New Ions for Therapy

Comparing efficacy of charged-particle therapy with brachytherapy in treatment of uveal melanoma

BACKGROUND

Uveal melanoma (UM) is the most common primary ocular tumour in adults. The most used eye-preserving treatments are charged-particle therapy (CPT) and brachytherapy. We performed a systematic review and meta-analysis to compare efficacies and complications of these two radiotherapies.

METHODS

We searched EMBASE, PubMed, MEDLINE, and the Cochrane Library from January 2012 to December 2022. Two independent reviewers identified controlled studies comparing outcomes of CPT versus brachytherapy. Case series that utilize either treatment modality were also reviewed.

RESULTS

One hundred fifty studies met the eligibility criteria, including 2 randomized control trials, 5 controlled cohort studies, and 143 case series studies. We found significant reduction in local recurrence rate among patients treated with CPT compared to brachytherapy (Odds ratio[OR] 0.38, 95% Confidence interval [CI] 0.24-0.60, p < 0.01). Analysis also showed a trend of increased risks of secondary glaucoma after CPT. No statistically significant differences were found in analyzing risks of mortality, enucleation, and cataract.

CONCLUSIONS

Our study suggested no difference in mortality, enucleation rate and cataract formation rate comparing the two treatments. Lower local recurrence rate and possibly higher secondary glaucoma incidence was noted among patients treated with CPT. Nevertheless, the overall level of evidence is limited, and further high-quality studies are necessary to provide a more definitive conclusion.

DNA-PKcs Inhibition Sensitizes Human Chondrosarcoma Cells to Carbon Ion Irradiation via Cell Cycle Arrest and Telomere Capping Disruption

In order to overcome the resistance to radiotherapy in human chondrosarcoma cells, the prevention from efficient DNA repair with a combined treatment with the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) inhibitor AZD7648 was explored for carbon ion (C-ion) as well as reference photon (X-ray) irradiation (IR) using gene expression analysis, flow cytometry, protein phosphorylation, and telomere length shortening. Proliferation markers and cell cycle distribution changed significantly after combined treatment, revealing a prominent G2/M arrest. The expression of the G2/M checkpoint genes cyclin B, CDK1, and WEE1 was significantly reduced by IR alone and the combined treatment. While IR alone showed no effects, additional AZD7648 treatment resulted in a dose-dependent reduction in AKT phosphorylation and an increase in Chk2 phosphorylation. Twenty-four hours after IR, the key genes of DNA repair mechanisms were reduced by the combined treatment, which led to impaired DNA repair and increased radiosensitivity. A time-dependent shortening of telomere length was observed in both cell lines after combined treatment with AZD7648 and 8 Gy X-ray/C-ion IR. Our data suggest that the inhibition of DNA-PKcs may increase sensitivity to X-rays and C-ion IR by impairing its functional role in DNA repair mechanisms and telomere end protection.

Integrating microdosimetricin vitroRBE models for particle therapy into TOPAS MC using the MicrOdosimetry-based modeliNg for RBE ASsessment (MONAS) tool

Objective.In this paper, we present MONAS (MicrOdosimetry-based modelliNg for relative biological effectiveness (RBE) ASsessment) toolkit. MONAS is a TOPAS Monte Carlo extension, that combines simulations of microdosimetric distributions with radiobiological microdosimetry-based models for predicting cell survival curves and dose-dependent RBE.Approach.MONAS expands TOPAS microdosimetric extension, by including novel specific energy scorers to calculate the single- and multi-event specific energy microdosimetric distributions at different micrometer scales. These spectra are used as physical input to three different formulations of themicrodosimetric kinetic model, and to thegeneralized stochastic microdosimetric model(GSM2), to predict dose-dependent cell survival fraction and RBE. MONAS predictions are then validated against experimental microdosimetric spectra andin vitrosurvival fraction data. To show the MONAS features, we present two different applications of the code: (i) the depth-RBE curve calculation from a passively scattered proton SOBP and monoenergetic12C-ion beam by using experimentally validated spectra as physical input, and (ii) the calculation of the 3D RBE distribution on a real head and neck patient geometry treated with protons.Main results.MONAS can estimate dose-dependent RBE and cell survival curves from experimentally validated microdosimetric spectra with four clinically relevant radiobiological models. From the radiobiological characterization of a proton SOBP and12C fields, we observe the well-known trend of increasing RBE values at the distal edge of the radiation field. The 3D RBE map calculated confirmed the trend observed in the analysis of the SOBP, with the highest RBE values found in the distal edge of the target.Significance.MONAS extension offers a comprehensive microdosimetry-based framework for assessing the biological effects of particle radiation in both research and clinical environments, pushing closer the experimental physics-based description to the biological damage assessment, contributing to bridging the gap between a microdosimetric description of the radiation field and its application in proton therapy treatment with variable RBE.

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.

Quasi-real-time range monitoring by in-beam PET: a case for 15O

A fast and reliable range monitoring method is required to take full advantage of the high linear energy transfer provided by therapeutic ion beams like carbon and oxygen while minimizing damage to healthy tissue due to range uncertainties. Quasi-real-time range monitoring using in-beam positron emission tomography (PET) with therapeutic beams of positron-emitters of carbon and oxygen is a promising approach. The number of implanted ions and the time required for an unambiguous range verification are decisive factors for choosing a candidate isotope. An experimental study was performed at the FRS fragment-separator of GSI Helmholtzzentrum für Schwerionenforschung GmbH, Germany, to investigate the evolution of positron annihilation activity profiles during the implantation of [Formula: see text]O and [Formula: see text]O ion beams in a PMMA phantom. The positron activity profile was imaged by a dual-panel version of a Siemens Biograph mCT PET scanner. Results from a similar experiment using ion beams of carbon positron-emitters [Formula: see text]C and [Formula: see text]C performed at the same experimental setup were used for comparison. Owing to their shorter half-lives, the number of implanted ions required for a precise positron annihilation activity peak determination is lower for [Formula: see text]C compared to [Formula: see text]C and likewise for [Formula: see text]O compared to [Formula: see text]O, but their lower production cross-sections make it difficult to produce them at therapeutically relevant intensities. With a similar production cross-section and a 10 times shorter half-life than [Formula: see text]C, [Formula: see text]O provides a faster conclusive positron annihilation activity peak position determination for a lower number of implanted ions compared to [Formula: see text]C. A figure of merit formulation was developed for the quantitative comparison of therapy-relevant positron-emitting beams in the context of quasi-real-time beam monitoring. In conclusion, this study demonstrates that among the positron emitters of carbon and oxygen, [Formula: see text]O is the most feasible candidate for quasi-real-time range monitoring by in-beam PET that can be produced at therapeutically relevant intensities. Additionally, this study demonstrated that the in-flight production and separation method can produce beams of therapeutic quality, in terms of purity, energy, and energy spread.

The Role of Carbon Ion Therapy in the Changing Oncology Landscape-A Narrative Review of the Literature and the Decade of Carbon Ion Experience at the Italian National Center for Oncological Hadrontherapy

BACKGROUND

Currently, 13 Asian and European facilities deliver carbon ion radiotherapy (CIRT) for preclinical and clinical activity, and, to date, 55 clinical studies including CIRT for adult and paediatric solid neoplasms have been registered. The National Center for Oncological Hadrontherapy (CNAO) is the only Italian facility able to accelerate both protons and carbon ions for oncological treatment and research.

METHODS

To summarise and critically evaluate state-of-the-art knowledge on the application of carbon ion radiotherapy in oncological settings, the authors conducted a literature search till December 2022 in the following electronic databases: PubMed, Web of Science, MEDLINE, Google Scholar, and Cochrane. The results of 68 studies are reported using a narrative approach, highlighting CNAO's clinical activity over the last 10 years of CIRT.

RESULTS

The ballistic and radiobiological hallmarks of CIRT make it an effective option in several rare, radioresistant, and difficult-to-treat tumours. CNAO has made a significant contribution to the advancement of knowledge on CIRT delivery in selected tumour types.

CONCLUSIONS

After an initial ramp-up period, CNAO has progressively honed its clinical, technical, and dosimetric skills. Growing engagement with national and international networks and research groups for complex cancers has led to increasingly targeted patient selection for CIRT and lowered barriers to facility access.

Radiation Therapy for Gestational Trophoblastic Neoplasia: Forward-Looking Lessons Learnt

Gestational trophoblastic neoplasia (GTN) includes several rare malignant diseases occurring after pregnancy: invasive moles, choriocarcinoma, placental site trophoblastic tumours, and epithelioid trophoblastic tumours. Multidisciplinary protocols including multi-agent chemotherapy, surgery, and occasionally radiotherapy achieve good outcomes for some high-risk metastatic patients. In this narrative review of the published studies on the topic, we have tried to identify the role of radiotherapy. The available studies are mainly small, old, and retrospective, with incomplete data regarding radiotherapy protocols delivering low doses (which can make this disease appear radioresistant in some cases despite high response rates with palliative doses) to wide fields (whole-brain, whole-liver, etc.), which can increase toxicity. Studies considering modern techniques are needed to overcome these limitations and determine the full potential of radiotherapy beyond its antihemorrhagic and palliative roles.

MC TRIM Algorithm in Mandibula Phantom in Helium Therapy

Helium ion beam therapy, one of the particle therapies developed and studied in the 1950s for cancer treatment, resulted in clinical trials starting at Lawrence Berkeley National Laboratory in 1975. While proton and carbon ion therapies have been implemented in research institutions and hospitals globally after the end of the trials, progress in comprehending the physical, biological, and clinical findings of helium ion beam therapy has been limited, particularly due to its limited accessibility. Ongoing efforts aim to establish programs that evaluate the use of helium ion beams for clinical and research purposes, especially in the treatment of sensitive clinical cases. Additionally, helium ions have superior physical properties to proton beams, such as lower lateral scattering and larger LET. Moreover, they exhibit similar physical characteristics to carbon, oxygen, and neon ions, which are all used in heavy ion therapy. However, they demonstrate a sharper lateral penumbra with a lower radiobiological absence of certainties and lack the degradation of variations in dose distributions caused by excessive fragmenting of heavier-ion beams, especially at greater depths of penetration. In this context, the status and the prospective advancements of helium ion therapy are examined by investigating ionization, recoil, and lateral scattering values using MC TRIM algorithms in mandible plate phantoms designed from both tissue and previously studied biomaterials, providing an overview for dental cancer treatment. An average difference of 1.9% in the Bragg peak positions and 0.211 mm in lateral scattering was observed in both phantoms. Therefore, it is suggested that the 4He ion beam can be used in the treatment of mandibular tumors, and experimental research is recommended using the proposed biomaterial mandible plate phantom.

Short-lived radioactive8Li and8He ions for hadrontherapy: a simulation study

Purpose.Although charged particle therapy (CPT) for cancer treatment has grown these past years, the use of protons and carbon ions for therapy remains debated compared to x-ray therapy. While a biological advantage of protons is not clearly demonstrated, therapy using carbon ions is often pointed out for its high cost. Furthermore, the nuclear interactions undergone by carbons inside the patient are responsible for an additional dose delivered after the Bragg peak, which deteriorates the ballistic advantage of CPT. Therefore, a renewed interest for lighter ions with higher biological efficiency than protons was recently observed. In this context, helium and lithium ions represent a good compromise between protons and carbons, as they exhibit a higher linear energy transfer (LET) than protons in the Bragg peak and can be accelerated by cyclotrons. The possibility of accelerating radioactive⁸Li, decaying in 2α-particles, and⁸He, decaying in⁸Li byβ^-decay, is particularly interesting.Methods. This work aims to assess the interest of the use of⁸Li and⁸He ions for therapy by Monte Carlo simulations carried out withGeant4.Results. It was calculated that the⁸Li and⁸He decay results in an increase of the LET of almost a factor 2 in the Bragg peak compared to stable⁷Li and⁴He. This results also in a higher dose deposited in the Bragg peak without an increase of the dose in the plateau region. It was also shown that both⁸He and⁸Li can have a potential interest for prompt-gamma monitoring techniques. Finally, the feasibility of accelerating facilities delivering⁸Li and⁸He was also discussed.Conclusion. In this study, we demonstrate that both⁸Li and⁸He have interesting properties for therapy. Indeed, simulations predict that⁸Li and⁸He are a good compromise between proton and¹²C, both in terms of LET and dose.

Neutron shielding assessment of a16O hadron therapy room by means of Monte Carlo simulations with the PHITS code

Hadron radiation therapy is of great interest worldwide. Heavy-ion beams provide ideal therapeutic conditions for deep-seated local tumours. At the Heidelberg Ion Beam Therapy Center (HIT, Germany), protons and carbon ions are already integrated into the clinical routine, while16O ions are still used for research only. To ensure the protection of the technical staff and members of the public, it is required to estimate the neutron dose distribution for optimal working conditions and at different locations. The Particle and Heavy Ion Transport Code System (PHITS) is used in this work to evaluate the dose rate distribution of secondary neutrons in a treatment room at HIT where16O ions are used: an equivalent target in soft tissue is considered in the shielding assessment to simulate the interaction of the beam with patients. The angular dependence of neutron fluences and energy spectra around the considered phantom were calculated. Alongside the spatial distribution of the neutron and photon fluence, a map of the effective dose rate was estimated using the ICRP fluence-to-effective dose conversion coefficients, exploiting the PHITS code's built-in capabilities. The capability of the actual shielding design of the studied HIT treatment room was approved.

Are charged particles a good match for combination with immunotherapy? Current knowledge and perspectives

Charged particle radiotherapy, mainly using protons and carbon ions, provides physical characteristics allowing for a volume conformal irradiation and a reduction of the integral dose to normal tissue. Carbon ion therapy additionally features an increased biological effectiveness resulting in peculiar molecular effects. Immunotherapy, mostly performed with immune checkpoint inhibitors, is nowadays considered a pillar in cancer therapy. Based on the advantageous features of charged particle radiotherapy, we review pre-clinical evidence revealing a strong potential of its combination with immunotherapy. We argue that the combination therapy deserves further investigation with the aim of translation in clinics, where a few studies have been set up already.

Good Timing Matters: The Spatially Fractionated High Dose Rate Boost Should Come First

Monoplanar microbeam irradiation (MBI) and pencilbeam irradiation (PBI) are two new concepts of high dose rate radiotherapy, combined with spatial dose fractionation at the micrometre range. In a small animal model, we have explored the concept of integrating MBI or PBI as a simultaneously integrated boost (SIB), either at the beginning or at the end of a conventional, low-dose rate schedule of 5x4 Gy broad beam (BB) whole brain radiotherapy (WBRT). MBI was administered as array of 50 µm wide, quasi-parallel microbeams. For PBI, the target was covered with an array of 50 µm × 50 µm pencilbeams. In both techniques, the centre-to-centre distance was 400 µm. To assure that the entire brain received a dose of at least 4 Gy in all irradiated animals, the peak doses were calculated based on the daily BB fraction to approximate the valley dose. The results of our study have shown that the sequence of the BB irradiation fractions and the microbeam SIB is important to limit the risk of acute adverse effects, including epileptic seizures and death. The microbeam SIB should be integrated early rather than late in the irradiation schedule.

Effects of dose and dose-averaged linear energy transfer on pelvic insufficiency fractures after carbon-ion radiotherapy for uterine carcinoma

BACKGROUND AND PURPOSE

The correlation between dose-averaged linear energy transfer (LETd) and its therapeutic or adverse effects, especially in carbon-ion radiotherapy (CIRT), remains controversial. This study aimed to investigate the effects of LETd and dose on pelvic insufficiency fractures after CIRT.

MATERIAL AND METHODS

Among patients who underwent CIRT for uterine carcinoma, 101 who were followed up for > 6 months without any other therapy were retrospectively analyzed. The sacrum insufficiency fractures (SIFs) were graded according to the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer toxicity criteria. The correlations between the relative biological effectiveness (RBE)-weighted dose, LETd, physical dose, clinical factors, and SIFs were evaluated. In addition, we analyzed the association of SIF with LETd, physical dose, and clinical factors in cases where the sacrum D50% RBE-weighted dose was above the median dose.

RESULTS

At the last follow-up, 19 patients developed SIFs. Receiver operating characteristic curve analysis revealed that the sacrum D50% RBE-weighted dose was a valuable predictor of SIF. Univariate analyses suggested that LETd V10 keV/µm, physical dose V5 Gy, and smoking status were associated with SIF. Cox regression analysis in patients over 50 years of age validated that current smoking habit was the sole risk factor for SIF. Therefore, LETd or physical dose parameters were not associated with SIF prediction.

CONCLUSION

The sacrum D50% RBE-weighted dose was identified as a risk factor for SIF. Additionally, neither LETd nor physical dose parameters were associated with SIF prediction.

Modelling the HPRT-gene mutation induction of particle beams: systematic in vitro data collection, analysis and microdosimetric kinetic model implementation

Objective. Since the early years, particle therapy treatments have been associated with concerns for late toxicities, especially secondary cancer risk (SCR). Nowadays, this concern is related to patients for whom long-term survival is expected (e.g. breast cancer, lymphoma, paediatrics). In the aim to contribute to this research, we present a dedicated statistical and modelling analysis aiming at improving our understanding of the RBE for mutation induction ( RBE M ˜ ) for different particle species. Approach. We built a new database based on a systematic collection of RBE data for mutation assays of the gene encoding for the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase from literature (105 entries, distributed among 3 cell lines and 16 particle species). The data were employed to perform statistical and modelling analysis. For the latter, we adapted the microdosimetric kinetic model (MKM) to describe the mutagenesis in analogy to lethal lesion induction. Main results. Correlation analysis between RBE for survival (RBES) and RBE M ˜ reveals significant correlation between these two quantities (ρ = 0.86, p < 0.05). The correlation gets stronger when looking at subsets of data based on cell line and particle species. We also show that the MKM can be successfully employed to describe RBE M ˜ , obtaining comparably good agreement with the experimental data. Remarkably, to improve the agreement with experimental data the MKM requires, consistently in all the analysed cases, a reduced domain size for the description of mutation induction compared to that adopted for survival. Significance. We were able to show that RBES and RBE M ˜ are strongly related quantities. We also showed for the first time that the MKM could be successfully applied to the description of mutation induction, representing an endpoint different from the more traditional cell killing. In analogy to the RBES, RBE M ˜ can be implemented into treatment planning system evaluations.

Physical aspects of Bragg curve of therapeutic oxygen-ion beam: Monte Carlo simulation

Introduction: Oxygen (16O) ion beams have been recommended for cancer treatment due to its physical Bragg curve feature and biological property. The goal of this research is to use Monte Carlo simulation to analyze the physical features of the 16O Bragg curve in water and tissue.

Material and methods: In order to determine the benefits and drawbacks of ion beam therapy, Monte Carlo simulation (PHITS code) was used to investigate the interaction and dose deposition properties of oxygen ions beam in water and human tissue medium. A benchmark study for the depth–dose distribution of a 16O ion beam in a water phantom was established using the PHITS code. Bragg’s peak location of 16O ions in water was simulated using the effect of water’s mean ionization potential. The contribution of secondary particles produced by nuclear fragmentation to the total dose has been calculated. The depth and radial dose profiles of 16O, 12C, 4He, and 1H beams were compared.

Results: It was shown that PHITS accurately reproduces the measured Bragg curves. The mean ionization potential of water was estimated. It has been found that secondary particles contribute 10% behind the Bragg peak for 16O energy of 300 MeV/u. The comparison of the depth and radial dose profiles of 16O, 12C, 4He, and 1H beams, shows clearly, that the oxygen beam has the greater deposited dose at Bragg peak and the minor lateral deflection.

Conclusions: The combination of these physical characteristics with radio-biological ones in the case of resistant organs located behind the tumor volume, leads to the conclusion that the 16O ion beams can be used to treat deep-seated hypoxic tumors.

Roadmap: helium ion therapy

Helium ion beam therapy for the treatment of cancer was one of several developed and studied particle treatments in the 1950s, leading to clinical trials beginning in 1975 at the Lawrence Berkeley National Laboratory. The trial shutdown was followed by decades of research and clinical silence on the topic while proton and carbon ion therapy made debuts at research facilities and academic hospitals worldwide. The lack of progression in understanding the principle facets of helium ion beam therapy in terms of physics, biological and clinical findings persists today, mainly attributable to its highly limited availability. Despite this major setback, there is an increasing focus on evaluating and establishing clinical and research programs using helium ion beams, with both therapy and imaging initiatives to supplement the clinical palette of radiotherapy in the treatment of aggressive disease and sensitive clinical cases. Moreover, due its intermediate physical and radio-biological properties between proton and carbon ion beams, helium ions may provide a streamlined economic steppingstone towards an era of widespread use of different particle species in light and heavy ion therapy. With respect to the clinical proton beams, helium ions exhibit superior physical properties such as reduced lateral scattering and range straggling with higher relative biological effectiveness (RBE) and dose-weighted linear energy transfer (LET_d) ranging from ∼4 keVμm^-1to ∼40 keVμm^-1. In the frame of heavy ion therapy using carbon, oxygen or neon ions, where LET_dincreases beyond 100 keVμm^-1, helium ions exhibit similar physical attributes such as a sharp lateral penumbra, however, with reduced radio-biological uncertainties and without potentially spoiling dose distributions due to excess fragmentation of heavier ion beams, particularly for higher penetration depths. This roadmap presents an overview of the current state-of-the-art and future directions of helium ion therapy: understanding physics and improving modeling, understanding biology and improving modeling, imaging techniques using helium ions and refining and establishing clinical approaches and aims from learned experience with protons. These topics are organized and presented into three main sections, outlining current and future tasks in establishing clinical and research programs using helium ion beams-A. Physics B. Biological and C. Clinical Perspectives.

A Review on the Recent Advancements on Therapeutic Effects of Ions in the Physiological Environments

This review focuses on the therapeutic effects of ions when released in physiological environments. Recent studies have shown that metallic ions like Ag+, Sr2+, Mg2+, Mn2+, Cu2+, Ca2+, P+5, etc., have shown promising results in drug delivery systems and regenerative medicine. These metallic ions can be loaded in nanoparticles, mesoporous bioactive glass nanoparticles (MBGNs), hydroxyapatite (HA), calcium phosphates, polymeric coatings, and salt solutions. The metallic ions can exhibit different functions in the physiological environment such as antibacterial, antiviral, anticancer, bioactive, biocompatible, and angiogenic effects. Furthermore, the metals/metalloid ions can be loaded into scaffolds to improve osteoblast proliferation, differentiation, bone development, fibroblast growth, and improved wound healing efficacy. Moreover, different ions possess different therapeutic limits. Therefore, further mechanisms need to be developed for the highly controlled and sustained release of these ions. This review paper summarizes the recent progress in the use of metallic/metalloid ions in regenerative medicine and encourages further study of ions as a solution to cure diseases.

Emerging Role of Carbon Ion Radiotherapy in Reirradiation of Recurrent Head and Neck Cancers: What Have We Achieved So Far?

Administering reirradiation for the treatment of recurrent head and neck cancers is extremely challenging. These tumors are hypoxic and radioresistant and require escalated radiation doses for adequate control. The obstacle to delivering this escalated dose of radiation to the target is its proximity to critical organs at risk (OARs) and possible development of consequent severe late toxicities. With the emergence of highly sophisticated technologies, intensity-modulated radiotherapy (IMRT) and stereotactic body radiotherapy have shown promising outcomes. Proton beam radiotherapy has been used for locally recurrent head and neck cancers because of its excellent physical dose distribution, exploring sharp Bragg peak properties with negligible entrance and exit doses. To further improve these results, carbon ion radiotherapy (CIRT) has been explored in several countries across Europe and Asia because of its favorable physical properties with minimal entrance and exit doses, sharper lateral penumbra, and much higher and variable relative biological efficacy, which cannot be currently achieved with any other form of radiation. Few studies have described the role of CIRT in recurrent head and neck cancers. In this article, we have discussed the different aspects of carbon ions in reirradiation of recurrent head and neck cancers, including European and Asian experiences, different dose schedules, dose constraints of OARs, outcomes, and toxicities, and a brief comparison with proton beam radiotherapy and IMRT.

MONTE-CARLO SIMULATION USING PHITS OF SECONDARY NEUTRONS PRODUCED IN-PATIENT DURING 16O ION THERAPY

In hadrontherapy, oxygen ions 16O can be currently considered as an alternative to carbon ions 12C designed specifically for the treatment of deep and radioresistant tumors. Secondary particles, particularly neutrons constitute a serious problem of undesirable additional irradiation to surrounding healthy tissue. The objective of this study is to evaluate, by Monte-Carlo simulation [code Particle and Heavy Ion Transport code System (PHITS)], the contribution in terms of dose of secondary neutrons produced during interaction 16O ion of 300 MeV u-1 in a soft tissue phantom. The dose of 16O ion, secondary particles and neutrons is evaluated, as well as the particle fluence and energy spectra of neutrons. The contribution to the total dose of secondary neutrons in a soft tissue phantom represents 0.1%. This dose, although apparently insignificant, is essential to conduct even more in-depth studies to understand the long-term effects of these secondary neutrons on the patient's body especially in pediatric case.

Treatment Planning Study for Microbeam Radiotherapy Using Clinical Patient Data

Microbeam radiotherapy (MRT) is a novel, still preclinical dose delivery technique. MRT has shown reduced normal tissue effects at equal tumor control rates compared to conventional radiotherapy. Treatment planning studies are required to permit clinical application. The aim of this study was to establish a dose comparison between MRT and conventional radiotherapy and to identify suitable clinical scenarios for future applications of MRT. We simulated MRT treatment scenarios for clinical patient data using an inhouse developed planning algorithm based on a hybrid Monte Carlo dose calculation and implemented the concept of equivalent uniform dose (EUD) for MRT dose evaluation. The investigated clinical scenarios comprised fractionated radiotherapy of a glioblastoma resection cavity, a lung stereotactic body radiotherapy (SBRT), palliative bone metastasis irradiation, brain metastasis radiosurgery and hypofractionated breast cancer radiotherapy. Clinically acceptable treatment plans were achieved for most analyzed parameters. Lung SBRT seemed the most challenging treatment scenario. Major limitations comprised treatment plan optimization and dose calculation considering the tissue microstructure. This study presents an important step of the development towards clinical MRT. For clinical treatment scenarios using a sophisticated dose comparison concept based on EUD and EQD2, we demonstrated the capability of MRT to achieve clinically acceptable dose distributions.

Future Developments in Charged Particle Therapy: Improving Beam Delivery for Efficiency and Efficacy

The physical and clinical benefits of charged particle therapy (CPT) are well recognized. However, the availability of CPT and complete exploitation of dosimetric advantages are still limited by high facility costs and technological challenges. There are extensive ongoing efforts to improve upon these, which will lead to greater accessibility, superior delivery, and therefore better treatment outcomes. Yet, the issue of cost remains a primary hurdle as utility of CPT is largely driven by the affordability, complexity and performance of current technology. Modern delivery techniques are necessary but limited by extended treatment times. Several of these aspects can be addressed by developments in the beam delivery system (BDS) which determines the overall shaping and timing capabilities enabling high quality treatments. The energy layer switching time (ELST) is a limiting constraint of the BDS and a determinant of the beam delivery time (BDT), along with the accelerator and other factors. This review evaluates the delivery process in detail, presenting the limitations and developments for the BDS and related accelerator technology, toward decreasing the BDT. As extended BDT impacts motion and has dosimetric implications for treatment, we discuss avenues to minimize the ELST and overview the clinical benefits and feasibility of a large energy acceptance BDS. These developments support the possibility of advanced modalities and faster delivery for a greater range of treatment indications which could also further reduce costs. Further work to realize methodologies such as volumetric rescanning, FLASH, arc, multi-ion and online image guided therapies are discussed. In this review we examine how increased treatment efficiency and efficacy could be achieved with improvements in beam delivery and how this could lead to faster and higher quality treatments for the future of CPT.

Physics and biomedical challenges of cancer therapy with accelerated heavy ions

Radiotherapy should have low toxicity in the entrance channel (normal tissue) and be very effective in cell killing in the target region (tumour). In this regard, ions heavier than protons have both physical and radiobiological advantages over conventional X-rays. Carbon ions represent an excellent combination of physical and biological advantages. There are a dozen carbon-ion clinical centres in Europe and Asia, and more under construction or at the planning stage, including the first in the USA. Clinical results from Japan and Germany are promising, but a heated debate on the cost-effectiveness is ongoing in the clinical community, owing to the larger footprint and greater expense of heavy ion facilities compared with proton therapy centres. We review here the physical basis and the clinical data with carbon ions and the use of different ions, such as helium and oxygen. Research towards smaller and cheaper machines with more effective beam delivery is necessary to make particle therapy affordable. The potential of heavy ions has not been fully exploited in clinics and, rather than there being a single 'silver bullet', different particles and their combination can provide a breakthrough in radiotherapy treatments in specific cases.

FLASH radiotherapy with carbon ion beams

FLASH radiotherapy is considered a new potential breakthrough in cancer treatment. Ultra-high dose rates (>40 Gy/s) have been shown to reduce toxicity in the normal tissue without compromising tumor control, resulting in a widened therapeutic window. These high dose rates are more easily achievable in the clinic with charged particles, and clinical trials are, indeed, ongoing using electrons or protons. FLASH could be an attractive solution also for heavier ions such as carbon and could even enhance the therapeutic window. However, it is not yet known whether the FLASH effect will be the same as for sparsely ionizing radiation when densely ionizing carbons ions are used. Here we discuss the technical challenges in beam delivery and present a promising solution using 3D range-modulators in order to apply ultra-high dose rates (UHDR) compatible with FLASH with carbon ions. Furthermore, we will discuss the possible outcome of C-ion therapy at UHDR on the level of the radiobiological and radiation chemical effects.

EVALUATION OF NEUTRON AMBIENT DOSE EQUIVALENT IN INTENSITY-MODULATED COMPOSITE PARTICLE THERAPY

Several studies have reported benefits derived from cancer treatment using various heavy-ion beams. Based on these reports, the National Institutes for Quantum and Radiological Science and Technology started developing intensity-modulated composite particle therapy (IMPACT) using He-, C-, O-, and Ne-ions. In ion beam therapy, nuclear interactions in the beamline devices or patient produce secondary neutrons. This study evaluated the characteristics of secondary neutrons in IMPACT. Neutron ambient dose equivalents were measured using WENDI-II. Measurements were performed under realistic case scenarios using He-, C-, O- and Ne-ion beams. Moreover, neutron ambient dose equivalents generated by He-, C-, O- and Ne-ion beams were compared with neutron ambient dose equivalents in proton therapy. No differences exist in the distance-dependence even when the primary ions are different. Neutrons generated by primary ion beams of high atomic numbers tend to emit forward. Moreover, in contrast with proton therapy, IMPACT can reduce neutron doses.

Inter-fractional monitoring of [Formula: see text]C ions treatments: results from a clinical trial at the CNAO facility

The high dose conformity and healthy tissue sparing achievable in Particle Therapy when using C ions calls for safety factors in treatment planning, to prevent the tumor under-dosage related to the possible occurrence of inter-fractional morphological changes during a treatment. This limitation could be overcome by a range monitor, still missing in clinical routine, capable of providing on-line feedback. The Dose Profiler (DP) is a detector developed within the INnovative Solution for In-beam Dosimetry in hadronthErapy (INSIDE) collaboration for the monitoring of carbon ion treatments at the CNAO facility (Centro Nazionale di Adroterapia Oncologica) exploiting the detection of charged secondary fragments that escape from the patient. The DP capability to detect inter-fractional changes is demonstrated by comparing the obtained fragment emission maps in different fractions of the treatments enrolled in the first ever clinical trial of such a monitoring system, performed at CNAO. The case of a CNAO patient that underwent a significant morphological change is presented in detail, focusing on the implications that can be drawn for the achievable inter-fractional monitoring DP sensitivity in real clinical conditions. The results have been cross-checked against a simulation study.

Carbon Ion Radiobiology

Radiotherapy using accelerated charged particles is rapidly growing worldwide. About 85% of the cancer patients receiving particle therapy are irradiated with protons, which have physical advantages compared to X-rays but a similar biological response. In addition to the ballistic advantages, heavy ions present specific radiobiological features that can make them attractive for treating radioresistant, hypoxic tumors. An ideal heavy ion should have lower toxicity in the entrance channel (normal tissue) and be exquisitely effective in the target region (tumor). Carbon ions have been chosen because they represent the best combination in this direction. Normal tissue toxicities and second cancer risk are similar to those observed in conventional radiotherapy. In the target region, they have increased relative biological effectiveness and a reduced oxygen enhancement ratio compared to X-rays. Some radiobiological properties of densely ionizing carbon ions are so distinct from X-rays and protons that they can be considered as a different "drug" in oncology, and may elicit favorable responses such as an increased immune response and reduced angiogenesis and metastatic potential. The radiobiological properties of carbon ions should guide patient selection and treatment protocols to achieve optimal clinical results.

Particle therapy in the future of precision therapy

The first hospital-based treatment facilities for particle therapy started operation about thirty years ago. Since then, the clinical experience with protons and carbon ions has grown continuously and more than 200,000 patients have been treated to date. The promising clinical results led to a rapidly increasing number of treatment facilities and many new facilities are planned or under construction all over the world. An inverted depth-dose profile combined with potential radiobiological advantages make charged particles a precious tool for the treatment of tumours that are particularly radioresistant or located nearby sensitive structures. A rising number of trials have already confirmed the benefits of particle therapy in selected clinical situations and further improvements in beam delivery, image guidance and treatment planning are expected. This review summarises some physical and biological characteristics of accelerated charged particles and gives some examples of their clinical application. Furthermore, challenges and future perspectives of particle therapy will be discussed.

Prompt gamma spectroscopy for absolute range verification of 12C ions at synchrotron-based facilities

The physical range uncertainty limits the exploitation of the full potential of charged particle therapy. In this work, we face this issue aiming to measure the absolute Bragg peak position in the target. We investigate p, ⁴He, ¹²C and ¹⁶O beams accelerated at the Heidelberg Ion-Beam Therapy Center. The residual range of the primary ¹²C ions is correlated to the energy spectrum of the prompt gamma radiation. The prompt gamma spectroscopy method was demonstrated for proton beams accelerated by cyclotrons and is developed here for the first time for heavier ions accelerated by a synchrotron. We develop a detector system that includes (i) a spectroscopic unit based on cerium(III) bromide and bismuth germanium oxide scintillating crystals, (ii) a beam trigger based on an array of scintillating fibers and (iii) a data acquisition system based on a FlashADC. We test the system in two different scenarios. In the first series of experiments, we detect and identify 19 independent spectral lines over a wide gamma energy spectrum in the presence of the four ion species for different targets, including a water target with a titanium insert. In the second series of experiments, we introduce a collimator aiming to relate the spectral information to the range of the primary particles. We perform extensive measurements for a ¹²C beam and demonstrate submillimetric precision for the measurement of its Bragg peak position in the experimental setup. The features of the energy and time spectra for gamma radiation induced by p, ⁴He and ¹⁶O are investigated upstream and downstream from the Bragg peak position. We conclude the analysis by extrapolating the required future developments, which would be needed to achieve range verification with a 2 mm accuracy during a single fraction delivery of [Formula: see text] physical dose.

Dose- rather than fluence-averaged LET should be used as a single-parameter descriptor of proton beam quality for radiochromic film dosimetry

PURPOSE

The dose response of Gafchromic EBT3 films exposed to proton beams depends on the dose, and additionally on the beam quality, which is often quantified with the linear energy transfer (LET) and, hence, also referred to as LET quenching. Fundamentally different methods to determine correction factors for this LET quenching effect have been reported in literature and a new method using the local proton fluence distribution differential in LET is presented. This method was exploited to investigate whether a more practical correction based on the dose- or fluence-averaged LET is feasible in a variety of clinically possible beam arrangements.

METHODS

The relative effectiveness (RE) was characterized within a high LET spread-out Bragg peak (SOBP) in water made up by the six lowest available energies (62.4-67.5 MeV, configuration " b 1 ") resulting in one of the highest clinically feasible dose-averaged LET distributions. Additionally, two beams were measured where a low LET proton beam (252.7 MeV) was superimposed on " b 1 ", which contributed either 50% of the initial particle fluence or 50% of the dose in the SOBP, referred to as configuration " b 2 " and " b 3 ," respectively. The proton LET spectrum was simulated with GATE/Geant4 at all measurement positions. The net optical density change differential in LET was integrated over the local proton spectrum to calculate the net optical density and therefrom the beam quality correction factor. The LET dependence of the film response was accounted for by an LET dependence of one of the three parameters in the calibration function and was determined from inverse optimization using measurement " b 1 ." This method was then validated on the measurements of " b 2 " and " b 3 " and subsequently used to calculate the RE at 900 positions in nine clinically relevant beams. The extrapolated RE set was used to derive a simple linear correction function based on dose-averaged LET ( L d ) and verify the validity in all points of the comprehensive RE set.

RESULTS

The uncorrected film dose deviated up to 26% from the reference dose, whereas the corrected film dose agreed within 3% in all three beams in water (" b 1 ", " b 2 " and " b 3 "). The LET dependence of the calibration function started to strongly increase around 5 keV/μm and flatten out around 30 keV/μm. All REs calculated from the proton fluence in the nine simulated beams could be approximated with a linear function of dose-averaged LET (RE = 1.0258-0.0211 μm/keV L d ). However, no functional relationship of RE- and fluence-averaged LET could be found encompassing all beam energies and modulations.

CONCLUSIONS

The film quenching was found to be nonlinear as a function of proton LET as well as of the dose-averaged LET. However, the linear relation of RE on dose-averaged LET was a good approximation in all cases. In contrast to dose-averaged LET, fluence-averaged LET could not describe the RE when multiple beams were applied.

Characterization of the Mixed Radiation Field Produced by Carbon and Oxygen Ion Beams of Therapeutic Energy: A Monte Carlo Simulation Study

Purpose:

The main advantages of charged particle radiotherapy compared to conventional X-ray external beam radiotherapy are a better tumor conformality coupled with the capability of treating deep-seated radio-resistant tumors. This work investigates the possibility to use oxygen beams for hadron therapy, as an alternative to carbon ions.

Materials and Methods:

Oxygen ions have the advantage of a higher relative biological effectiveness (RBE) and better conformality to the tumor target. This work describes the mixed radiation field produced by an oxygen beam in water and compares it to the one produced by a therapeutic carbon ion beam. The study has been performed using Geant4 simulations. The dose is calculated for incident carbon ions with energies of 162 MeV/u and 290 MeV/u, and oxygen ions with energies of 192 MeV/u and 245 MeV/u, and hence that the range of the primary oxygen ions projectiles in water was located at the same depth as the carbon ions.

Results:

The results show that the benefits of oxygen ions are more pronounced when using lower energies because of a slightly higher peak-to-entrance ratio, which allows either providing higher dose in tumor target or reducing it in the surrounding healthy tissues. It is observed that, per incident particle, oxygen ions deliver higher doses than carbon ions.

Conclusions:

This result coupled with the higher RBE shows that it may be possible to use a lower fluence of oxygen ions to achieve the same therapeutic dose in the patient as that obtained with carbon ion therapy.

The production of positron emitters with millisecond half-life during helium beam radiotherapy

Therapy with helium ions is currently receiving significantly increasing interest because helium ions have a sharper penumbra than protons and undergo less fragmentation than carbon ions and thus require less complicated dose calculations. For any ion of interest in hadron therapy, the accuracy of dose delivery is limited by range uncertainties. This has led to efforts by several groups to develop in vivo verification techniques, including positron emission tomography (PET), for monitoring of the dose delivery. Beam-on PET monitoring during proton therapy through the detection of short-lived positron emitters such as ¹²N (T _1/2  =  11 ms), an emerging PET technique, provides an attractive option given the achievable range accuracy, minimal susceptibility to biological washout and provision of near prompt feedback. Extension of this approach to helium ions requires information on the production yield of relevant short-lived positron emitters. This study presents the first measurements of the production of short-lived positron emitters in water, graphite, calcium and phosphorus targets irradiated with 59 MeV/u ³He and 50 MeV/u ⁴He beams. For these targets, the most produced short-lived nuclides are ¹³O/¹²N (T _1/2  =  8.6/11 ms) on water, ¹³O/¹²N on graphite, ⁴³Ti/⁴¹Sc/⁴²Sc (T _1/2  =  509-680 ms) on calcium, ²⁸P (T _1/2  =  268 ms) on phosphorus. A translation of the results from elemental targets to PMMA and representative tissues such as adipose tissue, muscle, compact and cortical bone, shows the dominance of ¹³O/¹²N in at least the first 20 s of an irradiation with ⁴He and somewhat longer with ³He. As the production of ¹³O/¹²N in a ³He irradiation is 3-4 times higher than in a ⁴He irradiation, from a statistical point of view, range verification using ¹³O/¹²N PET imaging will be about 2 times more precise for a ³He irradiation compared to a ⁴He irradiation.

Charge Transfer, Complexes Formation and Furan Fragmentation Induced by Collisions with Low-Energy Helium Cations

The present work focuses on unraveling the collisional processes leading to the fragmentation of the gas-phase furan molecules under the He⁺ and He²⁺ cations impact in the energy range 5–2000 eV. The presence of different mechanisms was identified by the analysis of the optical fragmentation spectra measured using the collision-induced emission spectroscopy (CIES) in conjunction with the ab initio calculations. The measurements of the fragmentation spectra of furan were performed at the different kinetic energies of both cations. In consequence, several excited products were identified by their luminescence. Among them, the emission of helium atoms excited to the 1s4d¹D₂, ³D_1,2,3 states was recorded. The structure of the furan molecule lacks an He atom. Therefore, observation of its emission lines is spectroscopic evidence of an impact reaction occurring via relocation of the electronic charge between interacting entities. Moreover, the recorded spectra revealed significant variations of relative band intensities of the products along with the change of the projectile charge and its velocity. In particular, at lower velocities of He⁺, the relative cross-sections of dissociation products have prominent resonance-like maxima. In order to elucidate the experimental results, the calculations have been performed by using a high level of quantum chemistry methods. The calculations showed that in both impact systems two collisional processes preceded fragmentation. The first one is an electron transfer from furan molecules to cations that leads to the neutralization and further excitation of the cations. The second mechanism starts from the formation of the He−C₄H₄O^+/2+ temporary clusters before decomposition, and it is responsible for the appearance of the narrow resonances in the relative cross-section curves.

First benchmarking of the BIANCA model for cell survival prediction in a clinical hadron therapy scenario

In the framework of RBE modelling for hadron therapy, the BIANCA biophysical model was extended to O-ions and was used to construct a radiobiological database describing the survival of V79 cells as a function of ion type (1  ⩽  Z  ⩽  8) and energy. This database allowed performing RBE predictions in very good agreement with experimental data. A method was then developed to construct analogous databases for different cell lines, starting from the V79 database as a reference. Following interface to the FLUKA Monte Carlo radiation transport code, BIANCA was then applied for the first time to predict cell survival in a typical patient treatment scenario, consisting of two opposing fields of range-equivalent protons or C-ions. The model predictions were found to be in good agreement with CHO cell survival data obtained at the Heidelberg ion-beam therapy (HIT) centre, as well as predictions performed by the local effect model (version LEM IV). This work shows that BIANCA can be used to predict cell survival and RBE not only for V79 and AG01522 cells, as shown previously, but also, in principle, for any cell line of interest. Furthermore, following interface to a transport code like FLUKA, BIANCA can provide predictions of 3D biological dose distributions for hadron therapy treatments, thus laying the foundations for future applications in clinics.

Secondary radiation measurements for particle therapy applications: Charged secondaries produced by 16O ion beams in a PMMA target at large angles

Particle therapy is a therapy technique that exploits protons or light ions to irradiate tumor targets with high accuracy. Protons and ¹²C ions are already used for irradiation in clinical routine, while new ions like ⁴He and ¹⁶O are currently being considered. Despite the indisputable physical and biological advantages of such ion beams, the planning of charged particle therapy treatments is challenged by range uncertainties, i.e. the uncertainty on the position of the maximal dose release (Bragg Peak - BP), during the treatment. To ensure correct 'in-treatment' dose deposition, range monitoring techniques, currently missing in light ion treatment techniques, are eagerly needed. The results presented in this manuscript indicate that charged secondary particles, mainly protons, produced by an ¹⁶O beam during target irradiation can be considered as candidates for ¹⁶O beam range monitoring. Hereafter, we report on the first yield measurements of protons, deuterons and tritons produced in the interaction of an ¹⁶O beam impinging on a PMMA target, as a function of detected energy and particle production position. Charged particles were detected at 90° and 60° with respect to incoming beam direction, and homogeneous and heterogeneous PMMA targets were used to probe the sensitivity of the technique to target inhomogeneities. The reported secondary particle yields provide essential information needed to assess the accuracy and resolution achievable in clinical conditions by range monitoring techniques based on secondary charged radiation.

Optimal modality selection in external beam radiotherapy

The goal in external beam radiotherapy (EBRT) for cancer is to maximize damage to the tumour while limiting toxic effects on the organs-at-risk. EBRT can be delivered via different modalities such as photons, protons and neutrons. The choice of an optimal modality depends on the anatomy of the irradiated area and the relative physical and biological properties of the modalities under consideration. There is no single universally dominant modality. We present the first-ever mathematical formulation of the optimal modality selection problem. We show that this problem can be tackled by solving the Karush-Kuhn-Tucker conditions of optimality, which reduce to an analytically tractable quartic equation. We perform numerical experiments to gain insights into the effect of biological and physical properties on the choice of an optimal modality or combination of modalities.

Review and performance of the Dose Profiler, a particle therapy treatments online monitor

Particle therapy (PT) can exploit heavy ions (such as He, C or O) to enhance the treatment efficacy, profiting from the increased Relative Biological Effectiveness and Oxygen Enhancement Ratio of these projectiles with respect to proton beams. To maximise the gain in tumor control probability a precise online monitoring of the dose release is needed, avoiding unnecessary large safety margins surroundings the tumor volume accounting for possible patient mispositioning or morphological changes with respect to the initial CT scan. The Dose Profiler (DP) detector, presented in this manuscript, is a scintillating fibres tracker of charged secondary particles (mainly protons) that will be operating during the treatment, allowing for an online range monitoring. Such monitoring technique is particularly promising in the context of heavy ions PT, in which the precision achievable by other techniques based on secondary photons detection is limited by the environmental background during the beam delivery. Developed and built at the SBAI department of "La Sapienza", within the INSIDE collaboration and as part of a Centro Fermi flagship project, the DP is a tracker detector specifically designed and planned for clinical applications inside a PT treatment room. The DP operation in clinical like conditions has been tested with the proton and carbon ions beams of Trento proton-therapy center and of the CNAO facility. In this contribution the detector performances are presented, in the context of the carbon ions monitoring clinical trial that is about to start at the CNAO centre.

Results from the experimental evaluation of CeBr 3 scintillators for 4 He prompt gamma spectroscopy

PURPOSE

The presence of range uncertainties hinders the exploitation of the full potential of charged particle therapy. Several range verification techniques have been proposed to mitigate this limitation. Prompt gamma spectroscopy (PGS) is among the most promising solutions for online and in vivo range verification. In this work, we present the experimental results of the detection of prompt gamma radiation, induced by 4 He beams at the Heidelberg Ion-Beam Therapy Center (HIT). The results were obtained, using a spectroscopic unit of which the design has been optimized using Monte Carlo simulations.

METHODS

The spectroscopic unit is composed by a primary cerium bromide (CeBr 3 ) crystal surrounded by a secondary bismuth germanate (BGO) crystal for anticoincidence detection (AC). The digitalization of the signals is performed with an advanced FADC/FPGA system. The 4 He beams at clinical energies and intensities are delivered to multiple targets in the experimental cave at the HIT. We analyze the production of prompt gamma on oxygen and carbon targets, as well as high Z materials such as titanium and aluminum. The quantitative analysis includes a systematic comparison of the signal-to-noise ratio (SNR) improvement for the spectral lines when introducing the AC detection. Moreover, the SNR improvement could provide a reduction of the number of events required to draw robust conclusions. We perform a statistic analysis to determine the magnitude of such an effect.

RESULTS

We present the energy spectra detected by the primary CeBr 3 and the secondary BGO. The combination of these two detectors leads to an average increase of the signal-to-noise ratio by a factor 2.1, which confirms the Monte Carlo predictions. The spectroscopic unit is capable of detecting efficiently the discrete gamma emission over the full energy spectrum. We identify and analyze 19 independent spectral lines in an energy range spacing from E γ = 0.718  MeV to E γ = 6.13  MeV. Moreover, when introducing the AC detection, the number of events required to determine robustly the intensity of the discrete lines decreases. Finally, the analysis of the low-energy reaction lines determines whether a thin metal insert is introduced in the beam direction.

CONCLUSIONS

This work provides the experimental characterization of the spectroscopy unit in development for range verification through PGS at the HIT. Excellent performances have been demonstrated over the full prompt gamma energy spectrum with 4 He beams at clinical energies and intensities.

Kill painting of hypoxic tumors with multiple ion beams

We report on a novel method for simultaneous biological optimization of treatment plans for hypoxic tumors using multiple ion species. Our previously introduced kill painting approach, where the overall cell killing is optimized on biologically heterogeneous targets, was expanded with the capability of handling different ion beams simultaneously. The current version (MIBO) of the research treatment planning system TRiP98 has now been augmented to handle 3D (voxel-by-voxel) target oxygenation data. We present a case of idealized geometries where this method can identify optimal combinations leading to an improved peak-to-entrance effective dose ratio. This is achieved by the redistribution of particle fluences, when the heavier ions are preferentially forwarded to hypoxic target areas, while the lighter ions deliver the remaining dose to its normoxic regions. Finally, we present an in silico skull base chordoma patient case study with a combination of ⁴He and ¹⁶O beams, demonstrating specific indications for its potential clinical application. In this particular case, the mean dose, received by the brainstem, was reduced by 3%-5% and by 10%-12% as compared to the pure ⁴He and ¹⁶O plans, respectively. The new method allows a full biological optimization of different ion beams, exploiting the capabilities of actively scanned ion beams of modern particle therapy centers. The possible experimental verification of the present approach at ion beam facilities disposing of fast ion switch is presented and discussed.

A comparison of mechanism-inspired models for particle relative biological effectiveness (RBE)

BACKGROUND AND SIGNIFICANCE

The application of heavy ion beams in cancer therapy must account for the increasing relative biological effectiveness (RBE) with increasing penetration depth when determining dose prescriptions and organ at risk (OAR) constraints in treatment planning. Because RBE depends in a complex manner on factors such as the ion type, energy, cell and tissue radiosensitivity, physical dose, biological endpoint, and position within and outside treatment fields, biophysical models reflecting these dependencies are required for the personalization and optimization of treatment plans.

AIM

To review and compare three mechanism-inspired models which predict the complexities of particle RBE for various ion types, energies, linear energy transfer (LET) values and tissue radiation sensitivities.

METHODS

The review of models and mechanisms focuses on the Local Effect Model (LEM), the Microdosimetric-Kinetic (MK) model, and the Repair-Misrepair-Fixation (RMF) model in combination with the Monte Carlo Damage Simulation (MCDS). These models relate the induction of potentially lethal double strand breaks (DSBs) to the subsequent interactions and biological processing of DSB into more lethal forms of damage. A key element to explain the increased biological effectiveness of high LET ions compared to MV x rays is the characterization of the number and local complexity (clustering) of the initial DSB produced within a cell. For high LET ions, the spatial density of DSB induction along an ion's trajectory is much greater than along the path of a low LET electron, such as the secondary electrons produced by the megavoltage (MV) x rays used in conventional radiation therapy. The main aspects of the three models are introduced and the conceptual similarities and differences are critiqued and highlighted. Model predictions are compared in terms of the RBE for DSB induction and for reproductive cell survival.

RESULTS AND CONCLUSIONS

Comparisons of the RBE for DSB induction and for cell survival are presented for proton (¹ H), helium (⁴ He), and carbon (¹² C) ions for the therapeutically most relevant range of ion beam energies. The reviewed models embody mechanisms of action acting over the spatial scales underlying the biological processing of potentially lethal DSB into more lethal forms of damage. Differences among the number and types of input parameters, relevant biological targets, and the computational approaches among the LEM, MK and RMF models are summarized and critiqued. Potential experiments to test some of the seemingly contradictory aspects of the models are discussed.

Combining Heavy-Ion Therapy with Immunotherapy: An Update on Recent Developments

Clinical trials and case reports of cancer therapies combining radiation therapy with immunotherapy have at times demonstrated total reduction or elimination of metastatic disease. While virtually all trials focus on the use of immunotherapy combined with conventional photon irradiation, the dose-distributive benefits of particles, in particular the distinct biological effects of heavy ions, have unknown potential vis-a-vis systemic disease response. Here, we review recent developments and evidence with a focus on the potential for heavy-ion combination therapy.

Biological effects of mixed-ion beams. Part 1: Effect of irradiation of the CHO-K1 cells with a mixed-ion beam containing the carbon and oxygen ions

Carbon and oxygen ions were accelerated simultaneously to estimate the effect of irradiation of living cells with the two different ions. This mixed ion beam was used to irradiate the CHO-K1 cells, and a survival test was performed. The type of the effect of the mixed ion beam on the cells was determined with the isobologram method, whereby survival curves for irradiations with individual ion beams were also used. An additive effect of irradiation with the two ions was found.

Adaptation of stochastic microdosimetric kinetic model for charged-particle therapy treatment planning

The microdosimetric kinetic (MK) model underestimates the cell-survival fractions for high linear energy transfer (LET) and high dose irradiations. To address the issue, some researchers previously extended the MK model to the stochastic microdosimetric kinetic (SMK) model. In the SMK model, the radiation induced cell-survival fractions were estimated from the specific energies z _d and z _n absorbed by a microscopic subnuclear structure domain and a cell nucleus, respectively. By taking the stochastic nature of z _n as well as that of z _d into account, the SMK model could reproduce the measured cell-survival fractions for radiations with wide LET and dose ranges. However, treatment planning based on the SMK model was unrealistic in clinical practice due to its long computation time and huge memory space required for the computation. In this study, we modified the SMK model to shorten the computation time and to reduce the memory space required for the computation. By using the dose-averaged cell-nucleus specific energy per event [Formula: see text] in the SMK formalism, the stochastic nature of z _n was reflected onto the estimated cell-survival fractions. The accuracy of the modified SMK model was examined through the comparison between the estimated and the measured survival fractions of human salivary gland tumor cells and V79 cells. We then implemented the modified SMK model into the in-house treatment planning software for scanned charged-particle therapy to validate its applicability in clinical practice. As examples, treatment plans of helium-, carbon-, and neon-ion beams were made for an orbital tumor case. The modified SMK model could reproduce the measured cell-survival fractions more accurately compared to the MK model especially for high-LET and high-dose irradiations. In summary, the modified SMK model offers the accuracy and simplicity required in treatment planning of scanned charged-particle therapy for wide LET and dose ranges.

Analysis of Radiation-Induced Chromosomal Aberrations on a Cell-by-Cell Basis after Alpha-Particle Microbeam Irradiation: Experimental Data and Simulations

There is a continued need for further clarification of various aspects of radiation-induced chromosomal aberration, including its correlation with radiation track structure. As part of the EMRP joint research project, Biologically Weighted Quantities in Radiotherapy (BioQuaRT), we performed experimental and theoretical analyses on chromosomal aberrations in Chinese hamster ovary cells (CHO-K1) exposed to α particles with final energies of 5.5 and 17.8 MeV (absorbed doses: ∼2.3 Gy and ∼1.9 Gy, respectively), which were generated by the microbeam at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany. In line with the differences in linear energy transfer (approximately 85 keV/μm for 5.5 MeV and 36 keV/μm for 17.8 MeV α particles), the 5.5 MeV α particles were more effective than the 17.8 MeV α particles, both in terms of the percentage of aberrant cells (57% vs. 33%) and aberration frequency. The yield of total aberrations increased by a factor of ∼2, although the increase in dicentrics plus centric rings was less pronounced than in acentric fragments. The experimental data were compared with Monte Carlo simulations based on the BIophysical ANalysis of Cell death and chromosomal Aberrations model (BIANCA). This comparison allowed interpretation of the results in terms of critical DNA damage [cluster lesions (CLs)]. More specifically, the higher aberration yields observed for the 5.5 MeV α particles were explained by taking into account that, although the nucleus was traversed by fewer particles (nominally, 11 vs. 25), each particle was much more effective (by a factor of ∼3) at inducing CLs. This led to an increased yield of CLs per cell (by a factor of ∼1.4), consistent with the increased yield of total aberrations observed in the experiments.