Nuclear interactions in proton therapy: dose and relative biological effect distributions originating from primary and secondary particles

A high-throughput focused collimator for OAR-sparing preclinical proton FLASH studies: commissioning and validation

Objective. To fabricate and validate a novel focused collimator designed to spare normal tissue in a murine hemithoracic irradiation model using 250 MeV protons delivered at ultra-high dose rates (UHDRs) for preclinical FLASH radiation therapy (FLASH-RT) studies.Approach. A brass collimator was developed to shape 250 MeV UHDR protons from our Varian ProBeam. Six 13 mm apertures, of equivalent size to kV x-ray fields historically used to perform hemithorax irradiations, were precisely machined to match beam divergence, allowing concurrent hemithoracic irradiation of six mice while sparing the contralateral lung and abdominal organs. The collimated field profiles were characterized by film dosimetry, and a radiation survey of neutron activation was performed to ensure the safety of staff positioning animals.Main results. The brass collimator produced 1.2 mm penumbrae radiation fields comparable to kV x-rays used in preclinical studies. The penumbrae in the six apertures are similar, with full-width half-maxima of 13.3 mm and 13.5 mm for the central and peripheral apertures, respectively. The collimator delivered a similar dose at an average rate of 52 Gy s-1for all apertures. While neutron activation produces a high (0.2 mSv h-1) initial ambient equivalent dose rate, a parallel work-flow in which imaging and setup are performed without the collimator ensures safety to staff.Significance. Scanned protons have the greatest potential for future translation of FLASH-RT in clinical treatments due to their ability to treat deep-seated tumors with high conformality. However, the Gaussian distribution of dose in proton spots produces wider lateral penumbrae compared to other modalities. This presents a challenge in small animal pre-clinical studies, where millimeter-scale penumbrae are required to precisely target the intended volume. Offering high-throughput irradiation of mice with sharp penumbrae, our novel collimator-based platform serves as an important benchmark for enabling large-scale, cost-effective radiobiological studies of the FLASH effect in murine models.

LETd Optimization Verification With an SOI Microdosimeter

PURPOSE

A first of its kind experimental verification of dose-averaged linear energy transfer (LETd) optimized treatment plans for proton therapy has been carried out using a silicon-on-insulator microdosimeter at the Massachusetts General Hospital (MGH), Boston, USA.

METHODS AND MATERIALS

Three clinical treatment plans of a typical ependymoma structure set were designed using the standard clinical approach, the proposed protocol approach, and a one-field approach. The plans were then reoptimized to reduce the LETd-weighted dose in the brain stem. All six plans were delivered in a solid water phantom and the experimental yD‾ measured.

RESULTS

After LETd optimization, a reduction in yD‾ was found within the brain stem by an average of 12%, 19%, and 4% for the clinical, protocol, and one-field plans, respectively, while maintaining adequate coverage of the tumor structure. The experimental LETd-weighted doses were in agreement with the treatment planning system calculations and Monte Carlo simulations and reinforced the improvement of the optimization.

CONCLUSIONS

This work demonstrates the first experimental verification of the clinical implementation of LETd optimization for patient treatment with proton therapy.

Calculation of biological effectiveness of SOBP proton beams: a TOPAS Monte Carlo study

Objective.This study aims to investigate the biological effectiveness of Spread-Out Bragg-Peak (SOBP) proton beams with initial kinetic energies 50-250 MeV at different depths in water using TOPAS Monte Carlo code.Approach.The study modelled SOBP proton beams using TOPAS time feature. Various LET-based models and Repair-Misrepair-Fixation model were employed to calculate Relative Biological Effectiveness (RBE) for V79 cell lines at different on-axis depths based on TOPAS. Microdosimetric Kinetic Model and biological weighting function-based models, which utilize microdosimetric distributions, were also used to estimate the RBE. A phase-space-based method was adopted for calculating microdosimetric distributions.Main results.The trend of variation of RBE with depth is similar in all the RBE models, but the absolute RBE values vary based on the calculation models. RBE sharply increases at the distal edge of SOBP proton beams. In the entrance region of all the proton beams, RBE values at 4 Gy i.e. RBE(4 Gy) resulting from different models are in the range of 1.04-1.07, comparable to clinically used generic RBE of 1.1. Moving from the proximal to distal end of the SOBP, RBE(4 Gy) is in the range of 1.15-1.33, 1.13-1.21, 1.11-1.17, 1.13-1.18 and 1.17-1.21, respectively for 50, 100, 150, 200 and 250 MeV SOBP beams, whereas at the distal dose fall-off region, these values are 1.68, 1.53, 1.44, 1.42 and 1.40, respectively.Significance.The study emphasises application of depth-, dose- and energy- dependent RBE values in clinical application of proton beams.

Modelling small block aperture in an in-house developed GPU-accelerated Monte Carlo-based dose engine for pencil beam scanning proton therapy

Purpose. To enhance an in-house graphic-processing-unit accelerated virtual particle (VP)-based Monte Carlo (MC) proton dose engine (VPMC) to model aperture blocks in both dose calculation and optimization for pencil beam scanning proton therapy (PBSPT)-based stereotactic radiosurgery (SRS).Methods and materials. A module to simulate VPs passing through patient-specific aperture blocks was developed and integrated in VPMC based on simulation results of realistic particles (primary protons and their secondaries). To validate the aperture block module, VPMC was first validated by an opensource MC code, MCsquare, in eight water phantom simulations with 3 cm thick brass apertures: four were with aperture openings of 1, 2, 3, and 4 cm without a range shifter, while the other four were with same aperture opening configurations with a range shifter of 45 mm water equivalent thickness. Then, VPMC was benchmarked with MCsquare and RayStation MC for 10 patients with small targets (average volume 8.4 c.c. with range of 0.4-43.3 c.c.). Finally, 3 typical patients were selected for robust optimization with aperture blocks using VPMC.Results. In the water phantoms, 3D gamma passing rate (2%/2 mm/10%) between VPMC and MCsquare was 99.71 ± 0.23%. In the patient geometries, 3D gamma passing rates (3%/2 mm/10%) between VPMC/MCsquare and RayStation MC were 97.79 ± 2.21%/97.78 ± 1.97%, respectively. Meanwhile, the calculation time was drastically decreased from 112.45 ± 114.08 s (MCsquare) to 8.20 ± 6.42 s (VPMC) with the same statistical uncertainties of ~0.5%. The robustly optimized plans met all the dose-volume-constraints (DVCs) for the targets and OARs per our institutional protocols. The mean calculation time for 13 influence matrices in robust optimization by VPMC was 41.6 s and the subsequent on-the-fly 'trial-and-error' optimization procedure took only 71.4 s on average for the selected three patients.Conclusion. VPMC has been successfully enhanced to model aperture blocks in dose calculation and optimization for the PBSPT-based SRS.

A GPU-based fast Monte Carlo code that supports proton transport in magnetic field for radiation therapy

This paper presents the effort to extend a previously reported code ARCHER, a GPU-based Monte Carlo (MC) code for coupled photon and electron transport, into protons including the consideration of magnetic fields. The proton transport is modeled using a Class-II condensed-history algorithm with continuous slowing-down approximation. The model includes ionization, multiple scattering, energy straggling, elastic and inelastic nuclear interactions, as well as deflection due to the Lorentz force in magnetic fields. An additional direction change is added for protons at the end of each step in the presence of the magnetic field. Secondary charge particles, except for protons, are terminated depositing kinetic energies locally, whereas secondary neutral particles are ignored. Each proton is transported step by step until its energy drops to below 0.5 MeV or when the proton leaves the phantom. The code is implemented using the compute unified device architecture (CUDA) platform for optimized GPU thread-level parallelism and efficiency. The code is validated by comparing it against TOPAS. Comparisons of dose distributions between our code and TOPAS for several exposure scenarios, ranging from single square beams in water to patient plan with magnetic fields, show good agreement. The 3D-gamma pass rate with a 2 mm/2% criterion in the region with dose greater than 10% of the maximum dose is computed to be over 99% for all tested cases. Using a single NVIDIA TITAN V GPU card, the computational time of ARCHER is found to range from 0.82 to 4.54 seconds for 1 × 107 proton histories. Compared to a few hours running on TOPAS, this speed improvement is significant. This work presents, for the first time, the performance of a GPU-based MC code to simulate proton transportation magnetic fields, demonstrating the feasibility of accurate and efficient dose calculations in potential magnetic resonance imaging (MRI)-guided proton therapy.

Distributions of energy imparted to a micro-volume by secondary charged fragments of nuclear interactions in space radiation field

The field of cosmic radiation at low-Earth orbit (LEO) has a complex composition. It always contains a component of secondary charged particles, formed by the products of nuclear interactions of the primary high-energy radiation with the nuclei of spacecraft's shielding material, electronic components and biological matter on board. Generation of this secondary radiation can be observed in some track detectors in the form of "stars" formed by tracks-fragments with a common vertex. The energy absorbed by the medium in the region adjacent to the interaction vertex can reach abnormally high values because of its intersection by several particle fragments. In the present paper, a methodology is considered to calculate the energy imparted by such fragments to a spherical sensitive volume in an aqueous medium. The energy distributions for three fragment events were calculated for different positions of the vertex relative to the spherical volume. The obtained data were analyzed and were compared with the distribution for a uniform fluence of secondary particles. It was shown that as the fragmentation vertex approaches the boundary of the sensitive micro-volume, the probability of events with anomalously high energy transfers, higher than the energies from single fragments, increases. The method can be applied to calculate absorbed energy distributions from secondary radiation in media of different elemental composition than that used in the present work. In the future, it is of interest to apply the method for example to study the energy imparted from secondary fragments to a silicon medium, to quantify the number of single event upsets in electronic components.

Investigation of In-Field and Out-of-Field Radiation Quality With Microdosimetry and Its Impact on Relative Biological Effectiveness in Proton Therapy

PURPOSE

Using microdosimetry, this study investigated the relative biological effectiveness (RBE) and quality factor (Q¯) variations in field and out of field as a function of radiation quality for clinical protons.

METHODS AND MATERIALS

A water phantom with a spread-out Bragg peak (SOBP) was irradiated to acquire microdosimetric spectra at several distal and lateral depths with a tissue equivalent proportional counter. The measurements were used as inputs to microdosimetric kinetic and Loncol models to determine the RBE spatial distribution and compare it with predictions from the dose-averaged linear energy transfer-based McNamara model. Q¯ values and biological and dose equivalent values were also calculated.

RESULTS

The data demonstrated that radiation quality changed more rapidly with depth than lateral distance from the SOBP. In beam, y_D ranged from approximately 4 keV/μm at the entrance to 8 keV/μm at the SOBP far end, reaching approximately 15 keV/μm at the penumbra. Out of field, the overall highest value of 23 ± 2 keV/μm was observed at the beam-edge penumbra. Radiation quality changes caused RBE deviations from the clinical value of 1.1, whose extent depends on the approach used for assessing radiation quality as well as on the radiobiological model. For RBE₁₀, microdosimetry-based models appeared to better reproduce the radiobiological data than the dose-averaged linear energy transfer model. Out of field, both the RBE and Q¯ values appeared to have limitations in describing the radiation biological effectiveness. This research also presents a first comprehensive benchmark of TOPAS code against in-field and out-of-field microdosimetric spectra of therapeutic protons.

CONCLUSIONS

Further investigation will be necessary to evaluate the quantitative effects of RBE variations on treatment planning and assess the clinical consequences in terms of both tumor control and normal-tissue toxicity. The achievement of this goal calls for accurate radiobiological data to validate the RBE models.

Application of a simple DNA damage model developed for electrons to proton irradiation

Proton beam therapy allows irradiating tumor volumes with reduced side effects on normal tissues with respect to conventional x-ray radiotherapy. Biological effects such as cell killing after proton beam irradiations depend on the proton kinetic energy, which is intrinsically related to early DNA damage induction. As such, DNA damage estimation based on Monte Carlo simulations is a research topic of worldwide interest. Such simulation is a mean of investigating the mechanisms of DNA strand break formations. However, past modellings considering chemical processes and DNA structures require long calculation times. Particle and heavy ion transport system (PHITS) is one of the general-purpose Monte Carlo codes that can simulate track structure of protons, meanwhile cannot handle radical dynamics simulation in liquid water. It also includes a simple model enabling the efficient estimation of DNA damage yields only from the spatial distribution of ionizations and excitations without DNA geometry, which was originally developed for electron track-structure simulations. In this study, we investigated the potential application of the model to protons without any modification. The yields of single-strand breaks, double-strand breaks (DSBs) and the complex DSBs were assessed as functions of the proton kinetic energy. The PHITS-based estimation showed that the DSB yields increased as the linear energy transfer (LET) increased, and reproduced the experimental and simulated yields of various DNA damage types induced by protons with LET up to about 30 keV μm−1. These results suggest that the current DNA damage model implemented in PHITS is sufficient for estimating DNA lesion yields induced after protons irradiation except at very low energies (below 1 MeV). This model contributes to evaluating early biological impacts in radiation therapy.

Using 157Gd doped carbon and 157GdF4 nanoparticles in proton-targeted therapy for effectiveness enhancement and thermal neutron reduction: a simulation study

There are two major problems in proton therapy. (1) In comparison with the gamma-ray therapy, proton therapy has only ~ 10% greater biological effectiveness, and (2) the risk of the secondary neutrons in proton therapy is another unsolved problem. In this report, the increase of biological effectiveness in proton therapy has been evaluated with better performance than ¹¹B in the presence of two proposed nanomaterials of ¹⁵⁷GdF4 and ¹⁵⁷Gd doped carbon with the thermal neutron reduction due to the presence of ¹⁵⁷Gd isotope. The present study is based on the microanalysis calculations using GEANT4 Monte Carlo tool and GEANT4-DNA package for the strand breaks measurement. It was found that the proposed method will increase the effectiveness corresponding to the alpha particles by more than 100% and also, potentially will decrease the thermal neutrons fluence, significantly. Also, in this work, a discussion is presented on a significant contribution of the secondary alpha particles in total effectiveness in proton therapy.

A systematic study of the contribution of counting statistics to the final lineal energy uncertainty in microdosimetry

Objectives. Microdosimetry is proving to be a reliable and powerful tool to be applied in different fields such as radiobiology, radiation protection and hadron therapy. However, accepted standard protocols and codes of practice are still missing. With this regard, a systematic and methodical uncertainty analysis is fundamental to build an accredited uncertainty budget of practical use. This work studied the contribution of counting statistics (i.e. number of events collected) to the final frequency-mean and dose-mean lineal energy uncertainties, aiming at providing guidelines for good experimental and simulation practice. The practical limitation of current technologies and the non-negligible probability of nuclear reactions require careful considerations and nonlinear approaches. Approach. Microdosimetric data were obtained by means of the particle tracking Monte Carlo code Geant4. The uncertainty analysis was carried out relying on a Monte Carlo based numerical analysis, as suggested by the BIPM's ‘Guide to the expression of uncertainty in measurement’. Final uncertainties were systematically investigated for proton, helium and carbon ions at an increasing number of detected events, for a range of different clinical-relevant beam energies. Main results. Rare events generated by nuclear interactions in the detector sensitive volume were found to massively degrade microdosimetric uncertainties unless a very high statistics is collected. The study showed an increasing impact of such events for increasing beam energy and lighter ions. For instance, in the entrance region of a 250 MeV proton beam, about 5 ∗ 107 events need to be collected to obtain a dose-mean lineal energy uncertainty below 10%. Significance. The results of this study help define the necessary conditions to achieve appropriate statistics in computational microdosimetry, pointing out the importance of properly taking into account nuclear interaction events. Their impact on microdosimetric quantities and on their uncertainty is significant and cannot be overlooked, particularly when characterising clinical beams and radiobiological response. This work prepared the ground for deeper investigations involving dedicated experiments and for the development of a method to properly evaluate the counting statistics uncertainty contribution in the uncertainty budget, whose accuracy is fundamental for the clinical transition of microdosimetry.

First application of the BIANCA biophysical model to carbon-ion patient cases

Objective. The main objective of this work consists of applying, for the first time, the BIANCA (BIophysical ANalysis of Cell death and chromosome Aberrations) biophysical model to the RBE calculation for C-ion cancer patients, and comparing the outcomes with those obtained by the LEM I model, which is applied in clinics. Indeed, the continuous development of heavy-ion cancer therapy requires modelling of biological effects of ion beams on tumours and normal tissues. The relative biological effectiveness (RBE) of heavy ions is higher than that of protons, with a significant variation along the beam path. Therefore, it requires a precise modelling, especially for the pencil-beam scanning technique. Currently, two radiobiological models, LEM I and MKM, are in use for heavy ions in scanned pencil-beam facilities. Approach. Utilizing an interface with the FLUKA Particle Therapy Tool, BIANCA was applied to re-calculate the RBE-weighted dose distribution for carbon-ion treatment of three patients (chordoma, head-and-neck and prostate) previously irradiated at CNAO, where radiobiological optimization was based on LEM I. The predictions obtained by BIANCA were based either on chordoma cell survival (RBE surv ), or on dicentric aberrations in peripheral blood lymphocytes (RBE ab ), which are indicators of late normal tissue damage, including secondary tumours. The simulation outcomes were then compared with those provided by LEM I. Main results. While in the target and in the entrance channel BIANCA predictions were lower than those obtained by LEM I, the two models provided very similar results in the considered OAR. The observed differences between RBE surv and RBE ab (which were also dependent on fractional dose and LET) suggest that in normal tissues the information on cell survival should be integrated by information more closely related to the induction of late damage, such as chromosome aberrations. Significance. This work showed that BIANCA is suitable for treatment plan optimization in ion-beam therapy, especially considering that it can predict both cell survival and chromosome aberrations and has previously shown good agreement with carbon-ion experimental data.

Ganetespib selectively sensitizes cancer cells for proximal and distal spread-out Bragg peak proton irradiation

Objective

Hypersensitivity towards proton versus photon irradiation was demonstrated in homologous recombination repair (HRR)-deficient cell lines. Hence, combined treatment concepts targeting HRR provide a rational for potential pharmaceutical exploitation. The HSP90 inhibitor ganetespib (STA-9090) downregulates a multitude of HRR-associated proteins and sensitizes for certain chemotherapeutics. Thus, the radiosensitizing effect of HSP90-inhibiting ganetespib was investigated for reference photon irradiation and proton irradiation at a proximal and distal position in a spread-out Bragg peak (SOBP).

Methods

A549 and FaDu cells were treated with low-dose (2 nM resp. 1 nM) ganetespib and irradiated with 200 kV photons. Proton irradiation was performed at a proximal and a distal position within a SOBP, with corresponding dose-averaged linear-energy transfer (LET_D) values of 2.1 and 4.5 keV/µm, respectively. Cellular survival data was fitted to the linear-quadratic model to calculate relative biological effectiveness (RBE) and the dose-modifying factor (DMF). Additionally, A549 cells were treated with increasing doses of ganetespib and investigated by flow cytometry, immunoblotting, and immunofluorescence microscopy to investigate cell cycle distribution, Rad51 protein levels, and γH2AX foci, respectively.

Results

Low-dosed ganetespib significantly sensitized both cancer cell lines exclusively for proton irradiation at both investigated LET_D, resulting in increased RBE values of 10–40%. In comparison to photon irradiation, the fraction of cells in S/G2/M phase was elevated in response to proton irradiation with 10 nM ganetespib consistently reducing this population. No changes in cell cycle distribution were detected in unirradiated cells by ganetespib alone. Protein levels of Rad51 are downregulated in irradiated A549 cells by 10 nM and also 2 nM ganetespib within 24 h. Immunofluorescence staining demonstrated similar induction and removal of γH2AX foci, irrespective of irradiation type or ganetespib administration.

Conclusion

Our findings illustrate a proton-specific sensitizing effect of low-dosed ganetespib in both employed cell lines and at both investigated SOBP positions. We provide additional experimental data on cellular response and a rational for future combinatorial approaches with proton radiotherapy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13014-022-02036-z.

Applications of Machine Learning to Improve the Clinical Viability of Compton Camera Based in vivo Range Verification in Proton Radiotherapy

We studied the application of a deep, fully connected Neural Network (NN) to process prompt gamma (PG) data measured by a Compton camera (CC) during the delivery of clinical proton radiotherapy beams. The network identifies 1) recorded "bad" PG events arising from background noise during the measurement, and 2) the correct ordering of PG interactions in the CC to help improve the fidelity of "good" data used for image reconstruction. PG emission from a tissue-equivalent target during irradiation with a 150 MeV proton beam delivered at clinical dose rates was measured with a prototype CC. Images were reconstructed from both the raw measured data and the measured data that was further processed with a neural network (NN) trained to identify "good" and "bad" PG events and predict the ordering of individual interactions within the good PG events. We determine if NN processing of the CC data could improve the reconstructed PG images to a level in which they could provide clinically useful information about the in vivo range and range shifts of the proton beams delivered at full clinical dose rates. Results showed that a deep, fully connected NN improved the achievable contrast to noise ratio (CNR) in our images by more than a factor of 8x. This allowed the path, range, and lateral width of the clinical proton beam within a tissue equivalent target to easily be identified from the PG images, even at the highest dose rates of a 150 MeV proton beam used for clinical treatments. On average, shifts in the beam range as small as 3 mm could be identified. However, when limited by the amount of PG data measured with our prototype CC during the delivery of a single proton pencil beam (~1 × 109 protons), the uncertainty in the reconstructed PG images limited the identification of range shift to ~5 mm. Substantial improvements in CC images were obtained during clinical beam delivery through NN pre-processing of the measured PG data. We believe this shows the potential of NNs to help improve and push CC-based PG imaging toward eventual clinical application for proton RT treatment delivery verification.

A Monte Carlo study of different LET definitions and calculation parameters for proton beam therapy

The strongin vitroevidence that proton Relative Biological Effectiveness (RBE) varies with Linear Energy Transfer (LET) has led to an interest in applying LET within treatment planning. However, there is a lack of consensus on LET definition, Monte Carlo (MC) parameters or clinical methodology. This work aims to investigate how common variations of LET definition may affect potential clinical applications. MC simulations (GATE/GEANT4) were used to calculate absorbed dose and different types of LET for a simple Spread Out Bragg Peak (SOBP) and for four clinical PBT plans covering a range of tumour sites. Variations in the following LET calculation methods were considered: (i) averaging (dose-averaged LET (LET_d) & track-averaged LET); (ii) scoring (LET_dto water, to medium and to mass density); (iii) particle inclusion (LET_dto all protons, to primary protons and to particles); (iv) MC settings (hit type and Maximum Step Size (MSS)). LET distributions were compared using: qualitative comparison, LET Volume Histograms (LVHs), single value criteria (maximum and mean values) and optimised LET-weighted dose models. Substantial differences were found between LET values in averaging, scoring and particle type. These differences depended on the methodology, but for one patient a difference of ∼100% was observed between the maximum LET_dfor all particles and maximum LET_dfor all protons within the brainstem in the high isodose region (4 keVμm^-1and 8 keVμm^-1respectively). An RBE model using LET_dincluding heavier ions was found to predict substantially different LET-weighted dose compared to those using other LET definitions. In conclusion, the selection of LET definition may affect the results of clinical metrics considered in treatment planning and the results of an RBE model. The authors' advocate for the scoring of dose-averaged LET to water for primary and secondary protons using a random hit type and automated MSS.

Handling of beam spectra in training and application of proton RBE models

Published data from cell survival experiments are frequently used as training data for models of proton relative biological effectiveness (RBE). The publications rarely provide full information about the primary particle spectrum of the used beam, or its content of heavy secondary particles. The purpose of this paper is to assess to what extent heavy secondary particles may have been present in published cell survival experiments, and to investigate the impact of non-primary protons for RBE calculations in treatment planning. We used the Monte Carlo code Geant4 to calculate the occurrence of non-primary protons and heavier secondary particles for clinical protons beams in water for four incident energies in the [100, 250] MeV interval. We used the resulting spectra together with a conservative RBE parameterization and an RBE model to map both the rise of RBE at the beam entry surface due to heavy secondary particle buildup, and the difference in estimated RBE if non-primary protons are included or not in the beam quality metric. If included, non-primary protons cause a difference of 2% of the RBE in the plateau region of an spread out Bragg peak and 1% in the Bragg peak. Including non-primary protons specifically for RBE calculations will consequently have a negligible impact and can be ignored. A buildup distance in water of one millimeter was sufficient to reach an equilibrium state of RBE for the four incident energies selected. For the investigated experimental data, 83 out of the 86 data points were found to have been determined with at least that amount of buildup material. Hence, RBE model training data should be interpreted to include the contribution of heavy secondaries.

High LET-Like Radiation Tracks at the Distal Side of Accelerated Proton Bragg Peak

Proton therapy is a type of hadron radiotherapy used for treating solid tumors. Unlike heavy charged elements, proton radiation is considered to be low LET (Linear Energy Transfer) radiation, like X-rays. However, the clinical SOBP (Spread Out Bragg Peak) proton radiation is considered to be higher in relative biological effectiveness (RBE) than both X-ray and their own entrance region. The RBE is estimated to be 1.1–1.2, which can be attributed to the higher LET at the SOBP region than at the entrance region. In order to clarify the nature of higher LET near the Bragg peak of proton radiation and its potential cytotoxic effects, we utilized a horizontal irradiation system with CHO cells. Additionally, we examined DNA repair mutants, analyzed cytotoxicity with colony formation, and assessed DNA damage and its repair with γ-H2AX foci assay in a high-resolution microscopic scale analysis along with the Bragg peak. Besides confirming that the most cytotoxic effects occurred at the Bragg peak, extended cytotoxicity was observed a few millimeters after the Bragg peak. γ-H2AX foci numbers reached a maximum at the Bragg peak and reduced dramatically after the Bragg peak. However, in the post-Bragg peak region, particle track-like structures were sporadically observed. This region contains foci that are more difficult to repair. The peak and post-Bragg peak regions contain rare high LET-like radiation tracks and can cause cellular lethality. This may have caused unwanted side effects and complexities of outputs for the proton therapy treatment.

A systematic review on the usage of averaged LET in radiation biology for particle therapy

Linear Energy Transfer (LET) is widely used to express the radiation quality of ion beams, when characterizing the biological effectiveness. However, averaged LET may be defined in multiple ways, and the chosen definition may impact the resulting reported value. We review averaged LET definitions found in the literature, and quantify which impact using these various definitions have for different reference setups. We recorded the averaged LET definitions used in 354 publications quantifying the relative biological effectiveness (RBE) of hadronic beams, and investigated how these various definitions impact the reported averaged LET using a Monte Carlo particle transport code. We find that the kind of averaged LET being applied is, generally, poorly defined. Some definitions of averaged LET may influence the reported averaged LET values up to an order of magnitude. For publications involving protons, most applied dose averaged LET when reporting RBE. The absence of what target medium is used and what secondary particles are included further contributes to an ill-defined averaged LET. We also found evidence of inconsistent usage of averaged LET definitions when deriving LET-based RBE models. To conclude, due to commonly ill-defined averaged LET and to the inherent problems of LET-based RBE models, averaged LET may only be used as a coarse indicator of radiation quality. We propose a more rigorous way of reporting LET values, and suggest that ideally the entire particle fluence spectra should be recorded and provided for future RBE studies, from which any type of averaged LET (or other quantities) may be inferred.

Report on G4-Med, a Geant4 benchmarking system for medical physics applications developed by the Geant4 Medical Simulation Benchmarking Group

BACKGROUND

Geant4 is a Monte Carlo code extensively used in medical physics for a wide range of applications, such as dosimetry, micro- and nanodosimetry, imaging, radiation protection, and nuclear medicine. Geant4 is continuously evolving, so it is crucial to have a system that benchmarks this Monte Carlo code for medical physics against reference data and to perform regression testing.

AIMS

To respond to these needs, we developed G4-Med, a benchmarking and regression testing system of Geant4 for medical physics.

MATERIALS AND METHODS

G4-Med currently includes 18 tests. They range from the benchmarking of fundamental physics quantities to the testing of Monte Carlo simulation setups typical of medical physics applications. Both electromagnetic and hadronic physics processes and models within the prebuilt Geant4 physics lists are tested. The tests included in G4-Med are executed on the CERN computing infrastructure via the use of the geant-val web application, developed at CERN for Geant4 testing. The physical observables can be compared to reference data for benchmarking and to results of previous Geant4 versions for regression testing purposes.

RESULTS

This paper describes the tests included in G4-Med and shows the results derived from the benchmarking of Geant4 10.5 against reference data.

DISCUSSION

Our results indicate that the Geant4 electromagnetic physics constructor G4EmStandardPhysics_option4 gives a good agreement with the reference data for all the tests. The QGSP_BIC_HP physics list provided an overall adequate description of the physics involved in hadron therapy, including proton and carbon ion therapy. New tests should be included in the next stage of the project to extend the benchmarking to other physical quantities and application scenarios of interest for medical physics.

CONCLUSION

The results presented and discussed in this paper will aid users in tailoring physics lists to their particular application.

Microdosimetry of a therapeutic proton beam with a mini-TEPC and a MicroPlus-Bridge detector for RBE assessment

Proton beams are widely used worldwide to treat localized tumours, the lower entrance dose and no exit dose, thus sparing surrounding normal tissues, being the main advantage of this treatment modality compared to conventional photon techniques. Clinical proton beam therapy treatment planning is based on the use of a general relative biological effectiveness (RBE) of 1.1 along the whole beam penetration depth, without taking into account the documented increase in RBE at the end of the depth dose profile, in the Bragg peak and beyond. However, an inaccurate estimation of the RBE can cause both underdose or overdose, in particular it can cause the unfavourable situation of underdosing the tumour and overdosing the normal tissue just beyond the tumour, which limits the treatment success and increases the risk of complications. In view of a more precise dose delivery that takes into account the variation of RBE, experimental microdosimetry offers valuable tools for the quality assurance of LET or RBE-based treatment planning systems. The purpose of this work is to compare the response of two different microdosimetry systems: the mini-TEPC and the MicroPlus-Bridge detector. Microdosimetric spectra were measured across the 62 MeV spread out Bragg peak of CATANA with the mini-TEPC and with the Bridge microdosimeter. The frequency and dose distributions of lineal energy were compared and the different contributions to the spectra were analysed, discussing the effects of different site sizes and chord length distributions. The shape of the lineal energy distributions measured with the two detectors are markedly different, due to the different water-equivalent sizes of the sensitive volumes: 0.85 μm for the TEPC and 17.3 μm for the silicon detector. When the Loncol's biological weighting function is applied to calculate the microdosimetric assessment of the RBE, both detectors lead to results that are consistent with biological survival data for glioma U87 cells. Both the mini-TEPC and the MicroPlus-Bridge detector can be used to assess the RBE variation of a 62 MeV modulated proton beam along its penetration depth. The microdosimetric assessment of the RBE based on the Loncol's weighting function is in good agreement with radiobiological results when the 10% biological uncertainty is taken into account.

FLUKA simulation of target fragmentation in proton therapy

In proton therapy, secondary fragments are created in nuclear interactions of the beam with the target nuclei. The secondary fragments have low kinetic energies and high atomic numbers as compared to primary protons. Fragments have a high LET and deposit all their energy close to the generation point. For their characteristics, secondary fragments can alter the dose distribution and lead to an increase of RBE for the same delivered physical dose. Moreover, the radiobiological impact of target fragmentation is significant mostly in the region before the Bragg peak, where generally healthy tissues are present, and immediately after Bragg peak. Considering the high biological impact of those particles, especially in the case of healthy tissues or organs at risk, the inclusion of target fragmentation processes in the dose calculation of a treatment planning system can be relevant to improve the treatment accuracy and for this reason it is one of the major tasks of the MoVe IT project. In this study, Monte Carlo simulations were employed to fully characterize the mixed radiation field generated by target fragmentation in proton therapy. The dose averaged LET has been evaluated in case of a Spread Out Bragg Peak (SOBP). Starting from LET distribution, RBE has been evaluated with two different phenomenological models. In order to characterize the mixed radiation field, the production cross section has been evaluated by means of the FLUKA code. The future development of present work is to generate a MC database of fragments fluence to be included in TPS.

Biological Effects of Scattered Versus Scanned Proton Beams on Normal Tissues in Total Body Irradiated Mice: Survival, Genotoxicity, Oxidative Stress and Inflammation

Side effects of proton therapy are poorly studied. Moreover, the differences in the method of dose delivery on normal tissues are not taken into account when proton beams are scanned instead of being scattered. We proposed here to study the effects of both modalities of proton beam delivery on blood; skin; lung and heart in a murine model. In that purpose; C57BL/6 mice were total body irradiated by 190.6 MeV proton beams either by Double Scattering (DS) or by Pencil Beam Scanning (PBS) in the plateau phase before the Bragg Peak. Mouse survival was evaluated. Blood and organs were removed three months after irradiation. Biomarkers of genotoxicity; oxidative stress and inflammation were measured. Proton irradiation was shown to increase lymphocyte micronucleus frequency; lung superoxide dismutase activity; erythrocyte and skin glutathione peroxidase activity; erythrocyte catalase activity; lung; heart and skin oxidized glutathione level; erythrocyte and lung lipid peroxidation and erythrocyte protein carbonylation even 3 months post-irradiation. When comparing both methods of proton beam delivery; mouse survival was not different. However, PBS significantly increased lymphocyte micronucleus frequency; erythrocyte glutathione peroxidase activity and heart oxidized glutathione level compared to DS. These results point out the necessity to take into account the way of delivering dose in PT as it could influence late side effects.

A Monte Carlo based radiation response modelling framework to assess variability of clinical RBE in proton therapy

The clinical implementation of a variable relative biological effectiveness (RBE) in proton therapy is currently controversially discussed. Initial clinical evidence indicates a variable proton RBE, which needs to be verified. In this study, a radiation response modelling framework for assessing clinical RBE variability is established. It was applied to four selected glioma patients (grade III) treated with adjuvant radio(chemo)therapy and who developed late morphological image changes on T1-weighted contrast-enhanced (T1w-CE) magnetic resonance (MR) images within approximately two years of recurrence-free follow-up. The image changes were correlated voxelwise with dose and linear energy transfer (LET) values using univariable and multivariable logistic regression analysis. The regression models were evaluated by the area-under-the-curve (AUC) method performing a leave-one-out cross validation. The tolerance dose TD₅₀ at which 50% of patient voxels experienced toxicity was interpolated from the models. A Monte Carlo (MC) model was developed to simulate dose and LET distributions, which includes variance reduction (VR) techniques to decrease computation time. Its reliability and accuracy were evaluated based on dose calculations of the clinical treatment planning system (TPS) as well as absolute dose measurements performed in the patient specific quality assurance. Morphological image changes were related to a combination of dose and LET. The multivariable models revealed cross-validated AUC values of up to 0.88. The interpolated TD₅₀ curves decreased with increasing LET indicating an increase in biological effectiveness. The MC model reliably predicted average TPS dose within the clinical target volume as well as absolute water phantom dose measurements within 2% accuracy using dedicated VR settings. The observed correlation of dose and LET with late brain tissue damage suggests considering RBE variability for predicting chronic radiation-induced brain toxicities. The MC model simulates radiation fields in patients precisely and time-efficiently. Hence, this study encourages and enables in-depth patient evaluation to assess the variability of clinical proton RBE.

Monte Carlo study of out-of-field exposure in carbon-ion radiotherapy: Organ doses in pediatric brain tumor treatment

PURPOSE

To estimate out-of-field doses during carbon-ion radiotherapy (CIRT) for pediatric cerebellar ependymoma.

METHODS

Given that the out-of-field dose of CIRT depends on beam parameters, we set them for treatment of typical pediatric cerebellar ependymoma based on a previous study. The out-of-field dose during CIRT for pediatric cerebellar ependymoma was then estimated using the Particle and Heavy-Ion Transport code System with Monte Carlo simulations and a computational phantom developed at the University of Florida. From the simulation results, out-of-field doses at dose equivalents of passive beam and active scanning beam CIRT were calculated and compared to the secondary neutron-equivalent dose of passive beam CIRT and proton therapy.

RESULTS

The out-of-field dose equivalent decreases from 1.45 mSv/Gy (relative biological effectiveness - RBE) at the thyroid to 0.06 mSv/Gy (RBE) at the bladder, verifying decay as the distance from the treatment target increases. The out-of-field neutron-equivalent dose in organs per prescribed dose for passive beam CIRT is lower than that for passive beam proton therapy. Moreover, the out-of-field organ dose equivalent per prescribed dose for the active scanning beam CIRT is lower than that for the passive beam CIRT.

CONCLUSIONS

Active scanning beam CIRT is promising for pediatric cerebellar ependymoma regarding out-of-field exposure, outperforming the comparison radiotherapy modalities.

Proton transport modeling in a realistic biological environment by using TILDA-V

Whether it is in radiobiology to identify DNA lesions or in medicine to adapt the radiotherapeutic protocols, a detailed understanding of the radiation-induced interactions in living matter is required. Monte Carlo track-structure codes have been successfully developed to describe these interactions and predict the radiation-induced energy deposits at the nanoscale level in the medium of interest. In this work, the quantum-mechanically based Monte Carlo track-structure code TILDA-V has been used to compute the slowing-down of protons in water and DNA. Stopping power and range are then reported and compared with existing data. Then, a first application of TILDA-V to cellular irradiations is also reported in order to highlight the absolute necessity of taking into account a realistic description of the cellular environment in microdosimetry.

Microdosimetry at the CATANA 62 MeV proton beam with a sealed miniaturized TEPC

A new mini-TEPC with cylindrical sensitive volume of 0.9 mm in diameter and height, and with external diameter of 2.7 mm, has been developed to work without gas flow. With such a mini counter we have measured the physical quality of the 62 MeV therapeutic proton beam of CATANA (Catania, Italy). Measurements were performed at six precise positions along the Spread-Out Bragg Peak (SOBP): 1.4, 19.4, 24.6, 29.0, 29.7 and 30.8 mm, corresponding to positions of clinical relevance (entrance, proximal, central, and distal-edge of the SOBP) or of high lineal energy transfer (LET) increment (distal-dose drop off). Without refilling the microdosimeter with new gas, the measurements were repeated at the same positions 4 months later, in order to study the stability of the response in sealed-mode operation. From the microdosimetric spectra the frequency-mean lineal energy y-_F and the dose-mean lineal energy y-_D were derived and compared with average LET values calculated by means of Geant4 simulations. The comparison points out, in particular, a good agreement between microdosimetric y-_D and the total dose-average LET¯_d, which is the average LET of the mixed radiation field, including the contribution by nuclear reactions.

Ionization quenching in scintillators used for dosimetry of mixed particle fields

Ionization quenching in ion beam dosimetry is often related to the fluence- or dose-averaged linear energy transfer (LET). Both quantities are however averaged over a wide LET range and a mixed field of primary and secondary ions. We propose a novel method to correct the quenched luminescence in scintillators exposed to ion beams. The method uses the energy spectrum of the primaries and accounts for the varying quenched luminescence in heavy, secondary ion tracks through amorphous track structure theory. The new method is assessed against more traditional approaches by correcting the quenched luminescence response from the BCF-12, BCF-60, and 81-0084 plastic scintillators exposed to a 100 MeV pristine proton beam in order to compare the effects of the averaged LET quantities and the secondary ions. Calculations and measurements show that primary protons constitute more than 92% of the energy deposition but account for more than 95% of the luminescence signal in the scintillators. The quenching corrected luminescence signal is in better agreement with the dose measurement when the secondary particles are taken into account. The Birks model provided the overall best quenching corrections, when the quenching corrected signal is adjusted for the number of free model parameters. The quenching parameter kB for the BCF-12 and BCF-60 scintillators is in agreement with literature values and was found to be [Formula: see text] [Formula: see text]m keV^-1 for the 81-0084 scintillator. Finally, a fluence threshold for the 100 MeV proton beam was calculated to be of the order of 10¹⁰ cm^-2, corresponding to 110 Gy, above which the quenching increases non-linearly and the Birks model no longer is applicable.

A novel methodology to assess linear energy transfer and relative biological effectiveness in proton therapy using pairs of differently doped thermoluminescent detectors

A new methodology for assessing linear energy transfer (LET) and relative biological effectiveness (RBE) in proton therapy beams using thermoluminescent detectors is presented. The method is based on the different LET response of two different lithium fluoride thermoluminescent detectors (LiF:Mg,Ti and LiF:Mg,Cu,P) for measuring charged particles. The relative efficiency of the two detector types was predicted using the recently developed Microdosimetric d(z) Model in combination with the Monte Carlo code PHITS. Afterwards, the calculated ratio of the expected response of the two detector types was correlated with the fluence- and dose- mean values of the unrestricted proton LET. Using the obtained proton dose mean LET as input, the RBE was assessed using a phenomenological biophysical model of cell survival. The aforementioned methodology was benchmarked by exposing the detectors at different depths within the spread out Bragg peak (SOBP) of a clinical proton beam at iThemba LABS. The assessed LET values were found to be in good agreement with the results of radiation transport computer simulations performed using the Monte Carlo code GEANT4. Furthermore, the estimated RBE values were compared with the RBE values experimentally determined by performing colony survival measurements with Chinese Hamster Ovary (CHO) cells during the same experimental run. A very good agreement was found between the results of the proposed methodology and the results of the in vitro study.

Contribution to dose in healthy tissue from secondary target fragments in therapeutic proton, He and C beams measured with CR-39 plastic nuclear track detectors

The linear energy transfer (LET) spectrum, absorbed dose and dose equivalent from secondary particles of LET∞H2O ≥15 keV/μm deposited within the plateau of the Bragg curve in primary particle-induced nuclear target fragmentation reactions in tissue during proton and heavy ion radiotherapy were measured using CR-39 plastic nuclear track detectors and analyzed by means of atomic force microscopy. It was found that secondary target fragments contributed 20% to dose equivalent for primary protons (157 MeV), 13% for primary helium ions (145 MeV/n) and 4% for primary carbon ions (383 MeV/n), respectively. Little research has been done on the contribution from these particles to primary given dose. The smaller contribution measured for energetic carbon ion beams compared to proton beams can be considered an advantage of carbon ion radiotherapy over proton radiotherapy.

A New Anthropomorphic Pediatric Spine Phantom for Proton Therapy Clinical Trial Credentialing

Purpose

To design and evaluate an anthropomorphic spine phantom for use in credentialing proton therapy facilities for clinical trial participation by the Imaging and Radiation Oncology Core Houston QA Center.

Materials and Methods

A phantom was designed to perform an end-to-end audit of the proton spine treatment process, including simulation, dose calculation, and proton treatment delivery. Because plastics that simulate bone in proton beams are unknown, 11 potential materials were tested to identify suitable phantom materials. Once built, preliminary testing using passive scattering and spot scanning treatment plans (including a field junction) were created in-house and delivered 3 times to test reproducibility. The following measured attributes were compared with the calculated values: absolute dose agreement using thermoluminescent dosimeters, planar gamma agreement, distal range, junction match, and right and left profile alignment using radiochromic film. Finally, credentialing results from 10 institutions were also assessed.

Results

A suitable bone substitute was identified (Techtron HPV Bearing Grade), which had a measured relative stopping power that agreed within 1.1% of its value calculated by Eclipse. In-house passive scatter testing of the phantom demonstrated that the phantom was suitable for assessing craniospinal irradiation dose delivery. However, the in-house scanning beam results were more mixed, highlighting challenges in treatment delivery. Seven of ten institutions passed the proposed criteria for this phantom, a pass rate consistent with other Imaging and Radiation Oncology phantoms.

Conclusions

An anthropomorphic proton spine phantom was developed to evaluate proton therapy delivery. This phantom provides a realistic challenge for centers wishing to participate in proton clinical trials and highlights the need for caution in applying advanced treatments.

Energy Spectra of Protons and Generated Secondary Electrons around the Bragg Peak in Materials of Interest in Proton Therapy

The number and energy of secondary electrons generated around the trajectories of swift protons interacting with biological materials are highly relevant in proton therapy, due to the prominent role of low-energy electrons in the production of biodamage. For a given material, electron energy distributions are determined by the proton energy; and it is imperative that the distribution of proton energy at depths around the Bragg peak region be described as accurately as possible. With this objective, we simulated the energy distributions of proton beams of clinically relevant energies (50-300 MeV) at depths around the Bragg peak in liquid water and the water-equivalent polymer poly(methyl methacrylate) (PMMA). By using a simple model, this simulation has been conveniently extended to account for nuclear fragmentation reactions, providing depth-dose curves in excellent agreement with available experimental data. Special care has been taken to describe the electronic excitation spectrum of the target, taking into account its condensed phase nature. A predictive formula has been obtained for the mean value and the width of the proton energy distribution at the Bragg peak depth, quantities which are found to grow linearly with the initial energy of the beam, in good agreement with available data. To accurately characterize (in number and energy) the electrons generated around the proton paths, the energy distributions of the latter at each depth have been convoluted with the energy-dependent ionization inverse mean free paths. This results in a number of low-energy electrons around the Bragg peak larger than when only the proton beam average energy at the given depths is considered. The convoluted ionization inverse mean free path closely resembles the Bragg curve shape. The average energy of the secondary electrons is nearly constant (∼55 eV for liquid water and ∼43 eV for PMMA) in the plateau of the Bragg curve, independent of the proton incident energy and suddenly decaying once the Bragg peak is reached. These findings highlight the importance of a precise calculation of the proton beam energy distribution as a function of the target depth to reliably characterize the secondary electrons generated around the Bragg peak region.

Characterization of a multilayer ionization chamber prototype for fast verification of relative depth ionization curves and spread-out-Bragg-peaks in light ion beam therapy

PURPOSE

To dosimetrically characterize a multilayer ionization chamber (MLIC) prototype for quality assurance (QA) of pristine integral ionization curves (ICs) and spread-out-Bragg-peaks (SOBPs) for scanning light ion beams.

METHODS

QUBE (De.Tec.Tor., Torino, Italy) is a modular detector designed for QA in particle therapy (PT). Its main module is a MLIC detector, able to evaluate particle beam relative depth ionization distributions at different beam energies and modulations. The charge collecting electrodes are made of aluminum, for a nominal water equivalent thickness (WET) of ~75 mm. The detector prototype was calibrated by acquiring the signals in the initial plateau region of a pristine BP and in terms of WET. Successively, it was characterized in terms of repeatability response, linearity, short-term stability and dose rate dependence. Beam-induced measurements of activation in terms of ambient dose equivalent rate were also performed. To increase the detector coarse native spatial resolution (~2.3 mm), several consecutive acquisitions with a set of certified 0.175-mm-thick PMMA sheets (Goodfellow, Cambridge Limited, UK), placed in front of the QUBE mylar entrance window, were performed. The ICs/SOBPs were achieved as the result of the sum of the set of measurements, made up of a one-by-one PMMA layer acquisition. The newly obtained detector spatial resolution allowed the experimental measurements to be properly comparable against the reference curves acquired in water with the PTW Peakfinder. Furthermore, QUBE detector was modeled in the FLUKA Monte Carlo (MC) code following the technical design details and ICs/SOBPs were calculated.

RESULTS

Measurements showed a high repeatability: mean relative standard deviation within ±0.5% for all channels and both particle types. Moreover, the detector response was linear with dose (R²  > 0.998) and independent on the dose rate. The mean deviation over the channel-by-channel readout respect to the reference beam flux (100%) was equal to 0.7% (1.9%) for the 50% (20%) beam flux level. The short-term stability of the gain calibration was very satisfying for both particle types: the channel mean relative standard deviation was within ±1% for all the acquisitions performed at different times. The ICs obtained with the MLIC QUBE at improved resolution satisfactorily matched both the MC simulations and the reference curves acquired with Peakfinder. Deviations from the reference values in terms of BP position, peak width and distal fall-off were submillimetric for both particle types in the whole investigated energy range. For modulated SOBPs, a submillimetric deviation was found when comparing both experimental MLIC QUBE data against the reference values and MC calculations. The relative dose deviations for the experimental MLIC QUBE acquisitions, with respect to Peakfinder data, ranged from ~1% to ~3.5%. Maximum value of 14.1 μSv/h was measured in contact with QUBE entrance window soon after a long irradiation with carbon ions.

CONCLUSION

MLIC QUBE appears to be a promising detector for accurately measuring pristine ICs and SOBPs. A simple procedure to improve the intrinsic spatial resolution of the detector is proposed. Being the detector very accurate, precise, fast responding, and easy to handle, it is therefore well suited for daily checks in PT.

Limitations in predicting the space radiation health risk for exploration astronauts

Despite years of research, understanding of the space radiation environment and the risk it poses to long-duration astronauts remains limited. There is a disparity between research results and observed empirical effects seen in human astronaut crews, likely due to the numerous factors that limit terrestrial simulation of the complex space environment and extrapolation of human clinical consequences from varied animal models. Given the intended future of human spaceflight, with efforts now to rapidly expand capabilities for human missions to the moon and Mars, there is a pressing need to improve upon the understanding of the space radiation risk, predict likely clinical outcomes of interplanetary radiation exposure, and develop appropriate and effective mitigation strategies for future missions. To achieve this goal, the space radiation and aerospace community must recognize the historical limitations of radiation research and how such limitations could be addressed in future research endeavors. We have sought to highlight the numerous factors that limit understanding of the risk of space radiation for human crews and to identify ways in which these limitations could be addressed for improved understanding and appropriate risk posture regarding future human spaceflight.

Present developments in reaching an international consensus for a model-based approach to particle beam therapy

Particle beam therapy (PBT), including proton and carbon ion therapy, is an emerging innovative treatment for cancer patients. Due to the high cost of and limited access to treatment, meticulous selection of patients who would benefit most from PBT, when compared with standard X-ray therapy (XRT), is necessary. Due to the cost and labor involved in randomized controlled trials, the model-based approach (MBA) is used as an alternative means of establishing scientific evidence in medicine, and it can be improved continuously. Good databases and reasonable models are crucial for the reliability of this approach. The tumor control probability and normal tissue complication probability models are good illustrations of the advantages of PBT, but pre-existing NTCP models have been derived from historical patient treatments from the XRT era. This highlights the necessity of prospectively analyzing specific treatment-related toxicities in order to develop PBT-compatible models. An international consensus has been reached at the Global Institution for Collaborative Research and Education (GI-CoRE) joint symposium, concluding that a systematically developed model is required for model accuracy and performance. Six important steps that need to be observed in these considerations include patient selection, treatment planning, beam delivery, dose verification, response assessment, and data analysis. Advanced technologies in radiotherapy and computer science can be integrated to improve the efficacy of a treatment. Model validation and appropriately defined thresholds in a cost-effectiveness centered manner, together with quality assurance in the treatment planning, have to be achieved prior to clinical implementation.

MEASUREMENT OF NEUTRON AMBIENT DOSE EQUIVALENT IN PROTON RADIOTHERAPY WITH LINE-SCANNING AND WOBBLING MODE TREATMENT SYSTEM

The primary objective of this study was to measure secondary neutron dose during proton therapy using a detector that covers the entire neutron energy range produced in proton therapy. We analyzed and compared the neutron dose during proton treatment with passive scattering and line scanning. The neutron ambient dose equivalents were measured with a 190 MeV wobbling and line-scanning proton beam. The center of a plastic water phantom (30 × 30 × 60 cm3) was placed at the isocenter. A Wide-Energy Neutron Detection Instrument (WENDI-2) was located 1m from the isocenter at four different angles (0°, 45°, 90° and 135°). Both wobbling and line-scanning modes of a multipurpose and pencil beam scanning dedicated nozzles were used to obtain a spread-out Bragg peak with 10-cm-width for the measurements. The ambient dose equivalent H*(10) value was normalized by the proton therapeutic dose at the isocenter. For wobbling mode and line-scanning mode, the highest H*(10) values were 1.972 and 0.099 mSv/Gy, respectively. We successfully measured the neutron ambient dose equivalents at six positions generated by a 190 MeV proton beam using wobbling and line-scanning mode with the WENDI-2. These reference data could be used for neutron dose reduction methods and other analysis for advanced proton treatment in the near future.

Monte Carlo simulation of secondary neutron dose for scanning proton therapy using FLUKA

Proton therapy is a rapidly progressing field for cancer treatment. Globally, many proton therapy facilities are being commissioned or under construction. Secondary neutrons are an important issue during the commissioning process of a proton therapy facility. The purpose of this study is to model and validate scanning nozzles of proton therapy at Samsung Medical Center (SMC) by Monte Carlo simulation for beam commissioning. After the commissioning, a secondary neutron ambient dose from proton scanning nozzle (Gantry 1) was simulated and measured. This simulation was performed to evaluate beam properties such as percent depth dose curve, Bragg peak, and distal fall-off, so that they could be verified with measured data. Using the validated beam nozzle, the secondary neutron ambient dose was simulated and then compared with the measured ambient dose from Gantry 1. We calculated secondary neutron dose at several different points. We demonstrated the validity modeling a proton scanning nozzle system to evaluate various parameters using FLUKA. The measured secondary neutron ambient dose showed a similar tendency with the simulation result. This work will increase the knowledge necessary for the development of radiation safety technology in medical particle accelerators.

Factors influencing the performance of patient specific quality assurance for pencil beam scanning IMPT fields

PURPOSE

A detailed analysis of 2728 intensity modulated proton therapy (IMPT) fields that were clinically delivered to patients between 2007 and 2013 at Paul Scherrer Institute (PSI) was performed. The aim of this study was to analyze the results of patient specific dosimetric verifications and to assess possible correlation between the quality assurance (QA) results and specific field metrics.

METHODS

Dosimetric verifications were performed for every IMPT field prior to patient treatment. For every field, a steering file was generated containing all the treatment unit information necessary for treatment delivery: beam energy, beam angle, dose, size of air gap, nuclear interaction (NI) correction factor, number of range shifter plates, number of Bragg peaks (BPs) with their position and weight. This information was extracted and correlated to the results of dosimetric verification of each field which was a measurement of two orthogonal profiles using an orthogonal ionization chamber array in a movable water column.

RESULTS

The data analysis has shown more than 94% of all verified plans were within defined clinical tolerances. The differences between measured and calculated dose depend critically on the number of BPs, total thickness of all range shifter plates inserted in the beam path, and maximal range. An increase of the dose difference was observed with smaller number of BPs (i.e., smaller tumor) and smaller ranges (i.e., superficial tumors). The results of the verification do not depend, however, on the prescribed dose, NI correction, or the size of the air gap. There is no dependency of the transversal and longitudinal spot position precision on the beam angle. The value of NI correction depends on the number of spots and number of range shifter plates.

CONCLUSIONS

The presented study has shown that the verification method used at Centre for Proton Therapy at Paul Scherrer Institute is accurate and reproducible for performing patient specific QA. The results confirmed that the dose discrepancy is dependent on the size and location of the tumor.

Proton therapy - Present and future

In principle, proton therapy offers a substantial clinical advantage over conventional photon therapy. This is because of the unique depth-dose characteristics of protons, which can be exploited to achieve significant reductions in normal tissue doses proximal and distal to the target volume. These may, in turn, allow escalation of tumor doses and greater sparing of normal tissues, thus potentially improving local control and survival while at the same time reducing toxicity and improving quality of life. Protons, accelerated to therapeutic energies ranging from 70 to 250MeV, typically with a cyclotron or a synchrotron, are transported to the treatment room where they enter the treatment head mounted on a rotating gantry. The initial thin beams of protons are spread laterally and longitudinally and shaped appropriately to deliver treatments. Spreading and shaping can be achieved by electro-mechanical means to treat the patients with "passively-scattered proton therapy" (PSPT) or using magnetic scanning of thin "beamlets" of protons of a sequence of initial energies. The latter technique can be used to treat patients with optimized intensity modulated proton therapy (IMPT), the most powerful proton modality. Despite the high potential of proton therapy, the clinical evidence supporting the broad use of protons is mixed. It is generally acknowledged that proton therapy is safe, effective and recommended for many types of pediatric cancers, ocular melanomas, chordomas and chondrosarcomas. Although promising results have been and continue to be reported for many other types of cancers, they are based on small studies. Considering the high cost of establishing and operating proton therapy centers, questions have been raised about their cost effectiveness. General consensus is that there is a need to conduct randomized trials and/or collect outcomes data in multi-institutional registries to unequivocally demonstrate the advantage of protons. Treatment planning and plan evaluation of PSPT and IMPT require special considerations compared to the processes used for photon treatment planning. The differences in techniques arise from the unique physical properties of protons but are also necessary because of the greater vulnerability of protons to uncertainties, especially from inter- and intra-fractional variations in anatomy. These factors must be considered in designing as well as evaluating treatment plans. In addition to anatomy variations, other sources of uncertainty in dose delivered to the patient include the approximations and assumptions of models used for computing dose distributions for planning of treatments. Furthermore, the relative biological effectiveness (RBE) of protons is simplistically assumed to have a constant value of 1.1. In reality, the RBE is variable and a complex function of the energy of protons, dose per fraction, tissue and cell type, end point, etc. These uncertainties, approximations and current technological limitations of proton therapy may limit the achievement of its true potential. Ongoing research is aimed at better understanding the consequences of the various uncertainties on proton therapy and reducing the uncertainties through image-guidance, adaptive radiotherapy, further study of biological properties of protons and the development of novel dose computation and optimization methods. However, residual uncertainties will remain in spite of the best efforts. To increase the resilience of dose distributions in the face of uncertainties and improve our confidence in dose distributions seen on treatment plans, robust optimization techniques are being developed and implemented. We assert that, with such research, proton therapy will be a commonly applied radiotherapy modality for most types of solid cancers in the near future.