Report of the AAPM TG-256 on the relative biological effectiveness of proton beams in radiation therapy

Salvage Therapy for Isolated Local-Regional Breast Cancer Recurrence

The overall number of breast cancer patients at risk of developing local-regional disease recurrence has been increasing due to long-term survivorship from improvements in multimodality breast cancer treatment and a steady increase in breast cancer incidence. Many patients receive radiotherapy as part of definitive multidisciplinary breast cancer treatment, and to date, practitioners have approached reirradiation delivery with reticence due to concern for serious toxicities that may be incurred with high cumulative radiation doses. However, a subset of patients with breast cancer recurrence may benefit from reirradiation for improved locoregional tumor control. An emerging body of evidence has demonstrated promising efficacy and safety of breast cancer reirradiation that are gradually redefining the treatment paradigm. In addition, an increase in systemic therapy options has further optimized the opportunity for successful salvage of breast cancer recurrence. In this critical review, we review breast cancer radiation and systemic therapy salvage options, available data, ongoing studies, and treatment delivery considerations.

Embracing the Future of Clinical Trials in Radiation Therapy: An NRG Oncology CIRO Technology Retreat Whitepaper on Pioneering Technologies and AI-Driven Solutions

This white paper examines the potential of pioneering technologies and artificial intelligence-driven solutions in advancing clinical trials involving radiation therapy. As the field of radiation therapy evolves, the integration of cutting-edge approaches such as radiopharmaceutical dosimetry, FLASH radiation therapy, image guided radiation therapy, and artificial intelligence promises to improve treatment planning, patient care, and outcomes. Additionally, recent advancements in quantum science, linear energy transfer/relative biological effect, and the combination of radiation therapy and immunotherapy create new avenues for innovation in clinical trials. The paper aims to provide an overview of these emerging technologies and discuss their challenges and opportunities in shaping the future of radiation oncology clinical trials. By synthesizing knowledge from experts across various disciplines, this white paper aims to present a foundation for the successful integration of these innovations into radiation therapy research and practice, ultimately enhancing patient outcomes and revolutionizing cancer care.

Comparison of Proton Versus Photon SBRT for Treatment of Spinal Metastases Using Variable RBE Models

Purpose

Study of proton stereotactic body radiation therapy (SBRT) for spinal metastasis has been limited, largely due to concerns of increased risk of spinal cord injury given the challenges of end of range relative biological effectiveness (RBE). Although the 1.1 RBE constant for proton beam has been adopted for clinical use, data indicate that proton RBE is variable and dependent on technical-, tissue-, and patient factors. To better understand the safety of proton SBRT for spinal metastasis, this dosimetric analysis compares plans using photon robotic techniques and proton therapy accounting for RBE-weighted dose (D_{RBE}).

Materials and Methods

Nine patients with spinal metastasis were selected to be representative of a broad range of complex clinical practice (3 cervical, 3 thoracic, 3 lumbar) that are uniquely challenging to treat with SBRT were identified. Each vertebral level contained a case with paraspinal extension, a reirradiation case, and a case with high-grade epidural disease (Bilsky grade ≥1c) given that such complex cases in current practice often require target volume under-coverage with photon SBRT (PH-SBRT) in order to meet organ at risk (OAR) dose constraints. All selected patients were treated with PH-SBRT using a robotic system to a prescription dose of 30 Gy in 5 fractions despite our institutional preference for further dose escalation, because further dose escalation was not feasible in the original planning process while keeping normal tissues below acceptable dose constraints. To see if superior target coverage could be achieved with proton treatment, comparative intensity modulated proton therapy (IMPT) plans were generated with the same prescription dose as what was clinically delivered using the 1.1 RBE constant. Dose escalated IMPT plans were then generated to 45 Gy(RBE) in 5 fractions. Variable RBE models (Carabe, McNamara, and Wedenberg) were then utilized to generate RBE-weighted dose D_{RBE} distribution for 30 Gy(RBE) and 45 Gy(RBE) plans using the α/β value (which was 3.76 in this study), physical dose, linear energy transfer (LET) value, and dose per fraction parameters. Proton plans used the robust optimization parameters of ±3.5% range and 2-mm setup uncertainties. Planning target volume (PTV) coverage and OARs sparing were compared using the Wilcoxon signed-rank test.

Results

Planning target volume coverage was significantly improved when comparing PH-SBRT at 30 Gy in 5 fractions (median: 25 Gy) to IMPT at 30 Gy[RBE] in 5 fractions (median: 30.3 Gy[RBE], P = .02) and 45 Gy(RBE) in 5 fractions (median 35.6 Gy[RBE], P = .001). Maximum dose of the spinal cord (cord Dmax) was significantly lower with IMPT at 30 Gy(RBE) (median: 17.6 Gy[RBE], P = .04) and 45 Gy(RBE) (median: 16.1 Gy[RBE], P = .04) compared to conventional plan at 30 Gy (median: 18 Gy). Spinal cord expansion (cord + 2 mm) maximum dose did not change in both photon (median 21.5 Gy) and proton plans (median 22.5, P = .27). Other OARs were better spared with the same and dose-escalated proton plans. No difference was seen in cord Dmax when comparing the PH-SBRT at 30 Gy to D_{RBE} at 30 and 45 Gy(RBE) using Carabe-, McNamara-, or Wedenberg models. However, for spinal cord expansion (cord + 2 mm), there was significant difference between PH-SBRT and D_{RBE} at 30 Gy(RBE) and 45 Gy(RBE) in 5 fractions using Carabe- (median: 25.4 Gy[RBE], P = .002), McNamara- (median: 25.1 Gy[RBE], P = .003), or Wedenberg (median: 24.8 Gy[RBE], P = .0001) models. The average increase in the spinal cord expansion maximum dose using these models compared to the fixed RBE plans was 5.3%.

Conclusion

We report the first dosimetric analysis of proton SBRT for spine metastasis using variable RBE dose models. Compared to photon SBRT, IMPT may provide improved target coverage and better spare adjacent OARs, though fixed RBE models can underestimate the maximum dose to adjacent OARs. Future prospective studies are needed to validate these results.

Relative biological effectiveness-weighted dose sparing utilizing a linear energy transfer optimization function for organs-at-risk in left breast proton treatment plans

Current proton treatment plans employ a constant relative biological effectiveness (RBE) of 1.1, which may underestimate organs-at-risk (OAR) doses due to high linear energy transfer (LET) at the distal end of the proton beam. When 2 anterior fields are used for left breast treatment using pencil beam scanning (PBS), the ipsilateral lung and heart are susceptible to high LET. Although studies have indicated a potential increase in OAR toxicity linked to LET effects, the strategy of optimizing LET to RBE-weighted doses (D_vRBE) to spare OAR is still underexplored in treatment planning research. Addressing this gap is crucial for improving treatment outcomes and minimizing toxicities. To evaluate the effectiveness of LET optimization in reducing doses to OAR, we aimed to achieve more than a 10% reduction in the mean or maximum D_vRBE for the heart, ipsilateral lung, and humeral joint, while ensuring adequate target coverage. In this study, 12 cases of left-sided breast cancer with locally advanced invasive carcinoma were randomly selected to assess the dose reduction of OAR using LET optimization. The OAR doses were compared to those of clinically accepted PBS plans that did not incorporate LET optimization. To validate the hypothesis, a statistical analysis was implemented using the Wilcoxon-Signed Rank test. The results demonstrated a significant reduction in mean D_vRBE for both the heart and ipsilateral lung, as well as the maximum D_vRBE for humeral joint (p < 0.05). A decrease in both maximum and mean LET was observed in the ipsilateral lung (p < 0.05). These findings indicate that optimizing LET has the potential to effectively reduce D_vRBE for OAR, which can lead to minimizing organ toxicity.

Systematic review of MRI alterations in the brain following proton and photon radiation therapy: Towards a uniform European Particle Therapy Network (EPTN) definition

Magnetic resonance imaging (MRI) often demonstrates alterations following cranial radiotherapy (RT), which may result in clinical symptoms and diagnostic uncertainty, and thus potentially impact treatment decisions. The potential differences in MRI alterations after proton and photon RT, has raised concerns regarding the relative biological effectiveness of proton therapy. To provide an overview of MRI alterations in the brain post-RT and to explore differences between photon and proton RT, a systematic review adhering to the PRISMA guidelines was conducted, focusing on the assessment methods and definitions across studies. A systematic search of three electronic databases was performed using the concepts 'normo-fractionated radiotherapy ', 'MRI alterations' and 'brain, skull base or head and neck tumours in adult and paediatric populations'. Data extraction and quality assessment was performed on articles meeting the predefined criteria by two independent reviewers. Out of 5887 screened studies, 94 met the inclusion criteria. These studies were categorized based on confinement of the MRI alterations to temporal lobe, brainstem, or across the entire brain. Additional subclassification was performed based on MRI sequences evaluated or by the nature of the alterations, with pseudoprogression generally reserved for glioma patients. While many papers exist on MRI alterations in the brain after RT, this review highlights significant inconsistencies in the terminology and definitions, limiting the comparability of findings across studies. Our results highlight the need for and facilitate the development of a standardized framework for describing MRI alterations after RT.

Steenbakkers R, Vogel WV, Both S. Relating proton LETd to biological response of parotid and submandibular glands using PSMA-PET in clinical patients

Background and purpose

A recent study investigated the use of PSMA-PET in monitoring loss of secretory cells in salivary glands of head and neck cancer (HNC) patients. Previously, a dose-effect relation has been formulated to the PSMA-PET uptake in salivary glands. The aim of this study was to derive a proton RBE model from the PSMA-PET uptake in salivary glands after proton therapy of HNC patients.

Materials and methods

Six patients treated with proton therapy were included. These patients received a PET-CT scan using 68Ga (N = 1) or 18F (N = 5) PSMA before treatment (baseline) and one month after the last fraction (follow-up). Physical dose (D), D·LETd and the follow-up PSMA-PET scan were deformed to the baseline PET-CT using deformable image registration. Parotid and submandibular gland delineations were adjusted to include voxels which had an uptake of ≥ 5 g/ml in the baseline PSMA-PET scan.

Results

The average RBE-LET slope was 0.075 [0.009; 0.125] (keV/μm)-1 (mean [95 %CI]) for parotid and submandibular glands combined. When analyzing parotid or submandibular glands separately the RBE-LET curve slope varies with two and five patients showing a positive RBE-LET slope when only analyzing parotid or submandibular glands respectively.

Conclusion

Our study did not find clear evidence of an increased RBE in parotid and submandibular glands with increasing LETd. On average an LETd effect was observed, however our sample size was too small to clearly define an RBE-LET relation. A larger cohort scanned at later time intervals could shed more light on this issue.

Study of linear energy transfer effect on rib fracture in breast cancer patients receiving pencil-beam-scanning proton therapy

BACKGROUND

In breast cancer patients treated with pencil-beam scanning proton therapy (PBS), the increased linear energy transfer (LET) near the end of the proton range can affect nearby ribs. This may associate with a higher risk of rib fractures.

PURPOSE

To study the effect of LET on rib fracture in breast cancer patients treated with PBS using a novel tool of dose-LET volume histogram (DLVH).

METHODS

From a prospective registry of patients treated with post-mastectomy proton therapy to the chest wall and regional lymph nodes for breast cancer between 2015 and 2020, we retrospectively identified rib fracture cases detected after completing treatment. Contemporaneously treated control patients who did not develop rib fracture were matched to patients 2:1 considering prescription dose, boost location, reconstruction status, laterality, chest wall thickness, and treatment year. The DLVH index, V(d, l), defined as volume(V) of the structure with at least dose(d) and dose-averaged LET (l) (LETd), was calculated. DLVH plots between the fracture and control group were compared. Conditional logistic regression (CLR) model was used to establish the relation of V(d, l) and the observed fracture at each combination of d and l. The p-value derived from CLR model shows the statistical difference between fracture patients and the matched control group. Using the 2D p-value map derived from CLR model, the DLVH features associated with the patient outcomes were extracted.

RESULTS

Seven rib fracture patients were identified, and fourteen matched patients were selected for the control group. The median time from the completion of proton therapy to rib fracture diagnosis was 12 months (range 5-14 months). Two patients had grade 2 symptomatic rib fracture while the remaining 5 were grade 1 incidentally detected on imaging. The derived p-value map demonstrated larger V(0-36 Gy[RBE], 4.0-5.0 keV/µm) in patients experiencing fracture (p < 0.1). For example, the p-value for V(30 Gy[RBE], 4.0 keV/um) was 0.069.

CONCLUSION

In breast cancer patients receiving PBS, a larger volume of chest wall receiving moderate dose and high LETd may result in an increased risk of rib fracture.

Efficient proton transport modelling for proton beam therapy and biological quantification

In this work, we present a fundamental mathematical model for proton transport, tailored to capture the key physical processes underpinning Proton Beam Therapy (PBT). The model provides a robust and computationally efficient framework for exploring various aspects of PBT, including dose delivery, linear energy transfer, treatment planning and the evaluation of relative biological effectiveness. Our findings highlight the potential of this model as a complementary tool to more complex and computationally intensive simulation techniques currently used in clinical practice.

Differentiation Stage Predicts Radiosensitivity in Mesenchymal-Like Pancreatic Cancer

PURPOSE

To derive a genomic classifier to predict radiosensitivity of pancreatic cancer cell lines and patients with pancreatic cancer to allow genomic-guided radiation therapy.

METHODS AND MATERIALS

We collected a comprehensive data set of full clonogenic cell survival curves of 45 pancreatic cancer cell lines irradiated with clinical photon and proton beams. We derived classifiers based on data from human embryonic and fetal pancreas single-cell RNA-sequencing to distinguish between epithelial and mesenchymal cells and to predict pancreas cell-line differentiation stage. Independent testing was done with an embryonic mouse pancreas single-cell RNA-sequencing data set. We then used bulk RNA-seq profiles from the Cancer Cell Line Encyclopedia to classify our pancreatic cancer cell lines using our epithelial-mesenchymal and differentiation stage classifiers. We then correlated the differentiation stage classifier with the radiosensitivity of the pancreatic cancer cell lines as well as with pancreatic cancer patient data from The Cancer Genome Atlas.

RESULTS

We found wide variability in radiosensitivity to both photons and protons among pancreatic cancer cell lines. We showed that the differentiation stage is predictive of radiosensitivity of mesenchymal pancreatic cancer cell lines but not epithelial pancreatic cancer cell lines. We found that chromatin compaction is associated with the differentiation stage and showed that the less differentiated mesenchymal pancreatic cancer cell lines tend to be radioresistant and with more compact chromatin than the radiosensitive differentiated cell lines. Patients with more differentiated tumors exhibit better overall survival.

CONCLUSIONS

We found that mesenchymal-like undifferentiated pancreatic cancer cell lines are more radioresistant than mesenchymal-like differentiated ones and that patients with pancreatic cancer with mesenchymal-like undifferentiated tumors treated with radiation therapy tend to have lower overall survival compared with patients with mesenchymal-like differentiated tumors. We show that it is feasibility to use the differentiation stage of mesenchymal pancreatic cancer cells to predict tumor specific radiosensitivity.

The Association of Linear Energy Transfer and Dose With Radiation Necrosis After Pencil Beam Scanning Proton Therapy in Pediatric Posterior Fossa Tumors

PURPOSE

Proton therapy is the preferred treatment modality for most pediatric central nervous system tumors. The risk of radiation necrosis may be increased at the distal end of the beam because of an increase in linear energy transfer (LET) and relative biological effectiveness (RBE) dose. We report on the association of LET and dose with radiation necrosis after pencil beam scanning proton therapy in pediatric posterior fossa tumors using a case-control framework.

MATERIALS AND METHODS

From 2019 to 2022, 33 patients less than or equal to 18 years of age treated with first-line proton therapy for primary tumors in the posterior fossa and with 6 or more months of follow-up magnetic resonance imaging were retrospectively identified. Nine patients with imaging changes consistent with necrosis were matched with controls in a 1:2 fashion based on age, sex, dose, and follow-up time from proton therapy. Dose (Gy [RBE]) and dose-averaged LET (LETd) values for target structures and organs at risk were computed and compared between cases and controls.

RESULTS

Within the whole cohort, the mean age was 6.6 years (SD, 4.77) with a median follow-up time of 24.1 months. Within the case-control matched cohort (18 controls and 9 cases), there were no significant differences in age, sex, time to follow-up, tumor location, dose, and use of concurrent chemotherapy. The mean time to necrotic imaging finding was 4.47 months (SD, 2.03). Cases demonstrated significantly higher brainstem D50 (P = .02). LETd was not different between cases and controls. However, when using a combined metric of higher brainstem dose {>47.5 (Gy [RBE])} and higher LETd (>3.5 keV/µm), a greater proportion of cases compared with controls met this metric (89% vs 39%, P = .02).

CONCLUSIONS

Combined effects of intermediate-to-high dose and LETd in the brainstem may contribute to greater necrosis risk after pencil beam scanning proton therapy in children with posterior fossa tumors.

In Silico Interim Adaptation of Proton Therapy in Head and Neck Cancer by Simultaneous Dose and Linear Energy Transfer Escalation

PURPOSE

The outcome of proton therapy for head and neck cancer (HNC) varies considerably. We investigated the feasibility of adapting proton therapy plans based on 18F-fluorodeoxyglucose-positron emission tomography-defined biologic tumor volumes (BTVs) reflecting remaining aggressive tumor subvolumes 2 weeks into treatment (interim). Recognizing the potential to improve proton therapy response with increasing linear energy transfer (LET), we simulated a combined dose-LET escalation to the BTVs and compared it to pure dose escalation. In addition, the impact of relative biological effectiveness (RBE) was evaluated by comparing the constant RBE of 1.1 (RBE1.1) with a variable-RBE model.

METHODS AND MATERIALS

A semiautomated method was used to segment the BTV from 18F-fluorodeoxyglucose-positron emission tomography-defined for 9 patients with HNC, assuming high standardized uptake value at interim to reflect tumor radioresistance. An in-house Monte Carlo-based recalculation and reoptimization tool simulated proton therapy plans with both constant RBE1.1 and variable-RBE, aimed to deliver 68 Gy (RBE) to high-risk target volumes, 10% dose escalation to the BTV, and a LET boost to the BTV. Dose distributions were prioritized over LET optimization goals. Results were quantified by dose and LET distributions to target volumes and organs at risk, as well as normal tissue complication probabilities (NTCPs) for xerostomia and dysphagia.

RESULTS

Dose-LET adapted proton therapy plans achieved 10% dose escalation and mean dose-averaged LET (LETd) increases to the BTV above 1.0 keV/μm, with no significant LET increases to organs at risk. NTCP for xerostomia and dysphagia from dose-LET and dose-only escalation were similar. However, NTCPs increased 6% to 10% when variable-RBE was used instead of the constant RBE1.1.

CONCLUSIONS

Our in silico study showed that dose-LET escalation in proton therapy integrating a variable-RBE model may improve proton therapy for patients with HNC. Clinical evaluation of such a biological image-based dose-LET escalation in proton therapy of HNC remains to be investigated.

The proton RBE and the distal edge effect for acute and late normal tissue damage in vivo

BACKGROUND AND PURPOSE

In proton therapy, a relative biological effectiveness (RBE) of 1.1 is used toreach an isoeffective biological response between photon and proton doses. However, the RBE varies with biological endpoints and linear energy transfer (LET), two key parameters in radiotherapy. Few in vivo studies have investigated the increasing RBE with increasing LET. This study aims to test the hypothesis that the RBE varies between endpoints and has a distal edge effect in vivo.

MATERIALS AND METHODS

Unanesthetized micewere restrainedin jigs where their right hind legs were irradiated with a single dose of protons at the center (LET, all = 5.3 keV/μm) and distal edge (LET, all = 7.6 keV/μm) of a spread-out Bragg peak (SOBP). 6 MV photons were used as reference. The acute damage and skin toxicity were scored daily until day 30, and the late damage was evaluated using a joint contracture assay for one year after treatment.

RESULTS

An acute damage RBE of 1.06 ± 0.02(1.02-1.10) and late damage RBE of 1.16 ± 0.08(1.00-1.32) were found, displaying an enhanced RBE for late damage in the center SOBP. The distal edge RBE for acute and late damage was 1.15 ± 0.02(1.10-1.19) and 1.26 ± 0.09(1.07-1.43), showing a similar center-to-distal edge RBE enhancement of 8 % and 9 % for acute and late damage.

CONCLUSION

The findings demonstrate an increased RBE for late damage than acute damage and the distal edge effect is evident with increased RBE at the distal end of the proton SOBP in vivo.

'Dirty dose'-based proton variable RBE models - performance assessment on in vitro data

BACKGROUND

In clinical proton radiotherapy, a constant relative biological effectiveness (RBE) of 1.1 is typically applied. Due to abundant evidence of variable RBE effects from in vitro data, multiple variable RBE models have been suggested, typically by describing the α $\alpha$ and β $\beta$ parameters in the linear quadratic (LQ) model as a function of dose averaged linear energy transfer (LET d $\text{LET}_d$ ).

PURPOSE

This work introduces a new variable RBE model based on the dirty dose concept, where dose deposited in voxels with a corresponding LET exceeding a specific threshold is considered "dirty" in the sense that it has a biological effect above the one predicted by a constant RBE of 1.1. As only one LET level, corresponding to a specific energy for a given particle in a given medium, needs to be monitored, this offers several advantages, such as simplified calculations by removing the need for intricate end of range LET calculations and averaging procedures, as well as opening up for more efficient experimental assessment of the cell specific model parameters.

METHODS

Previously published in vitro data were utilized, where surviving fraction (SF), dose andLET d $\text{LET}_d$ were reported for a pristine proton beam with varying physical PMMA thicknesses placed upstream of the cells. The setup was re-simulated to extract dirty dose metrics for the corresponding reportedLET d $\text{LET}_d$ -values. Models were created by setting the α $\alpha$ parameter of the LQ model as a function of the fraction of dirty dose and subsequently benchmarked against models based on other radiation quality metrics by comparing the root-mean-square-error (RMSE) of the predicted and actual cell surviving fraction.

RESULTS

Variable RBE models based on dirty dose perform on par with conventional radiation quality metrics with a RMSE of 0.38 for a dirty dose-based model with a threshold of 7keV / μ m $\mathrm{keV}/{\umu}\mathrm{m}$ , compared to 0.42 and 0.36 for aLET d $\text{LET}_d$ -based andQ eff , d $Q_{\mathrm{eff}, d}$ -based model, respectively. Higher chosen LET thresholds typically performed better and lower performed worse.

CONCLUSION

The results indicate that models based on dirty dose metrics perform equally well as conventional radiation quality metrics. Due to the simplified calculations involved and the potential for more efficient measurement techniques for data generation, dirty dose-based models might be the most conservative and practical approach for creating future proton variable RBE models.

Beyond a constant proton relative biological effectiveness: A survey of clinical and research perspectives among proton institutions in Europe and the United States

PURPOSE

Although proton relative biological effectiveness (RBE) depends on factors like linear energy transfer (LET), tissue properties, dose, and biological endpoint, a constant RBE of 1.1 is recommended in clinical practice. This study surveys proton institutions to explore activities using functionalities beyond a constant proton RBE.

METHODS

Research versions of RayStation integrate functionalities considering variable proton RBE, LET, proton track-ends, and dirty dose. A survey of 19 institutions in Europe and the United States, with these functionalities available, investigated clinical adoption and research prospects using a 25-question online questionnaire.

RESULTS

Of the 16 institutions that responded (84% response rate), 13 were clinically active. These clinical institutions prescribe RBE = 1.1 but also employ planning strategies centered around special beam arrangements to address potentially enhanced RBE effects in serially structured organs at risk (OARs). Clinical plan evaluation encompassed beam angles/spot position (69%), dose-averaged LET (LETd) (46%), and variable RBE distributions (38%). High ratings (discrete scale: 1-5) were reported for the research functionalities using linear LETd-RBE models, LETd, track-end frequency and dirty dose (averages: 4.0-4.8), while LQ-based phenomenological RBE models dependent on LETd scored lower for optimization (average: 2.2) but congruent for evaluation (average: 4.1). The institutions preferred LET reported as LETd (94%), computed in unit-density water (56%), for all protons (63%), and lean toward LETd-based phenomenological RBE models for clinical use (> 50%).

CONCLUSIONS

Proton institutions recognize RBE variability but adhere to a constant RBE while actively mitigating potential enhancements, particularly in serially structured OARs. Research efforts focus on planning techniques that utilize functionalities beyond a constant RBE, emphasizing standardized LET and RBE calculations to facilitate their adoption in clinical practice and improve clinical data collection. LETd calculated in unit-density water for all protons as input to adaptable phenomenological RBE models was the most suggested approach, aligning with predominant clinical LET and variable RBE reporting.

The LET enhancement of energy-specific collimation in pencil beam scanning proton therapy

PURPOSE

To computationally characterize the LET distribution during dynamic collimation in PBS and quantify its impact on the resultant dose distribution.

METHODS

Monte Carlo simulations using Geant4 were used to model the production of low-energy proton scatter produced in the collimating components of a novel PBS collimator. Custom spectral tallies were created to quantify the energy, track- and dose-averaged LET resulting from individual beamlet and composite fields simulated from a model of the IBA dedicated nozzle system. The composite dose distributions were optimized to achieve a uniform physical dose coverage of a cubical and pyramidal target, and the resulting dose-average LET distributions were calculated for uncollimated and collimated PBS deliveries and used to generate RBE-weighted dose distributions.

RESULTS

For collimated beamlets, the scattered proton energy fluence is strongly dependent on collimator position relative to the central axis of the beamlet. When delivering a uniform profile, the distribution of dose-average LET was nearly identical within the target and increased between 1 and2 keV / μ m $2 \,{\rm keV}/\mathrm{\umu }\mathrm{m}$ within 10 mm surrounding the target. Dynamic collimation resulted in larger dose-average LET changes: increasing the dose-average LET between 1 and3 keV / μ m $3 \,{\rm keV}/\mathrm{\umu }\mathrm{m}$ within 10 mm of a pyramidal target while reducing the dose-average LET outside this margin by as much as10 keV / μ m $10 \,{\rm keV}/\mathrm{\umu }\mathrm{m}$ . Biological dose distributions are improved with energy-specific collimation in reducing the lateral penumbra.

CONCLUSION

The presence of energy-specific collimation in PBS can lead to dose-average LET changes relative to an uncollimated delivery. In some clinical situations, the placement and application of energy-specific collimation may require additional planning considerations based on its reduction to the lateral penumbra and increase in high-dose conformity. Future applications may embody these unique dosimetric characteristics to redirect high-LET portions of a collimated proton beamlet from healthy tissues while enhancing the dose-average LET distribution within target.

Linear approximation of variable relative biological effectiveness models for proton therapy

The McNamara (MCN) and Wedenberg (WED) RBE weighted dose (DRBE), dose and dose-weighted average LET (LETd) were calculated in twenty brain cancer patients. A linear approximation was made for each RBE model to give best agreement to clinically relevant dosimetric parameters. Additional evaluations were done on twenty head and neck and twenty breast cancer patients.The R2 of the fits was ≥0.94 and ≥0.91 for MCN and WED respectively for α/β values ≥1.0 Gy. The graphs derived in this work can be used to convert RBE-LET slopes derived from clinical data to α/β values in the MCN or WED models.

Variable-RBE-induced NTCP predictions for various side-effects following proton therapy for brain tumors - Identification of high-risk patients and risk mitigation

BACKGROUND AND PURPOSE

Disregarding the increase of relative biological effectiveness (RBE) may raise the risk of acute and late adverse events after proton beam therapy (PBT). This study aims to explore the relationship between variable RBE (above 1.1)-induced normal tissue complication probabilities (NTCP) and patient-specific factors, identify patients at high risk of RBE-induced NTCP increase, and assess risk mitigation by incorporating RBE variability into treatment planning.

MATERIALS AND METHODS

We retrospectively analyzed 105 primary brain tumor patients treated with PBT (RBE = 1.1). We calculated differences in estimated NTCP (ΔNTCP) using a variable RBE-weighted dose (DRBE, Wedenberg model) and a constant RBE-weighted dose (DRBE=1.1), across 16 NTCP models. These differences were correlated with patient-specific characteristics. Based on ΔNTCP, patients were classified as high risk (32 %) or low risk (68 %) for adverse events due to RBE-induced NTCP. This classification was compared with alternative classifications based on (a) relevant patient-specific characteristics, (b) DRBE=1.1, and (c) the difference between DRBE and DRBE=1.1 (ΔD), assessing the balanced accuracy. The potential to reduce RBE-induced NTCP through track-end and linear energy transfer (LET) optimization was evaluated in six example patients.

RESULTS

Using a variable RBE instead of a constant one resulted in NTCP increases (up to 32 percentage points). Variable-RBE-induced NTCP increases were strongly negatively correlated with the distance between the clinical target volume (CTV) and the organ at risk (OAR) for most side-effects, and positively correlated with CTV volume for certain side-effects. High increases were associated with (a) specific patient factors, particularly the proximity of the CTV to OARs, (b) DRBE=1.1, and (c) ΔD, with a balanced accuracy of 0.88, 0.94, and 0.86, respectively. Optimization of track-ends and LET considerably reduced NTCP values, achieving a mean reduction of 31 % for optimized OARs.

CONCLUSION

The risk of variable-RBE-induced NTCP strongly depends on patient-specific factors and the considered side-effect. A small distance between the tumor and OARs notably increases the risk. Integrating biologically-guided objectives into treatment planning can effectively mitigate the risk.

Individualized evaluation of the total dose received by radiotherapy patients: Integrating in-field, out-of-field, and imaging doses

PURPOSE

To propose a methodology for integrating the out-of-field and imaging doses to the in-field dose received by radiotherapy (RT) patients. In addition, the impact of considering the total dose in planning and radiation-induced second malignancies (RISM) risk assessment will be evaluated in several scenarios comprising photon and proton treatments.

METHODS

The total dose is the voxel-wise sum of the doses from the different radiation sources (accounting for the radiobiological effectiveness) produced during the whole RT chain. The dose from the plan and imaging procedures were obtained by measurements for a photon prostate treatment and by calculation (combining treatment planning system, analytical models, and Monte Carlo simulations) for two lymphoma treatments, one using photons and the other, protons. Dose distributions, dose volume histograms (DVHs) metrics, mean organ doses, and RISM risks were evaluated for each radiation exposure in each treatment.

RESULTS

In general, the contribution of the imaging doses is low compared to the dose administered during RT treatment, being higher in proton therapy. However, for some organs, for instance testes in the prostate case, the imaging dose becomes higher than the scattered dose from the treatment fields. Plan evaluations revealed shifts in cumulative DVHs with the inclusion of out-of-field and imaging doses, though minimal clinical impact is expected. Risk assessment showed increased estimates with total dose.

CONCLUSIONS

The methodology enables accounting for the total dose for optimization of plans and imaging protocols, prospective risk predictions and retrospective epidemiological analyses.

NRG Oncology White Paper on the Relative Biological Effectiveness in Proton Therapy

This position paper, led by the NRG Oncology Particle Therapy Work Group, focuses on the concept of relative biologic effect (RBE) in clinical proton therapy (PT), with the goal of providing recommendations for the next-generation clinical trials with PT on the best practice of investigating and using RBE, which could deviate from the current standard proton RBE value of 1.1 relative to photons. In part 1, current clinical utilization and practice are reviewed, giving the context and history of RBE. Evidence for variation in RBE is presented along with the concept of linear energy transfer (LET). The intertwined nature of tumor radiobiology, normal tissue constraints, and treatment planning with LET and RBE considerations is then reviewed. Part 2 summarizes current and past clinical data and then suggests the next steps to explore and employ tools for improved dynamic models for RBE. In part 3, approaches and methods for the next generation of prospective clinical trials are explored, with the goal of optimizing RBE to be both more reflective of clinical reality and also deployable in trials to allow clinical validation and interpatient comparisons. These concepts provide the foundation for personalized biologic treatments reviewed in part 4. Finally, we conclude with a summary including short- and long-term scientific focus points for clinical PT. The practicalities and capacity to use RBE in treatment planning are reviewed and considered with more biological data in hand. The intermediate step of LET optimization is summarized and proposed as a potential bridge to the ultimate goal of case-specific RBE planning that can be achieved as a hypothesis-generating tool in near-term proton trials.

Recommendations for reporting and evaluating proton therapy beyond dose and constant relative biological effectiveness

Background and purpose

In proton therapy, a relative biological effectiveness (RBE) of 1.1 is used to convert proton dose into an equivalent photon dose. However, RBE varies with tissue type, fraction dose, and beam quality parameters beyond dose such as linear energy transfer (LET) raising concerns about increased local effectiveness and potential toxicity. This work aims to harmonize quantities used for clinical consideration of variable RBE for proton therapy.

Materials and methods

A survey was distributed to proton centres to determine agreement on RBE-related concerns and clinical implementations. A subsequent clinical expert meeting facilitated by the European Particle Therapy Network was held to achieve consensus and to make clinical recommendations how to prescribe and report beyond using dose and constant RBE.

Results

The survey was answered by 17 out of 23 centres contacted (74%). For proton RBE, most concerns existed regarding toxicity in serial organs, while the assumption of an RBE of 1.1 was considered valid for targets. Most physicists intended to consider a physical quantity beyond dose in clinical decision making.

Conclusions

A constant RBE of 1.1 was the consensus for prescribing dose. However, current practice of recording and reporting dose in proton therapy must be complemented: the recommended quantity beyond dose was the dose-averaged LET in water from primary and secondary protons, normalized to unit density. This will facilitate analyses of treatment data on effectiveness beyond dose and between centres. No consensus on a single variable RBE model was found. More clinical training on proton RBE is needed.

Evaluating and reporting LET and RBE-weighted dose in proton therapy for glioma - The Dutch approach

BACKGROUND AND PURPOSE

With proton therapy, the relative biological effectiveness (RBE) accounts for increased DNA damage caused by higher linear energy transfer (LET) compared to photons. However, the LET and hence the RBE varies along the proton range, particularly at the Bragg peak, introducing challenges in proton treatment planning for brain tumors. The aim of this paper is to standardize evaluating and reporting LET and RBE in proton therapy for patients with grade 2 and 3 IDH mutant gliomas among the Dutch proton therapy centers.

MATERIALS AND METHODS

A working group, comprising experts from three Dutch proton therapy centers, conducted nine meetings between 2020 and 2023. A joint literature review supported the standardized evaluation and reporting of LET and RBE. Questionnaires sent out to the three Dutch proton centers in 2020 and 2023 provided input for discussions on clinical practices. Three clinical examples were chosen to illustrate the application of the recommended methodology in treatment planning.

RESULTS

Following the literature review, a guideline on evaluation and reporting using the dose averaged LET (LETd) of primary and secondary protons calculated in water normalized to unit density was established. The McNamara variable RBE model with an α/β value of 2 Gy was selected for reporting.

CONCLUSION

The study presents a harmonization of approaches to evaluating and reporting LET and variable RBE in a guideline for the three Dutch proton therapy centers, providing clarity for future clinical interpretation. Having chosen a single variable RBE model offers practicality, although its accuracy remains a topic of ongoing research.

The first Korean carbon-ion radiation therapy facility: current status of the Heavy-ion Therapy Center at the Yonsei Cancer Center

PURPOSE

This report offers a detailed examination of the inception and current state of the Heavy-ion Therapy Center (HITC) at the Yonsei Cancer Center (YCC), setting it apart as the world's first center equipped with a fixed beam and two superconducting gantries for carbon-ion radiation therapy (CIRT).

MATERIALS AND METHODS

Preparations for CIRT at YCC began in 2013; accordingly, this center has completed a decade of meticulous planning and culminating since the operational commencement of the HITC in April 2023.

RESULTS

This report elaborates on the clinical preparation for adopting CIRT in Korea. It includes an extensive description of HITC's facility layout at YCC, which comprises the accelerator and treatment rooms. Furthermore, this report delineates the clinical workflow, criteria for CIRT application, and the rigorous quality assurance processes implemented at YCC. It highlights YCC's sophisticated radiation therapy infrastructure, collaborative initiatives, and the efficacious treatment of >200 prostate cancer cases utilizing CIRT.

CONCLUSION

This manuscript concludes by discussing the prospective influence of CIRT on the medical domain within Korea, spotlighting YCC's pioneering contribution and forecasting the widespread integration of this groundbreaking technology.

Derivation of a comprehensive semi-empirical proton RBE model from published experimental cell survival data collected in the PIDE database

We aimed to develop a comprehensive proton relative biological effectiveness (RBE) model based on accumulated cell survival data in the literature. Our approach includes four major components: (1) Eligible cell survival data with various linear energy transfers (LETs) in the Particle Irradiation Data Ensemble (PIDE) database (72 datasets in four cell lines); (2) a cell survival model based on Poisson equation, with α and β defined as the ability to generate and repair damage, respectively, to replace the classic linear-quadratic model for fitting the cell survival data; (3) hypothetical linear relations of α and β on LET, orα ( L E T ) α x = α α + b α   ∗   L E T andβ ( L E T ) β x = α β - b β   ∗   L E T ; and (4) a multi-curve fitting (MCF) approach to fit all cell survival data into the survival model and derive the aα , bα , aβ , and bβ values for each cell line. Dependences of these parameters on cell type were thus determined and finally a comprehensive RBE model was derived. MCF showed that (aα , bα , aβ , bβ ) = (1.09, 0.0010, 0.96, 0.033), (1.10, 0.0015, 1.03, 0.023), (1.12, 0.0025, 0.99, 0.0085), and (1.17, 0.0025, 0.99, 0.013) for the four cell lines, respectively. Thus, aα = 1.12 ± 0.04, bα = 0.0019 ± 0.0008, aβ = 0.99 ± 0.03, and bβ = 0.013 ∗ αx , and approximately α ∼ 1.12 ∗ α x and β = ( 0.99 - 0.013 ∗ α x ∗ L E T ) ∗ β x . Consequently, a relatively reliable and comprehensive RBE model with dependence on LET, αx , βx , and dose per fraction was finally derived for potential clinical application.

Characterizing devices for validation of dose, dose rate, and LET in ultra high dose rate proton irradiations

BACKGROUND

Ultra high dose rate (UHDR) radiotherapy using ridge filter is a new treatment modality known as conformal FLASH that, when optimized for dose, dose rate (DR), and linear energy transfer (LET), has the potential to reduce damage to healthy tissue without sacrificing tumor killing efficacy via the FLASH effect.

PURPOSE

Clinical implementation of conformal FLASH proton therapy has been limited by quality assurance (QA) challenges, which include direct measurement of UHDR and LET. Voxel DR distributions and LET spectra at planning target margins are paramount to the DR/LET-related sparing of organs at risk. We hereby present a methodology to achieve experimental validation of these parameters.

METHODS

Dose, DR, and LET were measured for a conformal FLASH treatment plan involving a 250-MeV proton beam and a 3D-printed ridge filter designed to uniformly irradiate a spherical target. We measured dose and DR simultaneously using a 4D multi-layer strip ionization chamber (MLSIC) under UHDR conditions. Additionally, we developed an "under-sample and recover (USRe)" technique for a high-resolution pixelated semiconductor detector, Timepix3, to avoid event pile-up and to correct measured LET at high-proton-flux locations without undesirable beam modifications. Confirmation of these measurements was done using a MatriXX PT detector and by Monte Carlo (MC) simulations.

RESULTS

MC conformal FLASH computed doses had gamma passing rates of >95% (3 mm/3% criteria) when compared to MatriXX PT and MLSIC data. At the lateral margin, DR showed average agreement values within 0.3% of simulation at 100 Gy/s and fluctuations ∼10% at 15 Gy/s. LET spectra in the proximal, lateral, and distal margins had Bhattacharyya distances of <1.3%.

CONCLUSION

Our measurements with the MLSIC and Timepix3 detectors shown that the DR distributions for UHDR scenarios and LET spectra using USRe are in agreement with simulations. These results demonstrate that the methodology presented here can be used effectively for the experimental validation and QA of FLASH treatment plans.

Interpreting the biological effects of protons as a function of physical quantity: linear energy transfer or microdosimetric lineal energy spectrum?

The choice of appropriate physical quantities to characterize the biological effects of ionizing radiation has evolved over time coupled with advances in scientific understanding. The basic hypothesis in radiation dosimetry is that the energy deposited by ionizing radiation initiates all the consequences of exposure in a biological sample (e.g., DNA damage, reproductive cell death). Physical quantities defined to characterize energy deposition have included dose, a measure of the mean energy imparted per unit mass of the target, and linear energy transfer (LET), a measure of the mean energy deposition per unit distance that charged particles traverse in a medium. The primary advantage of using the "dose and LET" physical system is its relative simplicity, especially for presenting and recording results. Inclusion of additional information such as the energy spectrum of charged particles renders this approach adequate to describe the biological effects of large dose levels from homogeneous sources. The primary disadvantage of this system is that it does not provide a unique description of the stochastic nature of radiation interactions. We and others have used dose-averaged LET (LETd) as a correlative physical quantity to the relative biological effectiveness (RBE) of proton beams. This approach is based on established experimental findings that proton RBE increases with LETd. However, this approach might not be applicable to intensity-modulated proton therapy or other applications in which the proton energy spectrum is highly heterogeneous. In the current study, we irradiated cancer cells with scanning proton beams with identical LETd (3.4 keV/µm) but arising from two different proton energy/LET spectra (a narrow spectrum in group 1 and a widespread heterogeneous spectrum in group 2). Clonogenic survival after irradiation revealed significant differences in RBE at any cell surviving fraction: e.g., at a surviving fraction of 0.1, the RBE was 0.97 ± 0.03 in group 1 and 1.16 ± 0.04 in group 2 (p≤0.01), validating our hypothesis that LETd alone may not adequately indicate proton RBE. Further analysis showed that microdosimetric spectrum (the probability density function of the stochastic physical quantity lineal energy y) was helpful for interpreting observed differences in biological effects. However, more accurate use of microdosimetric spectrum to quantify RBE requires a cell line-specific mechanistic model.

Real-time measurement of two-dimensional LET distributions of proton beams using scintillators

Objective.The linear energy transfer (LET) of proton therapy beams increases rapidly from the Bragg peak to the end of the beam. Although the LET can be determined using analytical or computational methods, a technique for efficiently measuring its spatial distribution has not yet been established. Thus, the purpose of this study is to develop a technique to measure the two-dimensional LET distribution in proton therapy in real time using a combination of multiple scintillators with different quenching.Approach.Inorganic and organic scintillator sheets were layered and irradiated with proton beams. Two-color signals of the CMOS sensor were obtained from the scintillation light and calibration curves were generated using LET. LET was calculated using Monte Carlo simulations asLETtandLETdweighted by fluence and dose, respectively. The accuracy of the calibration curve was evaluated by comparing the calculated and measured LET values for the 200 MeV monoenergetic and spread-out Bragg peak (SOBP) beams. LET distributions were obtained from the calibration curves.Main results.The deviation between the calculated and measured LET values was evaluated. For bothLETtandLETd, the deviation in the plateau region of the monoenergetic and SOBP beams tended to be larger than those in the peak region. The deviation was smaller forLETd. In the obtainedLETddistribution, the deviation between the calculated and measured values agreed within 3% in the peak region, while the deviation was larger in other regions.Significance.The LET distribution can be measured with a single irradiation using two scintillator sheets. This method may be effective for verifying LET in daily clinical practice and for quality control.

Particle arc therapy: Status and potential

There is a rising interest in developing and utilizing arc delivery techniques with charged particle beams, e.g., proton, carbon or other ions, for clinical implementation. In this work, perspectives from the European Society for Radiotherapy and Oncology (ESTRO) 2022 physics workshop on particle arc therapy are reported. This outlook provides an outline and prospective vision for the path forward to clinically deliverable proton, carbon, and other ion arc treatments. Through the collaboration among industry, academic, and clinical research and development, the scientific landscape and outlook for particle arc therapy are presented here to help our community understand the physics, radiobiology, and clinical principles. The work is presented in three main sections: (i) treatment planning, (ii) treatment delivery, and (iii) clinical outlook.

Variations in linear energy transfer distributions within a European proton therapy planning comparison of paediatric posterior fossa tumours

Background and Purpose

Radiotherapy for paediatric posterior fossa tumours may cause complications in the brainstem and upper spinal cord due to high doses. With proton therapy (PT) this risk may increase due to higher relative biological effectiveness (RBE) from elevated linear energy transfer (LET). This study assesses variations in LET in the brainstem and spinal cord in proton treatment plans from European centres.

Materials and Methods

Ten European PT centres using spot-scanning PT planned two paediatric posterior fossa cases: One overlapping partly with the brainstem and upper spinal cord, prescribed 54 Gy(RBE), and the second wrapping around these organs, prescribed 59.4 Gy(RBE). Dose-averaged LET distributions were assessed in volumes of the brainstem and spinal cord irradiated to over 50 Gy(RBE = 1.1). The maximum hinge angle effect on near-maximum RBE-weighted doses using the Unkelbach RBE model was also investigated.

Results

In the first case, the mean LET in brainstem volumes receiving more than 50 Gy(RBE = 1.1) ranged from 2.8 keV/µm to 3.6 keV/µm across centres (median: 3.3 keV/µm). In the second case, treatment plans showed a narrower range of mean LET in the brainstem, from 2.5 keV/µm to 2.8 keV/µm (median: 2.7 keV/µm). There was no statistically significant impact of the maximum hinge angle.

Conclusions

LET distributions vary across centres due to different techniques but are also influenced significantly by factors like shape and position of the target volume.

LET-based approximation of the microdosimetric kinetic model for proton radiotherapy

BACKGROUND

Phenomenological relative biological effectiveness (RBE) models for proton therapy, based on the dose-averaged linear energy transfer (LET), have been developed to address the apparent RBE increase towards the end of the proton range. The results of these phenomenological models substantially differ due to varying empirical assumptions and fitting functions. In contrast, more theory-based approaches are used in carbon ion radiotherapy, such as the microdosimetric kinetic model (MKM). However, implementing microdosimetry-based models in LET-based proton therapy treatment planning systems poses challenges.

PURPOSE

This work presents a LET-based version of the MKM that is practical for clinical use in proton radiotherapy.

METHODS

At first, we derived an approximation of the Mayo Clinic Florida (MCF) MKM for relatively-sparsely ionizing radiation such as protons. The mathematical formalism of the proposed model is equivalent to the original MKM, but it maintains some key features of the MCF MKM, such as the determination of model parameters from measurable cell characteristics. Subsequently, we carried out Monte Carlo calculations with PHITS in different simulated scenarios to establish a heuristic correlation between microdosimetric quantities and the dose averaged LET of protons.

RESULTS

A simple allometric function was found able to describe the relationship between the dose-averaged LET of protons and the dose-mean lineal energy, which includes the contributions of secondary particles. The LET-based MKM was used to model the in vitro clonogenic survival RBE of five human and rodent cell lines (A549, AG01522, CHO, T98G, and U87) exposed to pristine and spread-out Bragg peak (SOBP) proton beams. The results of the LET-based MKM agree well with the biological data in a comparable or better way with respect to the other models included in the study. A sensitivity analysis on the model results was also performed.

CONCLUSIONS

The LET-based MKM integrates the predictive theoretical framework of the MCF MKM with a straightforward mathematical description of the RBE based on the dose-averaged LET, a physical quantity readily available in modern treatment planning systems for proton therapy.

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.

Clinical practice in European centres treating paediatric posterior fossa tumours with pencil beam scanning proton therapy

BACKGROUND AND PURPOSE

As no guidelines for pencil beam scanning (PBS) proton therapy (PT) of paediatric posterior fossa (PF) tumours exist to date, this study investigated planning techniques across European PT centres, with special considerations for brainstem and spinal cord sparing.

MATERIALS AND METHODS

A survey and a treatment planning comparison were initiated across nineteen European PBS-PT centres treating paediatric patients. The survey assessed all aspects of the treatment chain, including but not limited to delineations, dose constraints and treatment planning. Each centre planned two PF tumour cases for focal irradiation, according to their own clinical practice but based on common delineations. The prescription dose was 54 Gy(RBE) for Case 1 and 59.4 Gy(RBE) for Case 2. For both cases, planning strategies and relevant dose metrics were compared.

RESULTS

Seventeen (89 %) centres answered the survey, and sixteen (80 %) participated in the treatment planning comparison. In the survey, thirteen (68 %) centres reported using the European Particle Therapy Network definition for brainstem delineation. In the treatment planning study, while most centres used three beam directions, their configurations varied widely across centres. Large variations were also seen in brainstem doses, with a brainstem near maximum dose (D2%) ranging from 52.7 Gy(RBE) to 55.7 Gy(RBE) (Case 1), and from 56.8 Gy(RBE) to 60.9 Gy(RBE) (Case 2).

CONCLUSION

This study assessed the European PBS-PT planning of paediatric PF tumours. Agreement was achieved in e.g. delineation-practice, while wider variations were observed in planning approach and consequently dose to organs at risk. Collaboration between centres is still ongoing, striving towards common guidelines.

Linear Energy Transfer Measurements and Estimation of Relative Biological Effectiveness in Proton and Helium Ion Beams Using Fluorescent Nuclear Track Detectors

PURPOSE

Our objective was to develop a methodology for assessing the linear energy transfer (LET) and relative biological effectiveness (RBE) in clinical proton and helium ion beams using fluorescent nuclear track detectors (FNTDs).

METHODS AND MATERIALS

FNTDs were exposed behind solid water to proton and helium (4He) ion spread-out Bragg peaks. Detectors were imaged with a confocal microscope, and the LET spectra were derived from the fluorescence intensity. The track- and dose-averaged LET (LETF and LETD, respectively) were calculated from the LET spectra. LET measurements were used as input on RBE models to estimate the RBE. Human alveolar adenocarcinoma cells (A549) were exposed at the same positions as the FNTDs. The RBE was calculated from the resulting survival curves. All measurements were compared with Monte Carlo simulations.

RESULTS

For protons, average relative differences between measurements and simulations were 6% and 19% for LETF and LETD, respectively. For helium ions, the same differences were 11% for both quantities. The position of the experimental LET spectra primary peaks agreed with the simulations within 9% and 14% for protons and helium ions, respectively. For the RBE models using LETD as input, FNTD-based RBE values ranged from 1.02 ± 0.01 to 1.25 ± 0.04 and from 1.08 ± 0.09 to 2.68 ± 1.26 for protons and helium ions, respectively. The average relative differences between these values and simulations were 2% and 4%. For A549 cells, the RBE ranged from 1.05 ± 0.07 to 1.47 ± 0.09 and from 0.89 ± 0.06 to 3.28 ± 0.20 for protons and helium ions, respectively. Regarding the RBE-weighted dose (2.0 Gy at the spread-out Bragg peak), the differences between simulations and measurements were below 0.10 Gy.

CONCLUSIONS

This study demonstrates for the first time that FNTDs can be used to perform direct LET measurements and to estimate the RBE in clinical proton and helium ion beams.

ACR-ARS Practice Parameter for the Performance of Proton Beam Therapy

Purpose

This practice parameter for the performance of proton beam radiation therapy was revised collaboratively by the American College of Radiology (ACR) and the American Radium Society (ARS). This practice parameter was developed to serve as a tool in the appropriate application of proton therapy in the care of cancer patients or other patients with conditions in which radiation therapy is indicated. It addresses clinical implementation of proton radiation therapy, including personnel qualifications, quality assurance (QA) standards, indications, and suggested documentation.

Materials and Methods

This practice parameter for the performance of proton beam radiation therapy was developed according to the process described under the heading The Process for Developing ACR Practice Parameters and Technical Standards on the ACR website (https://www.acr.org/Clinical-Resources/Practice-Parameters-and-Technical-Standards) by the Committee on Practice Parameters - Radiation Oncology of the ACR Commission on Radiation Oncology in collaboration with the ARS.

Results

The qualifications and responsibilities of personnel, such as the proton center Chief Medical Officer or Medical Director, Radiation Oncologist, Radiation Physicist, Dosimetrist and Therapist, are outlined, including the necessity for continuing medical education. Proton therapy standard clinical indications and methodologies of treatment management are outlined by disease site and treatment group (e.g. pediatrics) including documentation and the process of proton therapy workflow and equipment specifications. Additionally, this proton therapy practice parameter updates policies and procedures related to a quality assurance and performance improvement program (QAPI), patient education, infection control, and safety.

Conclusion

As proton therapy becomes more accessible to cancer patients, policies and procedures as outlined in this practice parameter will help ensure quality and safety programs are effectively implemented to optimize clinical care.

Reducing Radiation Dermatitis for PBS Proton Therapy Breast Cancer Patients Using SpotDelete

Purpose

The purpose of this work was to reduce the severity of radiation dermatitis for breast cancer patients receiving pencil beam scanning proton therapy. The hypothesis was that eliminating proton spots (SpotDelete) in the 0.5 cm skin rind would reduce the potentially higher relative biological effectiveness (RBE) known to occur at the Bragg Peak.

Patients and Methods

Our center has been using an in-house developed Python script in RayStation since 2021 to remove spots from the skin rind of breast patients. In this work, we retrospectively reviewed the on-treatment visit data from a cohort of breast patients treated with hypofractionation (16 fractions) before this technique (MinDepth) and after (SpotDelete) to acquire the physician-reported radiation dermatitis scores. We evaluated the delivered treatment plans, calculating the linear energy transfer (LET) and applying 3 variable RBE models, Carabe-Fernandez, Wedenberg, and McNamara. An α/β of 10 was assumed for the skin.

Results

In the MinDepth cohort (n = 28), grade 1, 2, and 3 dermatitis accounted for 57%, 36%, and 7% of the cases, respectively. For SpotDelete (n = 27), the incidence rate of grade 1 and 2 acute radiation dermatitis was 67% and 37%, respectively. There were 0 instances of grade 3 dermatitis observed in the SpotDelete cohort. The onset of radiation dermatitis in the SpotDelete cohort was delayed compared to MinDepth, occurring 1 week later in the course of treatment. There was no significant difference in LET or in any of the variable RBE models when analyzing the 0.5 cm skin rind between the cohorts.

Conclusion

Despite the lack of correlation in LET or RBE, SpotDelete has been shown to reduce the severity and onset of radiation dermatitis. Possibly, more research into the α/β for skin and RBE models based on skin cell lines could provide insight into the efficacy of the SpotDelete technique.

A deep-learning-based surrogate model for Monte-Carlo simulations of the linear energy transfer in primary brain tumor patients treated with proton-beam radiotherapy

Objective.This study explores the use of neural networks (NNs) as surrogate models for Monte-Carlo (MC) simulations in predicting the dose-averaged linear energy transfer (LETd) of protons in proton-beam therapy based on the planned dose distribution and patient anatomy in the form of computed tomography (CT) images. As LETdis associated with variability in the relative biological effectiveness (RBE) of protons, we also evaluate the implications of using NN predictions for normal tissue complication probability (NTCP) models within a variable-RBE context.Approach.The predictive performance of three-dimensional NN architectures was evaluated using five-fold cross-validation on a cohort of brain tumor patients (n= 151). The best-performing model was identified and externally validated on patients from a different center (n= 107). LETdpredictions were compared to MC-simulated results in clinically relevant regions of interest. We assessed the impact on NTCP models by leveraging LETdpredictions to derive RBE-weighted doses, using the Wedenberg RBE model.Main results.We found NNs based solely on the planned dose distribution, i.e. without additional usage of CT images, can approximate MC-based LETddistributions. Root mean squared errors (RMSE) for the median LETdwithin the brain, brainstem, CTV, chiasm, lacrimal glands (ipsilateral/contralateral) and optic nerves (ipsilateral/contralateral) were 0.36, 0.87, 0.31, 0.73, 0.68, 1.04, 0.69 and 1.24 keV µm-1, respectively. Although model predictions showed statistically significant differences from MC outputs, these did not result in substantial changes in NTCP predictions, with RMSEs of at most 3.2 percentage points.Significance.The ability of NNs to predict LETdbased solely on planned dose distributions suggests a viable alternative to compute-intensive MC simulations in a variable-RBE setting. This is particularly useful in scenarios where MC simulation data are unavailable, facilitating resource-constrained proton therapy treatment planning, retrospective patient data analysis and further investigations on the variability of proton RBE.

In vitro validation of helium ion irradiations as a function of linear energy transfer in radioresistant human malignant cells

PURPOSE

Based on considerable interest to enlarge the experimental database of radioresistant cells after their irradiation with helium ions, HTB140, MCF-7 and HTB177 human malignant cells are exposed to helium ion beams having different linear energy transfer (LET).

MATERIALS AND METHODS

The cells are irradiated along the widened 62 MeV/u helium ion Bragg peak, providing LET of 4.9, 9.8, 23.4 and 36.8 keV/µm. Numerical simulations with the Geant4 toolkit are used for the experimental design. Cell survival is evaluated and compared with reference γ-rays. DNA double strand breaks are assessed via γ-H2AX foci.

RESULTS

With the increase of LET, surviving fractions at 2 Gy decrease, while RBE (2 Gy, γ) gradually increase. For HTB140 cells, above the dose of 4 Gy, a slight saturation of survival is observed while the increase of RBE (2 Gy, γ) remains unaffected. With the increase of LET the increase of γ-H2AX foci is revealed at 0.5 h after irradiation. There is no significant difference in the number of foci between the cell lines for the same LET. From 0.5 to 24 h, the number of foci drops reaching its residual level. For each time point, there are small differences in DNA DSB among the three cell lines.

CONCLUSION

Analyses of data acquired for the three cell lines irradiated by helium ions, having different LET, reveal high elimination capacity and creation of a large number of DNA DSB with respect to γ-rays, and are between those reported for protons and carbon ions.

Enhancing adaptive proton therapy through CBCT images: Synthetic head and neck CT generation based on 3D vision transformers

BACKGROUND

Proton therapy is a form of radiotherapy commonly used to treat various cancers. Due to its high conformality, minor variations in patient anatomy can lead to significant alterations in dose distribution, making adaptation crucial. While cone-beam computed tomography (CBCT) is a well-established technique for adaptive radiation therapy (ART), it cannot be directly used for adaptive proton therapy (APT) treatments because the stopping power ratio (SPR) cannot be estimated from CBCT images.

PURPOSE

To address this limitation, Deep Learning methods have been suggested for converting pseudo-CT (pCT) images from CBCT images. In spite of convolutional neural networks (CNNs) have shown consistent improvement in pCT literature, there is still a need for further enhancements to make them suitable for clinical applications.

METHODS

The authors introduce the 3D vision transformer (ViT) block, studying its performance at various stages of the proposed architectures. Additionally, they conduct a retrospective analysis of a dataset that includes 259 image pairs from 59 patients who underwent treatment for head and neck cancer. The dataset is partitioned into 80% for training, 10% for validation, and 10% for testing purposes.

RESULTS

The SPR maps obtained from the pCT using the proposed method present an absolute relative error of less than 5% from those computed from the planning CT, thus improving the results of CBCT.

CONCLUSIONS

We introduce an enhanced ViT3D architecture for pCT image generation from CBCT images, reducing SPR error within clinical margins for APT workflows. The new method minimizes bias compared to CT-based SPR estimation and dose calculation, signaling a promising direction for future research in this field. However, further research is needed to assess the robustness and generalizability across different medical imaging applications.

A Novel Inverse Algorithm To Solve the Integrated Optimization of Dose, Dose Rate, and Linear Energy Transfer of Proton FLASH Therapy With Sparse Filters

PURPOSE

The recently proposed Integrated Physical Optimization Intensity Modulated Proton Therapy (IPO-IMPT) framework allows simultaneous optimization of dose, dose rate, and linear energy transfer (LET) for ultra-high dose rate (FLASH) treatment planning. Finding solutions to IPO-IMPT is difficult because of computational intensiveness. Nevertheless, an inverse solution that simultaneously specifies the geometry of a sparse filter and weights of a proton intensity map is desirable for both clinical and preclinical applications. Such solutions can reduce effective biologic dose to organs at risk in patients with cancer as well as reduce the number of animal irradiations needed to derive extra biologic dose models in preclinical studies.

METHODS AND MATERIALS

Unlike the initial forward heuristic, this inverse IPO-IMPT solution includes simultaneous optimization of sparse range compensation, sparse range modulation, and spot intensity. The daunting computational tasks vital to this endeavor were resolved iteratively with a distributed computing framework to enable Simultaneous Intensity and Energy Modulation and Compensation (SIEMAC). SIEMAC was demonstrated on a human patient with central lung cancer and a minipig.

RESULTS

SIEMAC simultaneously improves maps of spot intensities and patient-field-specific sparse range compensators and range modulators. For the patient with lung cancer, at our maximum nozzle current of 300 nA, dose rate coverage above 100 Gy/s increased from 57% to 96% in the lung and from 93% to 100% in the heart, and LET coverage above 4 keV/µm dropped from 68% to 9% in the lung and from 26% to <1% in the heart. For a simple minipig plan, the full-width half-maximum of the dose, dose rate, and LET distributions decreased by 30%, 1.6%, and 57%, respectively, again with similar target dose coverage, thus reducing uncertainty in these quantities for preclinical studies.

CONCLUSIONS

The inverse solution to IPO-IMPT demonstrated the capability to simultaneously modulate subspot proton energy and intensity distributions for clinical and preclinical studies.

Characterizing Proton-Induced Biological Effects in a Mouse Spinal Cord Model: A Comparison of Bragg Peak and Entrance Beam Response in Single and Fractionated Exposures

PURPOSE

Proton relative biological effectiveness (RBE) is a dynamic variable influenced by factors like linear energy transfer (LET), dose, tissue type, and biological endpoint. The standard fixed proton RBE of 1.1, currently used in clinical planning, may not accurately represent the true biological effects of proton therapy (PT) in all cases. This uncertainty can contribute to radiation-induced normal tissue toxicity in patients. In late-responding tissues such as the spinal cord, toxicity can cause devastating complications. This study investigated spinal cord tolerance in mice subjected to proton irradiation and characterized the influence of fractionation on proton- induced myelopathy at entrance (ENT) and Bragg peak (BP) positions.

METHODS AND MATERIALS

Cervical spinal cords of 8-week-old C57BL/6J female mice were irradiated with single- or multi-fractions (18x) using lateral opposed radiation fields at 1 of 2 positions along the Bragg curve: ENT (dose-mean LET = 1.2 keV/μm) and BP (LET = 6.9 keV/μm). Mice were monitored over 1 year for changes in weight, mobility, and general health, with radiation-induced myelopathy as the primary biological endpoint. Calculations of the RBE of the ENT and BP curve (RBEENT/BP) were performed.

RESULTS

Single-fraction RBEENT/BP for 50% effect probability (tolerance dose (TD50), grade II paresis, determined using log-logistic model fitting) was 1.10 ± 0.06 (95% CI) and for multifraction treatments it was 1.19 ± 0.05 (95% CI). Higher incidence and faster onset of paralysis were seen in mice treated at the BP compared with ENT.

CONCLUSIONS

The findings challenge the universally fixed RBE value in PT, indicating up to a 25% mouse spinal cord RBEENT/BP variation for multifraction treatments. These results highlight the importance of considering fractionation in determining RBE for PT. Robust characterization of proton-induced toxicity, aided by in vivo models, is paramount for refining clinical decision-making and mitigating potential patient side effects.

Quantifying the Dosimetric Impact of Proton Range Uncertainties on RBE-Weighted Dose Distributions in Intensity-Modulated Proton Therapy for Bilateral Head and Neck Cancer

BACKGROUND

In current clinical practice, intensity-modulated proton therapy (IMPT) head and neck cancer (HNC) plans are generated using a constant relative biological effectiveness (cRBE) of 1.1. The primary goal of this study was to explore the dosimetric impact of proton range uncertainties on RBE-weighted dose (RWD) distributions using a variable RBE (vRBE) model in the context of bilateral HNC IMPT plans.

METHODS

The current study included the computed tomography (CT) datasets of ten bilateral HNC patients who had undergone photon therapy. Each patient's plan was generated using three IMPT beams to deliver doses to the CTV_High and CTV_Low for doses of 70 Gy(RBE) and 54 Gy(RBE), respectively, in 35 fractions through a simultaneous integrated boost (SIB) technique. Each nominal plan calculated with a cRBE of 1.1 was subjected to the range uncertainties of ±3%. The McNamara vRBE model was used for RWD calculations. For each patient, the differences in dosimetric metrices between the RWD and nominal dose distributions were compared.

RESULTS

The constrictor muscles, oral cavity, parotids, larynx, thyroid, and esophagus showed average differences in mean dose (Dmean) values up to 6.91 Gy(RBE), indicating the impact of proton range uncertainties on RWD distributions. Similarly, the brachial plexus, brain, brainstem, spinal cord, and mandible showed varying degrees of the average differences in maximum dose (Dmax) values (2.78-10.75 Gy(RBE)). The Dmean and Dmax to the CTV from RWD distributions were within ±2% of the dosimetric results in nominal plans.

CONCLUSION

The consistent trend of higher mean and maximum doses to the OARs with the McNamara vRBE model compared to cRBE model highlighted the need for consideration of proton range uncertainties while evaluating OAR doses in bilateral HNC IMPT plans.

Assessment of fluence- and dose-averaged linear energy transfer with passive luminescence detectors in clinical proton beams

Objective.This work investigates the use of passive luminescence detectors to determine different types of averaged linear energy transfer (LET-) for the energies relevant to proton therapy. The experimental results are compared to reference values obtained from Monte Carlo simulations.Approach.Optically stimulated luminescence detectors (OSLDs), fluorescent nuclear track detectors (FNTDs), and two different groups of thermoluminescence detectors (TLDs) were irradiated at four different radiation qualities. For each irradiation, the fluence- (LET-f) and dose-averaged LET (LET-d) were determined. For both quantities, two sub-types of averages were calculated, either considering the contributions from primary and secondary protons or from all protons and heavier, charged particles. Both simulated and experimental data were used in combination with a phenomenological model to estimate the relative biological effectiveness (RBE).Main results.All types ofLET-could be assessed with the luminescence detectors. The experimental determination ofLET-fis in agreement with reference data obtained from simulations across all measurement techniques and types of averaging. On the other hand,LET-dcan present challenges as a radiation quality metric to describe the detector response in mixed particle fields. However, excluding secondaries heavier than protons from theLET-dcalculation, as their contribution to the luminescence is suppressed by ionization quenching, leads to equal accuracy betweenLET-fandLET-d. Assessment of RBE through the experimentally determinedLET-dvalues agrees with independently acquired reference values, indicating that the investigated detectors can determineLET-with sufficient accuracy for proton therapy.Significance.OSLDs, TLDs, and FNTDs can be used to determineLET-and RBE in proton therapy. With the capability to determine dose through ionization quenching corrections derived fromLET-, OSLDs and TLDs can simultaneously ascertain dose,LET-, and RBE. This makes passive detectors appealing for measurements in phantoms to facilitate validation of clinical treatment plans or experiments related to proton therapy.

Improving Pediatric Normal Tissue Radiation Dose-Response Modeling in Children With Cancer: A PENTEC Initiative

The development of normal tissue radiation dose-response models for children with cancer has been challenged by many factors, including small sample sizes; the long length of follow-up needed to observe some toxicities; the continuing occurrence of events beyond the time of assessment; the often complex relationship between age at treatment, normal tissue developmental dynamics, and age at assessment; and the need to use retrospective dosimetry. Meta-analyses of published pediatric outcome studies face additional obstacles of incomplete reporting of critical dosimetric, clinical, and statistical information. This report describes general methods used to address some of the pediatric modeling issues. It highlights previous single- and multi-institutional pediatric dose-response studies and summarizes how each PENTEC taskforce addressed the challenges and limitations of the reviewed publications in constructing, when possible, organ-specific dose-effect models.

Radiation Therapy Technology Advances and Mitigation of Subsequent Neoplasms in Childhood Cancer Survivors

PURPOSE

In this Pediatric Normal Tissue Effects in the Clinic (PENTEC) vision paper, challenges and opportunities in the assessment of subsequent neoplasms (SNs) from radiation therapy (RT) are presented and discussed in the context of technology advancement.

METHODS AND MATERIALS

The paper discusses the current knowledge of SN risks associated with historic, contemporary, and future RT technologies. Opportunities for research and SN mitigation strategies in pediatric patients with cancer are reviewed.

RESULTS

Present experience with radiation carcinogenesis is from populations exposed during widely different scenarios. Knowledge gaps exist within clinical cohorts and follow-up; dose-response and volume effects; dose-rate and fractionation effects; radiation quality and proton/particle therapy; age considerations; susceptibility of specific tissues; and risks related to genetic predisposition. The biological mechanisms associated with local and patient-level risks are largely unknown.

CONCLUSIONS

Future cancer care is expected to involve several available RT technologies, necessitating evidence and strategies to assess the performance of competing treatments. It is essential to maximize the utilization of existing follow-up while planning for prospective data collection, including standardized registration of individual treatment information with linkage across patient databases.

Current Status and Future Directions of Proton Therapy for Head and Neck Carcinoma

The growing interest in proton therapy (PT) in recent decades is justified by the evidence that protons dose distribution allows maximal dose release at the tumor depth followed by sharp distal dose fall-off. But, in the holistic management of head and neck cancer (HNC), limiting the potential of PT to a mere dosimetric advantage appears reductive. Indeed, the precise targeting of PT may help evaluate the effectiveness of de-escalation strategies, especially for patients with human papillomavirus associated-oropharyngeal cancer (OPC) and nasopharyngeal cancer (NPC). Furthermore, PT could have potentially greater immunogenic effects than conventional photon therapy, possibly enhancing both the radiotherapy (RT) capability to activate anti-tumor immune response and the effectiveness of immunotherapy drugs. Based on these premises, the aim of the present paper is to conduct a narrative review reporting the safety and efficacy of PT compared to photon RT focusing on NPC and OPC. We also provide a snapshot of ongoing clinical trials comparing PT with photon RT for these two clinical scenarios. Finally, we discuss new insights that may further develop clinical research on PT for HNC.

Helium Ion Therapy for Advanced Juvenile Nasopharyngeal Angiofibroma

Helium ion therapy (HRT) is a promising modality for the treatment of pediatric tumors and those located close to critical structures due to the favorable biophysical properties of helium ions. This in silico study aimed to explore the potential benefits of HRT in advanced juvenile nasopharyngeal angiofibroma (JNA) compared to proton therapy (PRT). We assessed 11 consecutive patients previously treated with PRT for JNA in a definitive or postoperative setting with a relative biological effectiveness (RBE) weighted dose of 45 Gy (RBE) in 25 fractions at the Heidelberg Ion-Beam Therapy Center. HRT plans were designed retrospectively for dosimetric comparisons and risk assessments of radiation-induced complications. HRT led to enhanced target coverage in all patients, along with sparing of critical organs at risk, including a reduction in the brain integral dose by approximately 27%. In terms of estimated risks of radiation-induced complications, HRT led to a reduction in ocular toxicity, cataract development, xerostomia, tinnitus, alopecia and delayed recall. Similarly, HRT led to reduced estimated risks of radiation-induced secondary neoplasms, with a mean excess absolute risk reduction of approximately 30% for secondary CNS malignancies. HRT is a promising modality for advanced JNA, with the potential for enhanced sparing of healthy tissue and thus reduced radiation-induced acute and long-term complications.

Impact of Relative Biologic Effectiveness for Proton Therapy for Head and Neck and Skull-Base Tumors: A Technical and Clinical Review

Proton therapy has emerged as a crucial tool in the treatment of head and neck and skull-base cancers, offering advantages over photon therapy in terms of decreasing integral dose and reducing acute and late toxicities, such as dysgeusia, feeding tube dependence, xerostomia, secondary malignancies, and neurocognitive dysfunction. Despite its benefits in dose distribution and biological effectiveness, the application of proton therapy is challenged by uncertainties in its relative biological effectiveness (RBE). Overcoming the challenges related to RBE is key to fully realizing proton therapy's potential, which extends beyond its physical dosimetric properties when compared with photon-based therapies. In this paper, we discuss the clinical significance of RBE within treatment volumes and adjacent serial organs at risk in the management of head and neck and skull-base tumors. We review proton RBE uncertainties and its modeling and explore clinical outcomes. Additionally, we highlight technological advancements and innovations in plan optimization and treatment delivery, including linear energy transfer/RBE optimizations and the development of spot-scanning proton arc therapy. These advancements show promise in harnessing the full capabilities of proton therapy from an academic standpoint, further technological innovations and clinical outcome studies, however, are needed for their integration into routine clinical practice.

A review and bibliometric analysis of global research on proton radiotherapy

Proton beam therapy (PBT) has great advantages as tumor radiotherapy and is progressively becoming a more prevalent choice for individuals undergoing radiation therapy. The objective of this review is to pinpoint collaborative efforts among countries and institutions, while also exploring the hot topics and future outlook in the field of PBT. Data from publications were downloaded from the Web of Science Core Collection. CiteSpace and Excel 2016 were used to conduct the bibliometric and knowledge map analysis. A total of 6516 publications were identified, with the total number of articles steadily increasing and the United States being the most productive country. Harvard University took the lead in contributing the highest number of publications. Paganetti Harald published the most articles and had the most cocitations. PHYS MED BIOL published the greatest number of PBT-related articles, while INT J RADIAT ONCOL received the most citations. Paganetti Harald, 2012, PHYS MED BIOL can be classified as classic literature due to its high citation rate. We believe that research on technology development, dose calculation and relative biological effectiveness were the knowledge bases in this field. Future research hotspots may include clinical trials, flash radiotherapy, and immunotherapy.

Efficient computational modeling of electronic stopping power of organic polymers for proton therapy optimization

This comprehensive study delves into the intricate interplay between protons and organic polymers, offering insights into proton therapy in cancer treatment. Focusing on the influence of the spatial electron density distribution on stopping power estimates, we employed real-time time-dependent density functional theory coupled with the Penn method. Surprisingly, the assumption of electron density homogeneity in polymers is fundamentally flawed, resulting in an overestimation of stopping power values at energies below 2 MeV. Moreover, the Bragg rule application in specific compounds exhibited significant deviations from experimental data around the stopping maximum, challenging established norms.