery time by 52% in the lung case. The biological target and OAR dose distributions were further enhanced via LET optimization. On the other hand, proton ARC LATTCE also substantially improved plan quality from VMAT LATTICE, such as (1) improved CI from 0.45 to 0.81 for the valley target dose and from 0.63 to 0.97 for the peak target dose, (2) reduced esophagus dose from 0.59 Gy to 0.38 Gy (a 10.5% reduction with respect to 2 Gy valley prescription dose) and (3) improved PVDR from 3.88 to 4.28 in the lung case.Significance.The feasibility of high-plan-quality proton LATTICE is demonstrated via proton ARC with substantially improved target dose coverage and OAR sparing compared to IMPT, while the plan delivery efficiency for ARC based proton LATTICE can be optimized using energy layer optimization.

A novel treatment planning method via scissor beams for uniform-target-dose proton GRID with peak-valley-dose-ratio optimization

BACKGROUND

Proton spatially fractionated RT (SFRT) can potentially synergize the unique advantages of using proton Bragg peak and SFRT peak-valley dose ratio (PVDR) to reduce the radiation-induced damage for normal tissues. Uniform-target-dose (UTD) proton GRID is a proton SFRT modality that can be clinically desirable and conveniently adopted since its UTD resembles target dose distribution in conventional proton RT (CONV). However, UTD proton GRID is not used clinically, which is likely due to the lack of an effective treatment planning method.

PURPOSE

This work will develop a novel treatment planning method using scissor beams (SB) for UTD proton GRID, with the joint optimization of PVDR and dose objectives.

METHODS

The SB method for spatial dose modulation in normal tissues with UTD has two steps: (1) a primary beam (PB) is halved with interleaved beamlets, to generate spatial dose modulation in normal tissues; (2) a complementary beam (CB) is added to fill in previously valley-dose positions in the target to generate UTD, while the CB is angled slightly from the PB, to maintain spatial dose modulation in normal tissues. A treatment planning method with PVDR optimization via the joint total variation and L1 (TVL1) regularization is developed to jointly optimize PVDR and dose objectives. The plan optimization solution is obtained using an iterative convex relaxation algorithm.

RESULTS

The new methods SB and SB-TVL1 were validated in comparison with CONV. Compared to CONV of relatively homogeneous dose distribution, SB had modulated spatial dose pattern in normal tissues with UTD and comparable plan quality. Compared to SB, SB-TVL1 further maximized PVDR, with comparable dose-volume parameters.

CONCLUSIONS

A novel SB method is proposed that can generate modulated spatial dose pattern in normal tissues to achieve UTD proton GRID. A treatment planning method with PVDR optimization capability via TVL1 regularization is developed that can jointly optimize PVDR and dose objectives for proton GRID.

Charged particle radiotherapy for thyroid cancer. A systematic review

The role of external beam radiotherapy (EBRT) in thyroid cancer (TC) remains contentious due to limited data. Retrospective studies suggest adjuvant EBRT benefits high-risk differentiated thyroid cancer (DTC) and limited-stage anaplastic thyroid carcinoma (ATC), enhancing locoregional control and progression-free survival when combined with surgery and chemotherapy. Intensity-modulated radiotherapy (IMRT) and particle therapy (PT), including protons, carbon ions, and Boron Neutron Capture Therapy (BNCT), represent advances in TC treatment. Following PRISMA guidelines, we reviewed 471 studies from January 2002 to January 2024, selecting 14 articles (10 preclinical, 4 clinical). Preclinical research focused on BNCT in ATC mouse models, showing promising local control rates. Clinical studies explored proton, neutron, or photon radiotherapy, reporting favorable outcomes and manageable toxicity. While PT shows promise supported by biological rationale, further research is necessary to clarify its role and potential combination with systemic treatments in TC management.

Modelling radiobiology

Radiotherapy has played an essential role in cancer treatment for over a century, and remains one of the best-studied methods of cancer treatment. Because of its close links with the physical sciences, it has been the subject of extensive quantitative mathematical modelling, but a complete understanding of the mechanisms of radiotherapy has remained elusive. In part this is because of the complexity and range of scales involved in radiotherapy-from physical radiation interactions occurring over nanometres to evolution of patient responses over months and years. This review presents the current status and ongoing research in modelling radiotherapy responses across these scales, including basic physical mechanisms of DNA damage, the immediate biological responses this triggers, and genetic- and patient-level determinants of response. Finally, some of the major challenges in this field and potential avenues for future improvements are also discussed.

Particle Beam Radiobiology Status and Challenges: A PTCOG Radiobiology Subcommittee Report

Particle therapy (PT) represents a significant advancement in cancer treatment, precisely targeting tumor cells while sparing surrounding healthy tissues thanks to the unique depth-dose profiles of the charged particles. Furthermore, their linear energy transfer and relative biological effectiveness enhance their capability to treat radioresistant tumors, including hypoxic ones. Over the years, extensive research has paved the way for PT's clinical application, and current efforts aim to refine its efficacy and precision, minimizing the toxicities. In this regard, radiobiology research is evolving toward integrating biotechnology to advance drug discovery and radiation therapy optimization. This shift from basic radiobiology to understanding the molecular mechanisms of PT aims to expand the therapeutic window through innovative dose delivery regimens and combined therapy approaches. This review, written by over 30 contributors from various countries, provides a comprehensive look at key research areas and new developments in PT radiobiology, emphasizing the innovations and techniques transforming the field, ranging from the radiobiology of new irradiation modalities to multimodal radiation therapy and modeling efforts. We highlight both advancements and knowledge gaps, with the aim of improving the understanding and application of PT in oncology.

Relative Biological Effectiveness of Carbon Ion Beams for Induction of Medulloblastoma with Radiation-specific Chromosome 13 Deletion in Ptch1+/- Mice

Carbon ion radiotherapy (CIRT) for pediatric cancer is currently limited because of the unknown risk of induction of secondary cancers. Medulloblastoma of Ptch1+/- mice offers a unique experimental system for radiation-induced carcinogenesis, in which tumors are classified into spontaneous and radiation-induced subtypes based on their features of loss of heterozygosity (LOH) that affect the wild-type Ptch1 allele. The present study aims to investigate in young Ptch1+/- mice the carcinogenic effect, and its age dependence, of the low-linear energy transfer (LET, ∼13 keV/µm) carbon ions, to which normal tissues in front of the tumor are exposed during therapy. We irradiated Ptch1+/- mice at postnatal day (P) 1, 4, or 10 with 290 MeV/u carbon ions (0.05-0.5 Gy; LET, 13 keV/µm) and monitored them for medulloblastoma development. Loss of heterozygosity of seven genetic markers on chromosome 13 (where Ptch1 resides) was studied to classify the tumors. Carbon ion exposure induced medulloblastoma most effectively at P1. The LOH patterns of tumors were either telomeric or interstitial, the latter occurring almost exclusively in the irradiated groups, allowing the use of interstitial LOH as a biomarker of radiation-induced tumors. Radiation-induced tumors developed during a narrow age window (most strongly at P1 and only moderately at P4, with suppressed tumorigenesis at P10). Calculated using previous results using 137Cs gamma rays, the values for relative biological effectiveness (RBE) regarding radiation-induced tumors were 4.1 (3.4, 4.8) and 4.3 (3.3, 5.2) (mean and 95% confidence interval) for exposure at P1 and 4, respectively. Thus, the RBE of carbon ions for medulloblastoma induction in Ptch1+/- mice was higher than the generally recognized RBE of 1-2 for cell killing, chromosome aberrations, and skin reactions.

Hypofractionated proton and carbon ion beam radiotherapy for sacrococcygeal chordoma (ISAC): An open label, randomized, stratified, phase II trial

INTRODUCTION

Sacrococcygeal chordomas have high recurrence rates and are challenging to treat.

METHODS

In this phase II prospective, randomized, stratified trial, the safety and feasibility of hypofractionated ion radiation therapy were investigated. The primary focus was monitored through the incidence of Grade 3-5 NCI-CTC-AE toxicity. Secondary endpoints included local progression-free (LPFS) and overall survival (OS).

RESULTS

The study enrolled 82 patients with primary (87 %) and recurrent (13 %) inoperable or incompletely resected sacral chordomas from January 2013 to July 2022, divided equally into proton therapy (Arm A) and carbon ion beam therapy (Arm B) groups, each receiving a total dose of 64 Gy (RBE) in 16 fractions, 5-6 fractions per week. Overall 74 % of patients received no previous surgery and 66 % of tumors were confirmed by a brachyury staining. The mean and median Gross Tumor Volume at the time of treatment (GTV) was 407 ml and 185 ml, respectively. The median follow-up of the surviving patients was 44.7 months, and the 2-year and 4-year OS rates were 96 % and 81 %, respectively. Factors such as smaller GTV and younger age trended towards better OS. The LPFS after 2-year and 4-year was 84 % and 70 %, respectively. Male gender emerged as a significant predictor of LPFS. There was no significant difference between the treatment groups. We observed five grade 4 wound healing disorders (6 %).

CONCLUSION

The initial response rates were promising; however local control was not sustained. More comparative research on fractionation schemes is essential to refine treatment approaches for inoperable sacral chordoma.

Specific spectral sub-images for machine learning evaluation of optical differences between carbon ion and X ray radiation effects

Advances in radiotherapy, particularly the exploration of alternative radiation types such as carbon ions have updated our understanding of its effects and applicability on chondrosarcoma cells. Here we compare the optical effects produced by carbon ions (CI) and X-rays (XR) radiations on chondrosarcoma cells nuclei and set an automated method for evaluating the radiation-induced alterations without the need of chemical marking. Hyperspectral images (HSI) of SW1353 chondrosarcoma line carry detectable optical changes of the cells irradiated either with CI or XR compared to non-irradiated ones (REF). The differences between the spectral profiles of CI, XR and REF nuclei classes led to partitioning the HSIs into spectral sub-images. The changes are detected by support vector machine (SVM) classifiers whose performances are evaluated by the most used point metrics: sensitivity (SEN), accuracy (ACC), and precision (PREC), applied on spatial feature values. Specific interaction mechanisms by radiation type reveal distinct subintervals where HSIs changes are more prominent, and the classifiers perform at best. For CI the best classifiers are obtained for sub-images in the interval (424-436 nm), while for XR the best classifiers are obtained for sub-images in the interval (436-445 nm). The classifiers work better with texture features than roughness features in both cases. The classifier with the best SEN point metric in the testing phase is the most suitable to measure the irradiation efficiency irrespective of the radiation type. The altered nuclei are easier to discriminate when irradiated with CI than with XR. The study proves that SVM with optical data offers a rapid, automated, and label-free method for evaluating radiation-induced alterations in chondrosarcoma nuclei, thereby enabling effective analysis of extensive data.

Dosimetric study for breathing-induced motion effects in an abdominal pancreas phantom for carbon ion mini-beam radiotherapy

BACKGROUND

Particle mini-beam therapy exhibits promise in sparing healthy tissue through spatial fractionation, particularly notable for heavy ions, further enhancing the already favorable differential biological effectiveness at both target and entrance regions. However, breathing-induced organ motion affects particle mini-beam irradiation schemes since the organ displacements exceed the mini-beam structure dimensions, decreasing the advantages of spatial fractionation.

PURPOSE

In this study, the impact of breathing-induced organ motion on the dose distribution was examined at the target and organs at risk(OARs) during carbon ion mini-beam irradiation for pancreatic cancer.

METHODS

As a first step, the carbon ion mini-beam pattern was characterized with Monte Carlo simulations. To analyze the impact of breathing-induced organ motion on the dose distribution of a virtual pancreas tumor as target and related OARs, the anthropomorphic Pancreas Phantom for Ion beam Therapy (PPIeT) was irradiated with carbon ions. A mini-beam collimator was used to deliver a spatially fractionated dose distribution. During irradiation, varying breathing motion amplitudes were induced, ranging from 5 to 15 mm. Post-irradiation, the 2D dose pattern was analyzed, focusing on the full width at half maximum (FWHM), center-to-center distance (ctc), and the peak-to-valley dose ratio (PVDR).

RESULTS

The mini-beam pattern was visible within OARs, while in the virtual pancreas tumor a more homogeneous dose distribution was achieved. Applied motion affected the mini-beam pattern within the kidney, one of the OARs, reducing the PVDR from 3.78  ± $\pm$  0.12 to 1.478  ± $\pm$  0.070 for the 15 mm motion amplitude. In the immobile OARs including the spine and the skin at the back, the PVDR did not change within 3.4% comparing reference and motion conditions.

CONCLUSIONS

This study provides an initial understanding of how breathing-induced organ motion affects spatial fractionation during carbon ion irradiation, using an anthropomorphic phantom. A decrease in the PVDR was observed in the right kidney when breathing-induced motion was applied, potentially increasing the risk of damage to OARs. Therefore, further studies are needed to explore the clinical viability of mini-beam radiotherapy with carbon ions when irradiating abdominal regions.

Secretome from iPSC-derived MSCs exerts proangiogenic and immunosuppressive effects to alleviate radiation-induced vascular endothelial cell damage

BACKGROUND

Radiation therapy is the standard of care for central nervous system tumours. Despite the success of radiation therapy in reducing tumour mass, irradiation (IR)-induced vasculopathies and neuroinflammation contribute to late-delayed complications, neurodegeneration, and premature ageing in long-term cancer survivors. Mesenchymal stromal cells (MSCs) are adult stem cells that facilitate tissue integrity, homeostasis, and repair. Here, we investigated the potential of the iPSC-derived MSC (iMSC) secretome in immunomodulation and vasculature repair in response to radiation injury utilizing human cell lines.

METHODS

We generated iPSC-derived iMSC lines and evaluated the potential of their conditioned media (iMSC CM) to treat IR-induced injuries in human monocytes (THP1) and brain vascular endothelial cells (hCMEC/D3). We further assessed factors in the iMSC secretome, their modulation, and the molecular pathways they elicit.

RESULTS

Increasing doses of IR disturbed endothelial tube and spheroid formation in hCMEC/D3. When IR-injured hCMEC/D3 (IR ≤ 5 Gy) were treated with iMSC CM, endothelial cell viability, adherence, spheroid compactness, and proangiogenic sprout formation were significantly ameliorated, and IR-induced ROS levels were reduced. iMSC CM augmented tube formation in cocultures of hCMEC/D3 and iMSCs. Consistently, iMSC CM facilitated angiogenesis in a zebrafish model in vivo. Furthermore, iMSC CM suppressed IR-induced NFκB activation, TNF-α release, and ROS production in THP1 cells. Additionally, iMSC CM diminished NF-kB activation in THP1 cells cocultured with irradiated hCMEC/D3, iMSCs, or HMC3 microglial lines. The cytokine array revealed that iMSC CM contains the proangiogenic and immunosuppressive factors MCP1/CCL2, IL6, IL8/CXCL8, ANG (Angiogenin), GROα/CXCL1, and RANTES/CCL5. Common promoter regulatory elements were enriched in TF-binding motifs such as androgen receptor (ANDR) and GATA2. hCMEC/D3 phosphokinome profiling revealed increased expression of pro-survival factors, the PI3K/AKT/mTOR modulator PRAS40 and β-catenin in response to CM. The transcriptome analysis revealed increased expression of GATA2 in iMSCs and the enrichment of pathways involved in RNA metabolism, translation, mitochondrial respiration, DNA damage repair, and neurodevelopment.

CONCLUSIONS

The iMSC secretome is a comodulated composite of proangiogenic and immunosuppressive factors that has the potential to alleviate radiation-induced vascular endothelial cell damage and immune activation.

Experimental evidence that carbon-ion radiotherapy utilizes cytotoxic T lymphocyte-mediated anti-tumor immunity for shrinking tumors compared to X-ray therapy

The therapeutic efficacy of radiotherapy (RT) is primarily driven by two factors: biophysical DNA damage in cancer cells and radiation-induced anti-tumor immunity. However, Anti-tumor immune responses between X-ray RT (XRT) and carbon-ion RT (CIRT) remain unclear. In this study, we, employed mouse models to assess the immunological contribution, especially cytotoxic T-lymphocyte (CTL)-mediated immunity, to the therapeutic effectiveness of XRT and CIRT in shrinking tumors. We irradiated mouse intradermal tumors of B16F10-ovalbumin (OVA) mouse melanoma cells and 3LL-OVA mouse lung cancer cells with carbon-ion beams or X-rays in the presence or absence of CTLs. CTL removal was performed by administration of anti-CD8 monoclonal antibody (mAb) in mice. Based on tumor growth delay, we determined the tumor growth and regression curves. The enhancement ratio (ER) of the slope of regression lines in the presence of CTLs, relative to the absence of CTLs, indicates the dependency of RT on CTLs for shrinking mouse tumors, and the biological effectiveness (RBE) of CIRT relative to XRT were calculated. Tumor growth curves revealed that the elimination of CD8+ CTLs by administrating anti-CD8 mAb accelerated tumor growth compared to the presence of CTLs in both RTs. The ERs were larger in CIRT compared to XRT in the B16F10-OVA tumor models, but not in the 3LL-OVA models, suggesting a greater contribution of CTL-mediated anti-tumor immunity to tumor reduction in CIRT compared to XRT in the B16F10-OVA tumor model. In addition, the RBE values for both models were larger in the presence of CTLs compared to models without CTLs, suggesting that CIRT may utilize CTL-mediated anti-tumor immunity more than X-ray. The findings from this study suggest that although immunological contribution to therapeutic efficacy may vary depending on the type of tumor cell, CIRT utilizes CTL-mediated immunity to a greater extent compared to XRT.

Deep learning-based voxel sampling for particle therapy treatment planning

Objective.Scanned particle therapy often requires complex treatment plans, robust optimization, as well as treatment adaptation. Plan optimization is especially complicated for heavy ions due to the variable relative biological effectiveness. We present a novel deep-learning model to select a subset of voxels in the planning process thus reducing the planning problem size for improved computational efficiency.Approach.Using only a subset of the voxels in target and organs at risk (OARs) we produced high-quality treatment plans, but heuristic selection strategies require manual input. We designed a deep-learning model based onP-Net to obtain an optimal voxel sampling without relying on patient-specific user input. A cohort of 70 head and neck patients that received carbon ion therapy was used for model training (50), validation (10) and testing (10). For training, a total of 12 500 carbon ion plans were optimized, using a highly efficient artificial intelligence (AI) infrastructure implemented into a research treatment planning platform. A custom loss function increased sampling density in underdosed regions, while aiming to reduce the total number of voxels.Main results.On the test dataset, the number of voxels in the optimization could be reduced by 84.8% (median) at <1% median loss in plan quality. When the model was trained to reduce sampling in the target only while keeping all voxels in OARs, a median reduction up to 71.6% was achieved, with 0.5% loss in the plan quality. The optimization time was reduced by a factor of 7.5 for the total AI selection model and a factor of 3.7 for the model with only target selection.Significance.The novel deep-learning voxel sampling technique achieves a significant reduction in computational time with a negligible loss in the plan quality. The reduction in optimization time can be especially useful for future real-time adaptation strategies.

Balancing benefits and limitations of linear energy transfer optimization in carbon ion radiotherapy for large sacral chordomas

Background and Purpose

A low linear energy transfer (LET) in the target can reduce the effectiveness of carbon ion radiotherapy (CIRT). This study aimed at exploring benefits and limitations of LET optimization for large sacral chordomas (SC) undergoing CIRT.

Materials and Methods

Seventeen cases were used to tune LET-based optimization, and seven to independently test interfraction plan robustness. For each patient, a reference plan was optimized on biologically-weighted dose cost functions. For the first group, 7 LET-optimized plans were obtained by increasing the gross tumor volume (GTV) minimum LETd (minLETd) in the range 37-55 keV/μm, in steps of 3 keV/μm. The optimal LET-optimized plan (LETOPT) was the one maximizing LETd, while adhering to clinical acceptability criteria. Reference and LETOPT plans were compared through dose and LETd metrics (D x , L x to x% volume) for the GTV, clinical target volume (CTV), and organs at risk (OARs). The 7 held-out cases were optimized setting minLETd to the average GTV L98% of the investigation cohort. Both reference and LETOPT plans were recalculated on re-evaluation CTs and compared.

Results

GTV L98% increased from (31.8 ± 2.5)keV/μm to (47.6 ± 3.1)keV/μm on the LETOPT plans, while the fraction of GTV receiving over 50 keV/μm increased on average by 36% (p < 0.001), without affecting target coverage goals, or impacting LETd and dose to OARs. The interfraction analysis showed no significant worsening with minLETd set to 48 keV/μm.

Conclusion

LETd optimization for large SC could boost the LETd in the GTV without significantly compromising plan quality, potentially improving the therapeutic effects of CIRT for large radioresistant tumors.

The first real-world study on the role of carbon ion radiotherapy for oligo-metastatic, persistent, or recurrent (MPR) ovarian/fallopian tube cancer

Introduction

In the multidisciplinary management of oligometastatic, persistent, or recurrent (MPR) ovarian cancer, radiotherapy (RT) is becoming a more and more worthwhile treatment to potentially improve the chronicity of the disease. Particle beam RT has proved to be effective in several gynecological malignancies, but so far no data are available for ovarian cancer.

Material and Methods

This is a real-world, retrospective, bi-institutional, single-arm study aimed to assess the effectiveness and the safety of carbon ion RT (CIRT) in this setting. The co-first endpoints are 1-year and 2-year actuarial local control (LC) rates and the objective response rate (ORR) defined on a "per lesion" basis. The secondary endpoint was toxicity. Actuarial outcomes were evaluated using the Kaplan-Meier method while potential predictors were explored using the Log-rank test. Bi-variable logistic regression was employed in the analysis of factors predicting the complete response on a per-lesion basis.

Results

26 patients accounting for a total of 36 lesions underwent CIRT with a total median dose of 52.8 Gy[RBE] (range: 39-64 Gy[RBE]). Five patients received CIRT for re-irradiation. No concomitant systemic therapies were administered during CIRT. Within 12 months after the treatment, 17 lesions (47 %) achieved complete response while 18 (50 %) obtained a partial response with an ORR of 97 %. The achievement of a complete response is related to the dose per fraction (>4.2 Gy[RBE], p = 0.04) and total dose (>52,8 Gy[RBE], p = 0.05). The 1-year LC was 92 % and the 2-year LC was 83 %, according to the achievement of a CR (p = 0.007) and GTV ≤ 14 cm3 (p = 0.024). No grade > 3 toxicities were recorded both in naïve and re-irradiated patients. PARP-i and anti-VEGF seemed not to exacerbate the risk of severe toxicities.

Conclusions

CIRT was effective and safe in MPR ovarian cancers, even in the case of re-irradiation. Largest cohort studies and longer follow-up are needed to confirm these data.

Rat Models of Hormone Receptor-Positive Breast Cancer

Hormone receptor-positive (HR+) breast cancer (BC) is the most common type of breast cancer among women worldwide, accounting for 70-80% of all invasive cases. Patients with HR+ BC are commonly treated with endocrine therapy, but intrinsic or acquired resistance is a frequent problem, making HR+ BC a focal point of intense research. Despite this, the malignancy still lacks adequate in vitro and in vivo models for the study of its initiation and progression as well as response and resistance to endocrine therapy. No mouse models that fully mimic the human disease are available, however rat mammary tumor models pose a promising alternative to overcome this limitation. Compared to mice, rats are more similar to humans in terms of mammary gland architecture, ductal origin of neoplastic lesions and hormone dependency status. Moreover, rats can develop spontaneous or induced mammary tumors that resemble human HR+ BC. To date, six different types of rat models of HR+ BC have been established. These include the spontaneous, carcinogen-induced, transplantation, hormone-induced, radiation-induced and genetically engineered rat mammary tumor models. Each model has distinct advantages, disadvantages and utility for studying HR+ BC. This review provides a comprehensive overview of all published models to date.

Development and characterization of the first proton minibeam system for single-gantry proton facility

BACKGROUND

Minibeam represents a preclinical spatially fractionated radiotherapy modality with great translational potential. The advantage lies in its high therapeutic index (compared to GRID and LATTICE) and ability to treat at greater depth (compared to microbeam). Proton minibeam radiotherapy (pMBRT) is a synergy of proton and minibeam. While the single-gantry proton facility has gained popularity due to its affordability and compact design, it often has limited beam time available for research purposes. Conversely, given the current requirement of pMBRT on specific minibeam hardware collimators, necessitates a reproducible and fast setup to minimize pMBRT treatment time and streamline the switching time between pMBRT and conventional treatment for clinically translation.

PURPOSE

The contribution of this work is the development and characterization of the first pMBRT system tailored for single-gantry proton facility. The system allows for efficient and reproducible plug-and-play setup, achievable within minutes.

METHODS

The single room pMBRT system is constructed based on IBA ProteusONE proton machine. The end of nozzle is attached with beam modifying accessories though an accessory drawer. A small snout is attached to the accessory drawer and used to hold apertures and range shifters. The minibeam aperture consists of two components: a fitting ring and an aperture body. Three minibeam apertures were manufactured. The first-generation apertures underwent qualitatively analysis with film, and the second generation aperture underwent more comprehensive quantitative measurement. The reproducibility of the setup is accessed, and the film measurements are performed to characterize the pMBRT system in cross validation with Monte Carlo (MC) simulations.

RESULTS

We presented initial results of large field pMBRT aperture and the film measurements indicates the effect of source-to-isocenter distance = 930 cm in Y proton scanning direction. Consistent with TOPAS MC simulation, the dose uniformity of pMBRT field <2 cm is demonstrated to be better than 2%, rendering its suitability for pre-clinical studies. Subsequently, we developed the second generation of aperture with five slits and characterized the aperture with film dosimetry studies and compared the results to the benchmark MC. Comprehensive film measurements were also performed to evaluate the effect of divergence, air gap and gantry-angle dependency and repeatability and revealing a consistent performance within 5%. Furthermore, the 2D gamma analysis indicated a passing rate exceeding 99% using 3% dose difference and 0.2 mm distance agreement criteria. We also establish the peak valley dose ratio and the depth dose profile measurements, and the results are within 10% from MC simulation.

CONCLUSIONS

We have developed the first pMBRT system tailored for a single-gantry proton facility, which has demonstrated accuracy in benchmark with MC simulations, and allows for efficient plug-and-play setup, emphasizing efficiency.

Dose averaged linear energy transfer optimization for large sacral chordomas in carbon ion therapy

BACKGROUND

Carbon ion beams are well accepted as densely ionizing radiation with a high linear energy transfer (LET). However, the current clinical practice does not fully exploit the highest possible dose-averaged LET (LETd) and, consequently, the biological potential in the target. This aspect becomes worse in larger tumors for which inferior clinical outcomes and corresponding lower LETd was reported.

PURPOSE

The vicinity to critical organs in general and the inferior overall survival reported for larger sacral chordomas treated with carbon ion radiotherapy (CIRT), makes the treatment of such tumors challenging. In this work it was aimed to increase the LETd in large volume tumors while maintaining the relative biological effectiveness (RBE)-weighted dose, utilizing the LETd optimization functions of a commercial treatment planning system (TPS).

METHODS

Ten reference sequential boost carbon ion treatment plans, designed to mimic clinical plans for large sacral chordoma tumors, were generated. High dose clinical target volumes (CTV-HD) larger than250 cm 3 $250 \,{\rm cm}^{3}$ were considered as large targets. The total RBE-weighted median dose prescription with the local effect model (LEM) wasD RBE , 50 % = 73.6 Gy $\textrm {D}_{\rm RBE, 50\%}=73.6 \,{\rm Gy}$ in 16 fractions (nine to low dose and seven to high dose planning target volume). No LETd optimization was performed in the reference plans, while LETd optimized plans used the minimum LETd (Lmin) optimization function in RayStation 2023B. Three different Lmin values were investigated and specified for the seven boost fractions:L min = 60 keV / μ m $\textrm {L}_{\rm min}=60 \,{\rm keV}/{\umu }{\rm m}$ ,L min = 80 keV / μ m $\textrm {L}_{\rm min}=80 \,{\rm keV}/{\umu }{\rm m}$ andL min = 100 keV / μ m $\textrm {L}_{\rm min}=100 \,{\rm keV}/{\umu }{\rm m}$ . To compare the LETd optimized against reference plans, LETd and RBE-weighted dose based goals similar to and less strict than clinical ones were specified for the target. The goals for the organs at risk (OAR) remained unchanged. Robustness evaluation was studied for eight scenarios (± 3.5 % $\pm 3.5\%$ range uncertainty and± 3 mm $\pm 3 \,{\rm mm}$ setup uncertainty along the main three axes).

RESULTS

The optimization method withL min = 60 keV / μ m $\textrm {L}_{\rm min}=60 \,{\rm keV}/{\umu }{\rm m}$ resulted in an optimal LETd distribution with an average increase ofLET d , 98 % ${\rm {LET}}_{{\rm {d,}}98\%}$ (andLET d , 50 % ${\rm {LET}}_{{\rm {d,}}50\%}$ ) in the CTV-HD by8.9 ± 1.5 keV / μ m $8.9\pm 1.5 \,{\rm keV}/{\umu }{\rm m}$ (27 % $27\%$ ) (and6.9 ± 1.3 keV / μ m $6.9\pm 1.3 \,{\rm keV}/{\umu }{\rm m}$ (17 % $17\%$ )), without significant difference in the RBE-weighted dose. By allowing± 5 % $\pm 5\%$ over- and under-dosage in the target, theLET d , 98 % ${\rm {LET}}_{{\rm {d,}}98\%}$ (andLET d , 50 % ${\rm {LET}}_{{\rm {d,}}50\%}$ ) can be increased by11.3 ± 1.2 keV / μ m $11.3\pm 1.2 \,{\rm keV}/{\umu }{\rm m}$ (34 % $34\%$ ) (and11.7 ± 3.4 keV / μ m $11.7\pm 3.4 \,{\rm keV}/{\umu }{\rm m}$ (29 % $29\%$ )), using the optimization parametersL min = 80 keV / μ m $\textrm {L}_{\rm min}=80 \,{\rm keV}/{\umu }{\rm m}$ . The pass rate for the OAR goals in the LETd optimized plans was in the same level as the reference plans. LETd optimization lead to less robust plans compared to reference plans.

CONCLUSIONS

Compared to conventionally optimized treatment plans, the LETd in the target was increased while maintaining the RBE-weighted dose using TPS LETd optimization functionalities. Regularly assessing RBE-weighted dose robustness and acquiring more in-room images remain crucial and inevitable aspects during treatment.

Optical probing of ultrafast laser-induced solid-to-overdense-plasma transitions

Understanding the solid target dynamics resulting from the interaction with an ultrashort laser pulse is a challenging fundamental multi-physics problem involving atomic and solid-state physics, plasma physics, and laser physics. Knowledge of the initial interplay of the underlying processes is essential to many applications ranging from low-power laser regimes like laser-induced ablation to high-power laser regimes like laser-driven ion acceleration. Accessing the properties of the so-called pre-plasma formed as the laser pulse's rising edge ionizes the target is complicated from the theoretical and experimental point of view, and many aspects of this laser-induced transition from solid to overdense plasma over picosecond timescales are still open questions. On the one hand, laser-driven ion acceleration requires precise control of the pre-plasma because the efficiency of the acceleration process crucially depends on the target properties at the arrival of the relativistic intensity peak of the pulse. On the other hand, efficient laser ablation requires, for example, preventing the so-called "plasma shielding". By capturing the dynamics of the initial stage of the interaction, we report on a detailed visualization of the pre-plasma formation and evolution. Nanometer-thin diamond-like carbon foils are shown to transition from solid to plasma during the laser rising edge with intensities < 1016 W/cm². Single-shot near-infrared probe transmission measurements evidence sub-picosecond dynamics of an expanding plasma with densities above 1023 cm-3 (about 100 times the critical plasma density). The complementarity of a solid-state interaction model and kinetic plasma description provides deep insight into the interplay of initial ionization, collisions, and expansion.

A perspective on tumor radiation resistance following high-LET radiation treatment

High-linear energy transfer (LET) radiation is a promising alternative to conventional low-LET radiation for therapeutic gain against cancer owing to its ability to induce complex and clustered DNA lesions. However, the development of radiation resistance poses a significant barrier. The potential molecular mechanisms that could confer resistance development are translesion synthesis (TLS), replication gap suppression (RGS) mechanisms, autophagy, epithelial-mesenchymal transition (EMT) activation, release of exosomes, and epigenetic changes. This article will discuss various types of complex clustered DNA damage, their repair mechanisms, mutagenic potential, and the development of radiation resistance strategies. Furthermore, it highlights the importance of careful consideration and patient selection when employing high-LET radiotherapy in clinical settings.

Review of Recent Improvements in Carbon Ion Radiation Therapy in the Treatment of Glioblastoma

Purpose

This article provides an overview of the physical and biologic properties of carbon ions, followed by an examination of the latest clinical outcomes in patients with glioma who have received carbon ion radiation therapy.

Methods and Materials

According to thee articles that have been reviewed, glioma represents the predominant form of neoplastic growth in the brain, accounting for approximately 51% of all malignancies affecting the nervous system. Currently, high-grade glioma, specifically glioblastoma, comprises 15% of cases and is associated with a high mortality rate. The development of novel drugs for the treatment of high-grade tumors has been impeded by various factors, such as the blood-brain barrier and tumor heterogeneity, despite numerous endeavors. According to the definition of tumor grade established by the World Health Organization, the conventional treatment involves surgical resection followed by adjuvant radiation and chemotherapy. Despite numerous attempts in photon radiation therapy to apply the highest possible dose to the tumor site while minimizing damage to healthy tissue, there has been no success in increasing patient survival. The primary cause of resistance to conventional radiation therapy methods, namely x-ray and gamma-ray, is attributed to the survival of radio-resistant glioma stem cells, which have the potential to trigger a recurrence of tumors. Particle beams, such as protons and carbon ions, can deposit the highest dose to a confined region, thus offering a more accurate dose distribution compared with photon beams.

Results

Carbon ions exhibit higher linear energy transfer and relative biologic effectiveness compared with photons, potentially enabling them to overcome radio-resistant tumor cells.

Conclusions

Therefore, it can be hypothesized that carbon ion radiation therapy may show superior efficacy in destroying neoplastic cells with reduced negative outcomes compared with x-ray radiation therapy.

Is motherhood still possible after pelvic carbon ion radiotherapy? A promising combined fertility-preservation approach

INTRODUCTION

Preserving the endocrine and reproductive function in young female cancer patients undergoing pelvic radiation is a significant challenge. While the photon beam radiation's adverse effects on the uterus and ovaries are well established, the impact of pelvic carbon ion radiotherapy on women's reproductive function is largely unexplored. Strategies such as oocyte cryopreservation and ovarian transposition are commonly recommended for safeguarding future fertility.

METHODS

This study presents a pioneering case of successful pregnancy after carbon ion radiotherapy for locally advanced sacral chondrosarcoma.

RESULTS

A multidisciplinary approach facilitated the displacement of ovaries and uterus before carbon ion radiotherapy, resulting in the preservation of endocrine and reproductive function.

CONCLUSION

The patient achieved optimal oncological response and delivered a healthy infant following the completion of cancer treatment.

The rationale for a carbon ion radiation therapy facility in Australia

Australia has taken a collaborative nationally networked approach to achieve particle therapy capability. This supports the under-construction proton therapy facility in Adelaide, other potential proton centres and an under-evaluation proposal for a hybrid carbon ion and proton centre in western Sydney. A wide-ranging overview is presented of the rationale for carbon ion radiation therapy, applying observations to the case for an Australian facility and to the clinical and research potential from such a national centre.

Hypoxia-informed RBE-weighted beam orientation optimization for intensity modulated proton therapy

BACKGROUND

Variable relative biological effectiveness (RBE) models in treatment planning have been proposed to optimize the therapeutic ratio of proton therapy. It has been reported that proton RBE decreases with increasing tumor oxygen level, offering an opportunity to address hypoxia-related radioresistance with RBE-weighted optimization.

PURPOSE

Here, we obtain a voxel-level estimation of partial oxygen pressure to weigh RBE values in a single biologically informed beam orientation optimization (BOO) algorithm.

METHODS

Three glioblastoma patients with [18 F]-fluoromisonidazole (FMISO)-PET/CT images were selected from the institutional database. Oxygen values were derived from tracer uptake using a nonlinear least squares curve fitting. McNamara RBE, calculated from proton dose, was then weighed using oxygen enhancement ratios (OER) for each voxel and incorporated into the dose fidelity term of the BOO algorithm. The nonlinear optimization problem was solved using a split-Bregman approach, with FISTA as the solver. The proposed hypoxia informed RBE-weighted method (HypRBE) was compared to dose fidelity terms using the constant RBE of 1.1 (cRBE) and the normoxic McNamara RBE model (RegRBE). Tumor homogeneity index (HI), maximum biological dose (Dmax), and D95%, as well as OAR therapeutic index (TI = gEUDCTV /gEUDOAR ) were evaluated along with worst-case statistics after normalization to normal tissue isotoxicity.

RESULTS

Compared to [cRBE, RegRBE], HypRBE increased tumor HI, Dmax, and D95% across all plans by on average [31.3%, 31.8%], [48.6%, 27.1%], and [50.4%, 23.8%], respectively. In the worst-case scenario, the parameters increase on average by [12.5%, 14.7%], [7.3%,-8.9%], and [22.3%, 2.1%]. Despite increased OAR Dmean and Dmax by [8.0%, 3.0%] and [13.1%, -0.1%], HypRBE increased average TI by [22.0%, 21.1%]. Worst-case OAR Dmean, Dmax, and TI worsened by [17.9%, 4.3%], [24.5%, -1.2%], and [9.6%, 10.5%], but in the best cases, HypRBE escalates tumor coverage significantly without compromising OAR dose, increasing the therapeutic ratio.

CONCLUSIONS

We have developed an optimization algorithm whose dose fidelity term accounts for hypoxia-informed RBE values. We have shown that HypRBE selects bE:\Alok\aaeams better suited to deliver high physical dose to low RBE, hypoxic tumor regions while sparing the radiosensitive normal tissue.

Modification of BCLX pre-mRNA splicing has antitumor efficacy alone or in combination with radiotherapy in human glioblastoma cells

Dysregulation of anti-apoptotic and pro-apoptotic protein isoforms arising from aberrant splicing is a crucial hallmark of cancers and may contribute to therapeutic resistance. Thus, targeting RNA splicing to redirect isoform expression of apoptosis-related genes could lead to promising anti-cancer phenotypes. Glioblastoma (GBM) is the most common type of malignant brain tumor in adults. In this study, through RT-PCR and Western Blot analysis, we found that BCLX pre-mRNA is aberrantly spliced in GBM cells with a favored splicing of anti-apoptotic Bcl-xL. Modulation of BCLX pre-mRNA splicing using splice-switching oligonucleotides (SSOs) efficiently elevated the pro-apoptotic isoform Bcl-xS at the expense of the anti-apoptotic Bcl-xL. Induction of Bcl-xS by SSOs activated apoptosis and autophagy in GBM cells. In addition, we found that ionizing radiation could also modulate the alternative splicing of BCLX. In contrast to heavy (carbon) ion irradiation, low energy X-ray radiation-induced an increased ratio of Bcl-xL/Bcl-xS. Inhibiting Bcl-xL through splicing regulation can significantly enhance the radiation sensitivity of 2D and 3D GBM cells. These results suggested that manipulation of BCLX pre-mRNA alternative splicing by splice-switching oligonucleotides is a novel approach to inhibit glioblastoma tumorigenesis alone or in combination with radiotherapy.

Curative carbon ion radiotherapy in a head and neck mucosal melanoma series: Facing the future within multidisciplinarity

PURPOSE

To evaluate efficacy and toxicity of carbon ion radiotherapy (CIRT) in locally advanced head and neck mucosal melanoma (HNMM) patients treated at our Institute.

MATERIALS AND METHODS

Between June 2013 and June 2020, 40 HNMM patients were treated with CIRT. Prescription dose was 65.6-68.8 Gy relative biological effectiveness [RBE] in 16 fractions. Twelve (30%) patients received only biopsy, 28 (70%) surgical resection before CIRT. Immunotherapy was administered before and/or after CIRT in 45% of patients, mainly for distant progression (89%).

RESULTS

Median follow-up was 18 months. 2-year Local Relapse Free Survival (LRFS), Overall Survival (OS), Progression Free Survival (PFS) and Distant Metastasis Free Survival (DMFS) were 84.5%, 58.6%, 33.2% and 37.3%, respectively. At univariate analysis, LRFS was significantly better for non-recurrent status, < 2 surgeries before CIRT and treatment started < 9 months from the initial diagnosis, with no significant differences for operated versus unresected patients. After relapse, immunotherapy provided longer median OS (17 months vs 3.6, p-value<0.001). Late toxicity ≥ G3 (graded with CTCAE 5.0 scale) was reported in 10% of patients.

CONCLUSION

CIRT in advanced HNMM patients is safe and locally effective. Prospective trials are warranted to assess the role of targeted/immune- systemic therapy to improve OS.

Investigation on the physical dose filtered by linear energy transfer for treatment plan evaluation in carbon ion therapy

BACKGROUND

Large tumor size has been reported as a predicting factor for inferior clinical outcome in carbon ion radiotherapy (CIRT). Besides the clinical factors accompanied with such tumors, larger tumors receive typically more low linear energy transfer (LET) contributions than small ones which may be the underlying physical cause. Although dose averaged LET is often used as a single parameter descriptor to quantify the beam quality, there is no evidence that this parameter is the optimal clinical predictor for the complex mixed radiation fields in CIRT.

PURPOSE

Purpose of this study was to investigate on a novel dosimetric quantity, namely high-LET-dose (D > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ , the physical dose filtered based on an LET threshold) as a single parameter estimator to differentiate between carbon ion treatment plans (cTP) with a small and large tumor volume.

METHODS

Ten cTPs with a planning target volume,PTV ≥ 500 cm 3 $\mathrm{PTV}\ge {500}\,{{\rm cm}^{3}}$ (large) and nine with aPTV < 500 cm 3 $\mathrm{PTV}<{500}\,{{\rm cm}^{3}}$ (small) were selected for this study. To find a reasonable LET threshold (L thr $\textrm {L}_{\textrm {thr}}$ ) that results in a significant difference in terms ofD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ , the voxel based normalized high-LET-dose (D ̂ > L thr $\hat{\textrm {D}}_{>\textrm {L}_{\textrm {thr}}}$ ) distribution in the clinical target volume (CTV) was studied on a subset (12 out of 19 cTPs) for 18 LET thresholds, using standard distribution descriptors (mean, variance and skewness). The classical dose volume histogram concept was used to evaluate theD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ andD ̂ > L thr $\hat{\textrm {D}}_{>\textrm {L}_{\textrm {thr}}}$ distributions within the target of all 19 cTPs at the before determinedL thr $\textrm {L}_{\textrm {thr}}$ . Statistical significance of the difference between the two groups in terms of meanD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ andD ̂ > L thr $\hat{\textrm {D}}_{>\textrm {L}_{\textrm {thr}}}$ volume histogram parameters was evaluated by means of (two-sided) t-test or Mann-Whitney-U-test. In addition, the minimum target coverage at the above determinedL thr $\textrm {L}_{\textrm {thr}}$ was compared and validated against three other thresholds to verify its potential in differentiation between small and large volume tumors.

RESULTS

AnL thr $\textrm {L}_{\textrm {thr}}$ of approximately30 keV / μ m ${30}\,{\rm keV/}\umu {\rm m}$ was found to be a reasonable threshold to classify the two groups. At this threshold, theD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ andD ̂ > L thr $\hat{\textrm {D}}_{>\textrm {L}_{\textrm {thr}}}$ were significantly larger (p < 0.05 $p<0.05$ ) in small CTVs. For the small tumor group, the near-minimum and medianD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ (andD ̂ > L thr $\hat{\textrm {D}}_{>\textrm {L}_{\textrm {thr}}}$ ) in the CTV were in average9.3 ± 1.5 Gy $9.3\pm {1.5}\,{\rm Gy}$ (0.31 ± 0.08) and13.6 ± 1.6 Gy $13.6\pm {1.6}\,{\rm Gy}$ (0.46 ± 0.06), respectively. For the large tumors, these parameters were6.6 ± 0.2 Gy $6.6\pm {0.2}\,{\rm Gy}$ (0.20 ± 0.01) and8.6 ± 0.4 Gy $8.6\pm {0.4}\,{\rm Gy}$ (0.28 ± 0.02). The difference between the two groups in terms of mean near-minimum and medianD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ (D ̂ > L thr $\hat{\textrm {D}}_{>\textrm {L}_{\textrm {thr}}}$ ) was 2.7 Gy (11%) and 5.0 Gy (18%), respectively.

CONCLUSIONS

The feasibility of high-LET-dose based evaluation was shown in this study where a lowerD > L thr $\textrm {D}_{>\textrm {L}_{\textrm {thr}}}$ was found in cTPs with a large tumor size. Further investigation is needed to draw clinical conclusions. The proposed methodology in this work can be utilized for future high-LET-dose based studies.

Pencil Beam Scanning Carbon Ion Radiotherapy for Hepatocellular Carcinoma

Purpose

Carbon ion radiotherapy (CIRT) has emerged as a promising treatment modality for hepatocellular carcinoma (HCC). However, evidence of using the pencil beam scanning (PBS) technique to treat moving liver tumors remains lacking. The present study investigated the efficacy and toxicity of PBS CIRT in patients with HCC.

Methods

Between January 2016 and October 2021, 90 consecutive HCC patients treated with definitive CIRT in our center were retrospectively analyzed. Fifty-eight patients received relative biological effectiveness-weighted doses of 50-70 Gy in 10 fractions, and 32 received 60-67.5 Gy in 15 fractions, which were determined by the tumor location and normal tissue constraints. Active motion-management techniques and necessary strategies were adopted to mitigate interplay effects efficiently. Oncologic outcomes and toxicities were evaluated.

Results

The median follow-up time was 28.6 months (range 5.7-74.6 months). The objective response rate was 75.0% for all 90 patients with 100 treated lesions. The overall survival rates at 1-, 2- and 3-years were 97.8%, 83.3% and 75.4%, respectively. The local control rates at 1-, 2- and 3-years were 96.4%, 96.4% and 93.1%, respectively. Radiation-induced liver disease was not documented, and 4 patients (4.4%) had their Child-Pugh score elevated by 1 point after CIRT. No grade 3 or higher acute non-hematological toxicities were observed. Six patients (6.7%) experienced grade 3 or higher late toxicities.

Conclusion

The active scanning technique was clinically feasible to treat HCC by applying necessary mitigation measures for interplay effects. The desirable oncologic outcomes as well as favorable toxicity profiles presented in this study will be a valuable reference for other carbon-ion centers using the PBS technique and local effect model-based system, and add to a growing body of evidence about the role of CIRT in the management of HCC.

Variable RBE in proton radiotherapy: a comparative study with the predictive Mayo Clinic Florida microdosimetric kinetic model and phenomenological models of cell survival

Objectives. (1) To examine to what extent the cell- and exposure- specific information neglected in the phenomenological proton relative biological effectiveness (RBE) models could influence the computed RBE in proton therapy. (2) To explore similarities and differences in the formalism and the results between the linear energy transfer (LET)-based phenomenological proton RBE models and the microdosimetry-based Mayo Clinic Florida microdosimetric kinetic model (MCF MKM). (3) To investigate how the relationship between the RBE and the dose-mean proton LET is affected by the proton energy spectrum and the secondary fragments.Approach. We systematically compared six selected phenomenological proton RBE models with the MCF MKM in track-segment simulations, monoenergetic proton beams in a water phantom, and two spread-out Bragg peaks. A representative comparison within vitrodata for human glioblastoma cells (U87 cell line) is also included.Main results. Marked differences were observed between the results of the phenomenological proton RBE models, as reported in previous studies. The dispersion of these models' results was found to be comparable to the spread in the MCF MKM results obtained by varying the cell-specific parameters neglected in the phenomenological models. Furthermore, while single cell-specific correlation between RBE and the dose-mean proton LET seems reasonable above 2 keVμm-1, caution is necessary at lower LET values due to the relevant contribution of secondary fragments. The comparison within vitrodata demonstrates comparable agreement between the MCF MKM predictions and the results of the phenomenological models.Significance. The study highlights the importance of considering cell-specific characteristics and detailed radiation quality information for accurate RBE calculations in proton therapy. Furthermore, these results provide confidence in the use of the MCF MKM for clonogenic survival RBE calculations in proton therapy, offering a more mechanistic approach compared to phenomenological models.

High-LET charged particles: radiobiology and application for new approaches in radiotherapy

The number of patients treated with charged-particle radiotherapy as well as the number of treatment centers is increasing worldwide, particularly regarding protons. However, high-linear energy transfer (LET) particles, mainly carbon ions, are of special interest for application in radiotherapy, as their special physical features result in high precision and hence lower toxicity, and at the same time in increased efficiency in cell inactivation in the target region, i.e., the tumor. The radiobiology of high-LET particles differs with respect to DNA damage repair, cytogenetic damage, and cell death type, and their increased LET can tackle cells' resistance to hypoxia. Recent developments and perspectives, e.g., the return of high-LET particle therapy to the US with a center planned at Mayo clinics, the application of carbon ion radiotherapy using cost-reducing cyclotrons and the application of helium is foreseen to increase the interest in this type of radiotherapy. However, further preclinical research is needed to better understand the differential radiobiological mechanisms as opposed to photon radiotherapy, which will help to guide future clinical studies for optimal exploitation of high-LET particle therapy, in particular related to new concepts and innovative approaches. Herein, we summarize the basics and recent progress in high-LET particle radiobiology with a focus on carbon ions and discuss the implications of current knowledge for charged-particle radiotherapy. We emphasize the potential of high-LET particles with respect to immunogenicity and especially their combination with immunotherapy.

The LET trilemma: Conflicts between robust target coverage, uniform dose, and dose-averaged LET in carbon therapy

BACKGROUND

Linear energy transfer (LET) is closely related to the biological effect of ionizing radiation. Increasing the dose-averaged LET (LETd ) within the target volume has been proposed as a means to improve clinical outcome for hypoxic tumors. However, doing so can lead to reduced robustness to range uncertainty.

PURPOSE

To quantify the relationship between robust target coverage, target dose uniformity, and LETd , we employ robust optimization using dose-based and LETd -based functions and allow varying amounts of target non-uniformity.

METHODS AND MATERIALS

Robust carbon therapy optimization is used to create plans for phantom cases with increasing target sizes (radii 1, 3, and 5 cm). First, the influence of respectively range and setup uncertainty on the LETd in the target is studied. Second, we employ strategies allowing overdosage in the clinical target volume (CTV) or gross tumor volume (GTV), which enable increased LETd in the target. The relationship between robust target coverage and LETd in the target is illustrated by tradeoff curves generated by optimization using varying weights for the LETd -based functions.

RESULTS

As the range uncertainty used in the robust optimization increased from 0% to 5%, the near-minimum nominal LETd decreased by 17%-29% (9-21 keV/µm) for the different target sizes. The effect of increasing setup uncertainty was marginal. Allowing 10% overdosage in the CTV enabled 9%-29% (6-12 keV/µm) increased near-minimum worst case LETd for the different target sizes, compared to uniform dose plans. When 10% overdosage was allowed in the GTV only, the increase was 1%-20% (1-8 keV/µm).

CONCLUSIONS

There is an inherent conflict between range uncertainty robustness and high LETd in the target, which is aggravated with increasing target size. For large tumors, it is possible to simultaneously achieve two of the three qualities range robustness, uniform dose, and high LETd in the target.

Definitive carbon ion re-irradiation with pencil beam scanning in the treatment of unresectable locally recurrent rectal cancer

This study aimed to evaluate the oncological outcomes and safety of carbon ion re-irradiation with pencil beam scanning (PBS) delivery technique for previously irradiated and unresectable locally recurrent rectal cancer (LRRC). Between June 2017 and September 2021, 24 patients of unresectable LRRC with prior pelvic photon radiotherapy who underwent carbon ion re-irradiation at our institute were retrospectively analyzed. Carbon ion radiotherapy was delivered by raster scanning with a median relative biological effectiveness-weighted dose of 72 Gy in 20 fractions. Weekly CT reviews were carried out, and offline adaptive replanning was performed whenever required. The median follow-up duration was 23.8 months (range, 6.2-47.1 months). At the last follow-up, two patients had a local disease progression, and 11 patients developed distant metastases. The 1- and 2-year local control, progression-free survival and overall survival rates were 100 and 93.3%, 70.8 and 45.0% and 86.7 and 81.3%, respectively. There were no Grade 3 or higher acute toxicities observed. Three patients developed Grade 3 late toxicities, one each with gastrointestinal toxicity, skin reaction and pelvic infection. In conclusion, definitive carbon ion re-irradiation with PBS provided superior oncologic results with tolerable toxicities and may be served as a curative treatment strategy in unresectable LRRC.

An international approach to estimating the indications and number of eligible patients for carbon ion radiation therapy (CIRT) in Australia

BACKGROUND AND PURPOSE

To establish the treatment indications and potential patient numbers for carbon ion radiation therapy (CIRT) at the proposed national carbon ion (and proton) therapy facility in the Westmead precinct, New South Wales (NSW), Australia.

METHODS

An expert panel was convened, including representatives of four operational and two proposed international carbon ion facilities, as well as NSW-based CIRT stakeholders. They met virtually to consider CIRT available evidence and experience. Information regarding Japanese CIRT was provided pre- and post- the virtual meeting. Published information for South Korea was included in discussions.

RESULTS

There was jurisdictional variation in the tumours treated by CIRT due to differing incidences of some tumours, referral patterns, differences in decisions regarding which tumours to prioritise, CIRT resources available and funding arrangements. The greatest level of consensus was reached that CIRT in Australia can be justified currently for patients with adenoid cystic carcinomas and mucosal melanomas of the head and neck, hepatocellular cancer and liver metastases, base of skull meningiomas, chordomas and chondrosarcomas. Almost 1400 Australian patients annually meet the consensus-derived indications now.

CONCLUSION

A conservative estimate is that 1% of cancer patients in Australia (or 2% of patients recommended for radiation therapy) may preferentially benefit from CIRT for initial therapy of radiation resistant tumours, or to boost persistently active disease after other therapies, or for re-irradiation of recurrent disease. On this basis, one national carbon ion facility with up to four treatment rooms is justified for Australian patients.