Spread-out Bragg Peak Width Research Articles

Mini-ridge filter designs for conformal FLASH proton therapy

The present study designed a mini-ridge filter capable of generating a 30 mm spread-out Bragg peak (SOBP) specific to the proton FLASH beamline for use in proton FLASH mouse studies. Proton FLASH therapy delivers ultra-high dose rates exceeding 40 Gy/s, necessitating instantaneous SOBP generation via static method. The TOol for PArticle Simulation (TOPAS) was used to model the FLASH beamline, which enabled accurate replication of beam characteristics identified from measurements, and ensured accurate dose calculations. Comparisons between simulated and measured peak widths and R80d values, accurate to within 1 mm, verified the accuracy and reliability of Monte Carlo code. Two mini-ridge filters of different materials were designed and optimized using MATLAB, simulated with TOPAS for SOBP verification, and fabricated using 3D-printing or computer numerical control manufacturing. The results indicated strong agreement between the simulated and measured SOBP widths for the polylactic acid mini-ridge filter but with discrepancies observed in the range and flatness measurements, which may be due to the use of 3D printing. By contrast, the aluminium alloy mini-ridge filter achieved excellent results, generating a 31.6 mm SOBP width with 0.89% flatness. Therefore, the present study validated the accuracy of our Monte Carlo code in simulating the FLASH beamline and the feasibility of our mini-ridge filters.

Design, optimization, and testing of ridge filters for proton FLASH radiotherapy at TRIUMF: The HEDGEHOG

TRIUMF has recently developed and implemented a workflow to design and fabricate application-specific ridge filters, which is a key upgrade to enable research using ultra-high (FLASH) dose rate beam delivery at its Proton Therapy Research Centre. These static ridge filters, called HEDGEHOGs, replace the rotating modulator wheels used to create a spread-out Bragg peak (SOBP) during conventional proton radiotherapy beam delivery, which are incompatible with the fast irradiation times of clinical doses at FLASH dose rates at TRIUMF. Initial beam commissioning of HEDGEHOGs demonstrated that they produce spread-out Bragg peaks matching their design parameters. Additional studies of some of the design parameters, including SOBP widths as well as different pin diameters and fabrication options (material, 3D printer technology) have now been completed. Based on those studies, the use of liquid medium printers (resin, jet) instead of filament printers is recommended. These studies also demonstrate that the lateral dose uniformity of the beam at 36.5 cm from the HEDGEHOG location is acceptable even with relatively large 7 mm base-diameter pins. The code base and workflow are made publicly available and can easily be adapted to other facilities. The feasibility of adapting the HEDGEHOG workflow to other beam lines or facilities was validated on a second proton beam line at the same facility at which the HEDGEHOG workflow was developed.

A generalized model for monitor units determination in ocular proton therapy using machine learning: A proof-of-concept study

Objective. Determining and verifying the number of monitor units is crucial to achieving the desired dose distribution in radiotherapy and maintaining treatment efficacy. However, current commercial treatment planning system(s) dedicated to ocular passive eyelines in proton therapy do not provide the number of monitor units for patient-specific plan delivery. Performing specific pre-treatment field measurements, which is time and resource consuming, is usually gold-standard practice. This proof-of-concept study reports on the development of a multi-institutional-based generalized model for monitor units determination in proton therapy for eye melanoma treatments. Approach. To cope with the small number of patients being treated in proton centers, three European institutes participated in this study. Measurements data were collected to address output factor differences across the institutes, especially as function of field size, spread-out Bragg peak modulation width, residual range, and air gap. A generic model for monitor units prediction using a large number of 3748 patients and broad diversity in tumor patterns, was evaluated using six popular machine learning algorithms: (i) decision tree; (ii) random forest, (iii) extra trees, (iv) K-nearest neighbors, (v) gradient boosting, and (vi) the support vector regression. Features used as inputs into each machine learning pipeline were: Spread-out Bragg peak width, range, air gap, fraction and calibration doses. Performance measure was scored using the mean absolute error, which was the difference between predicted and real monitor units, as collected from institutional gold-standard methods. Main results. Predictions across algorithms were accurate within 3% uncertainty for up to 85.2% of the plans and within 10% uncertainty for up to 98.6% of the plans with the extra trees algorithm. Significance. A proof-of-concept of using machine learning-based generic monitor units determination in ocular proton therapy has been demonstrated. This could trigger the development of an independent monitor units calculation tool for clinical use.

Deep learning proton beam range estimation model for quality assurance based on two-dimensional scintillated light distributions in simulations.

Many studies have utilized optical camera systems with volumetric scintillators for quality assurances (QA) to estimate the proton beam range. However, previous analytically driven range estimation methods have the difficulty to derive the dose distributions from the scintillation images with quenching and optical effects. In this study, a deep learning method utilized to QA was used to predict the beam range and spread-out Bragg peak (SOBP) for two-dimensional (2D) map conversion from the scintillation light distribution (LD) into the dose distribution in a water phantom. The 2D residual U-net modeling for deep learning was used to predict the 2D water dose map from a 2D scintillation LD map. Monte Carlo simulations for dataset preparation were performed with varying monoenergetic proton beam energies, field sizes, and beam axis shifts. The LD was reconstructed using photons backpropagated from the aperture as a virtual lens. The SOBP samples were constructed based on monoenergetic dose distributions. The training set, including the validation set, consisted of 8659 image pairs of LD and water dose maps. After training, dose map prediction was performed using a 300 image pair test set generated under random conditions. The pairs of simulated and predicted dose maps were analyzed by Bragg peak fitting and gamma index maps to evaluate the model prediction. The estimated beam range and SOBP width resolutions were 0.02and 0.19mm respectively for varying beam conditions, and the beam range and SOBP width deviations from the reference simulation result were less than 0.1and 0.8mm respectively. The simulated and predicted distributions showed good agreement in the gamma analysis, except for rare cases with failed gamma indices in the proximal and field-marginal regions. The deep learning conversion method using scintillation LDs in an optical camera system with a scintillator is feasible for estimating proton beam range and SOBP width with high accuracy.

Validation of the relative biological effectiveness of active-energy scanning carbon-ion radiotherapy on a commercial treatment planning system with a microdosimetic kinetic model

BackgroundThe study objective was to validate the relative biological effectiveness (RBE) calculated by the modified microdosimetric kinetic model in RayStation (Ray-MKM) for active-energy scanning carbon-ion radiotherapy.MethodsThe Ray-MKM was benchmarked using a spread-out Bragg-peak (SOBP) plan, which was suggested in literature from the National Institute of Radiobiological Science (NIRS) in Japan. The residual RBE differences from the MKM at NIRS (NIRS-MKM) were derived using several SOBP plans with different ranges, SOBP widths, and prescriptions. To investigate the origins of the differences, we compared the saturation-corrected dose-mean specific energy Z_{1D}^{*} of the aforementioned SOBPs. Furthermore, we converted the RBE-weighted doses with the Ray-MKM to those with local effect model I (LEM doses). The purpose was to investigate whether the Ray-MKM could reproduce the RBE-weighted conversion study.ResultsThe benchmark determined the value of the clinical dose scaling factor, F_{clin}, as 2.40. The target mean RBE deviations between the Ray-MKM and NIRS-MKM were median: 0.6 (minimum: 0.0 to maximum: 1.69) %. The Z_{1D}^{*} difference in-depth led to the RBE difference in-depth and was remarkable at the distal end. The converted LEM doses from the Ray-MKM doses were comparable (the deviation being − 1.8–0.7%) to existing literature.ConclusionThis study validated the Ray-MKM based on our active-energy scanning carbon-ion beam via phantom studies. The Ray-MKM could generate similar RBEs as the NIRS-MKM after benchmarking. Analysis based on Z_{1D}^{*} indicated that the different beam qualities and fragment spectra caused the RBE differences. Since the absolute dose differences at the distal end were small, we neglected them. Furthermore, each centre may determine its centre-specific F_{clin} based on this approach.

Analysis of the relationship between LET, γH2AX foci volume and cell killing effect of carbon ions using high-resolution imaging technology.

The strong cell killing effect of high linear energy transfer (LET) carbon ions is dependent on lethal DNA damage. Our recent studies suggest that induction of clusters of double-strand breaks (DSBs) in close proximity is one of the potential mechanisms. However, the relationship between LET, the degree of DSB clustering and the cell killing effect of carbon ions remains unclear. Here, we used high-resolution imaging technology to analyze the volume of γH2AX foci induced by monoenergetic carbon ions with a clinically-relevant range of LET (13-100keV/μm). We obtained data from 3317 γH2AX foci and used a gaussian function to approximate the probability (p) that 1Gy-carbon ions induce γH2AX foci of a given volume (vth) or greater per nucleus. Cell killing effects were assessed in clonogenic assays. The cell killing effect showed high concordance with p at vth = 0.7μm3 across various LET values; the difference between the two was 4.7% ± 2.2%. This relationship was also true for clinical carbon ion beams harboring a mixed LET profile throughout a spread-out Bragg peak width (30-120mm), with the difference at vth = 0.7μm3 being 1.6% ± 1.2% when a Monte Carlo simulation-derived dose-averaged LET was used to calculate p. These data indicate that the cell killing effect of carbon ions is predictable by the ability of carbon ions to induce γH2AX foci containing clustered DSBs, which is linked to LET, providing the biological basis for LET modulation in the planning of carbon ion radiotherapy.

Decomposition of the weight fractions for modelling ridge filters in a proton wobbling nozzle

In proton therapy, the width of a pristine Bragg peak is too narrow to cover the entire tumor volume, and should be spread out using the Spread out Bragg peak (SOBP) techniques such as energy stacking, range modulator wheel and ridge filter. The main purpose of this study was to demonstrate feasibility of using a simple least square technique to decompose the simulated weight fractions of elemental Bragg peaks of ridge filters used in a clinical wobbling beam nozzle. First, initial simulated beam parameters including mean energy and energy spread at the nozzle entrance for all available clinical energies were commissioned against measurements for the open middle wobbling field at the Proton and Radiation Center of the Linkou Chang Gung Memorial Hospital (LCGMH) using the Particle Therapy Simulation Framework (PTSim) Monte Carlo package. Material composition and geometry of nozzle components were provided by the vendor. Depth dose distribution of beam energies from 70 to 230 MeV at 20-MeV increment were simulated and validated with measurement. Curve fitting to those beam parameters was performed to extend simulation energy range to all available clinical energies. For the reconstructed ridge filter simulation, elemental Bragg peaks (BPs) with layer filters corresponding to ridge filter step thicknesses were obtained as bases of the weight factor decomposition. The least square technique was implemented to decompose weight factors for individual ridge filter at a selected beam energy within its applicable energy range. Finally, the reconstructed ridge filters using decomposed weight factors were tested at one other energy to validate the study design. Four different ridge filters (5 cm-LE, 5 cm-HE, 10 cm and 11 cm SOBP) were examined in this study. Commission results for tuned beam parameter settings showed maximum disagreement of BP characteristics, represented by the 80% dose depth (R80d) and width of distal fall-off (W80-20), occurred at 230 MeV and was smaller than 0.2 and 0.5 mm respectively. Furthermore, analyzed outcome for SOBPs from the four ridge filters revealed general improvement of agreement to measurement with decomposition and satisfactory agreement between measurements and those with least square technique. Deviation from measurement of SOBP characteristics evaluated for R90d, W80-20 and SOBP flatness after weight decomposition showed values smaller than 1mm/1%. Similar evaluation of SOBP width generated larger difference with maximum value of 2.7 mm for the 11-cm ridge filter. Results of the validation testes were similar with maximum deviation of 1.4 mm, 0.6 mm, 0.2% and 3.8 mm for R90d, W80-20, flatness and width respectively.

Commissioning a newly developed treatment planning system, VQA Plan, for fast-raster scanning of carbon-ion beams.

In this study, we report our experience in commissioning a commercial treatment planning system (TPS) for fast-raster scanning of carbon-ion beams. This TPS uses an analytical dose calculation algorithm, a pencil-beam model with a triple Gaussian form for the lateral-dose distribution, and a beam splitting algorithm to consider lateral heterogeneity in a medium. We adopted the mixed beam model as the relative biological effectiveness (RBE) model for calculating the RBE values of the scanned carbon-ion beam. To validate the modeled physical dose, we compared the calculations with measurements of various relevant quantities as functions of the field size, range and width of the spread-out Bragg peak (SOBP), and depth-dose and lateral-dose profiles for a 6-mm SOBP in water. To model the biological dose, we compared the RBE calculated with the newly developed TPS to the RBE calculated with a previously validated TPS that is in clinical use and uses the same RBE model concept. We also performed patient-specific measurements to validate the dose model in clinical situations. The physical beam model reproduces the measured absolute dose at the center of the SOBP as a function of field size, range, and SOBP width and reproduces the dose profiles for a 6-mm SOBP in water. However, the profiles calculated for a heterogeneous phantom have some limitations in predicting the carbon-ion-beam dose, although the biological doses agreed well with the values calculated by the validated TPS. Using this dose model for fast-raster scanning, we successfully treated more than 900 patients from October 2018 to October 2020, with an acceptable agreement between the TPS-calculated and measured dose distributions. We conclude that the newly developed TPS can be used clinically with the understanding that it has limited accuracies for heterogeneous media.

Monitor unit prediction model for wobbling proton therapy with ridge filters.

We introduced an output factor (cGy/MU) prediction model for wobbling proton beams over the full range of proton energy, scatterer thickness, and the width of spread-out Bragg peak (SOBP). From December 2015 to August 2020, 1990 wobbling proton fields were used to treat patients, where 1714 fields had a diameter smaller than 11cm and 276 had a diameter between 11 and 16cm, which were designated as small and middle wobbling radius cases, respectively. The output factor is defined as the ratio of proton absorbed dose at mid-depth of SOBP to monitor unit (MU). It depends dominantly on proton energy, scatterer thickness, and the width of SOBP. We established the prediction model using the polynomial fitting function and determined its coefficients for the small and middle wobbling radius cases. We evaluated the accuracy of our prediction model by calculating the difference between predicted and measured output factors. For the small wobbling radius cases, the mean value of the output factor difference was 0.22% with a standard deviation of 1.3%. For the middle wobbling radius cases, the mean value was 0.20% and with a standard deviation of 0.79%. The large deviation was especially observed for wobbling proton beams having small field size and small width of SOBP. We made a prediction model of output factor for wobbling proton beams, thereby determining MU of each beam. This included the dependency of the output factor on the proton energy between 70 and 230 MeV, scatterer thickness, and the width of SOBP. For 93.6% of the small and 95.5% of the middle wobbling radius cases, the deviation between predicted and measured output factor was below 3%. The cases with deviations of predicted and measured output factor above 3% had small field size and small width of SOBP. The accuracy of our prediction model would be improved by adopting the field size effect and measuring more cases of small field size and small SOBP width in the future.

A quality assurance for respiratory gated proton irradiation with range modulation wheel.

The purpose of this study was to provide periodic quality assurance (QA) methods for respiratory‐gated proton beam with a range modulation wheel (RMW) and to clarify the characteristics and long‐term stability of the respiratory‐gated proton beam. A two‐dimensional detector array and a solid water phantom were used to measure absolute dose, spread‐out Bragg peak (SOBP) width and proton range for monthly QA. SOBP width and proton range were measured using an oblique incidence beam to the lateral side of a solid water phantom and compared between with and without a gating proton beam. To measure the delay time of beam‐on/off for annual QA, we collected the beam‐on/off signals and the dose monitor‐detected pulse. We analyzed the results of monthly QA over a 15‐month period and investigated the delay time by machine signal analysis. The dose deviations at proximal, SOBP center and distal points were −0.083 ± 0.25%, 0.026 ± 0.20%, and −0.083 ± 0.35%, respectively. The maximum dose deviation between with and without respiratory gating was −0.95% at the distal point and other deviations were within ±0.5%. Proximal and SOBP center doses showed the same trend over a 15‐month period. Delay times of beam‐on/off for 200 MeV/SOBP 16 cm were 140.5 ± 0.8 ms and 22.3 ± 13.0 ms, respectively. Delay times for 160 MeV/SOBP 10 cm were 167.5 ± 15.1 ms and 19.1 ± 9.8 ms. Our beam delivery system with the RMW showed sufficient stability for respiratory‐gated proton therapy and the system did not show dependency on the energy and the respiratory wave form. The delay times of beam‐on/off were within expectations. The proposed QA methods will be useful for managing the quality of respiratory‐gated proton beams and other beam delivery systems.

A dosimetric analysis of proton beam therapy using different snouts

AbstractPurposeThis exploration is intended to analyse the dosimetric characteristics of proton beams of multiple energies using different snout sizes.Materials and methodsA synchrotron was used for the extraction of eight proton beam energies (100–250 MeV). Dosimetric measurements were taken in a water phantom that was irradiated with a proton beam emanating from the gantry system at angles 0, 90, 180 and 270 degree using a large and a medium snout. The range of beam energies in the phantom, their corresponding centre modulation depth (CMD) and the width of spread out Bragg peak (SOBP) were measured by Markus chamber. Double scattering technique was employed for the creation of SOBPs.ResultsThe range of proton beams varied from 4·3 cm for 100 MeV beam to 28·5 cm for 250 MeV beam with the medium snout and from 4·3 cm for 100 MeV to 25 cm for 250 MeV beam with large snout in the water phantom. SOBP width showed a variation from 4 to 10 cm with medium and large snout. While determining the output with medium snout, the discrepancy of 1·1% was observed between the maximum and minimum mean values of output for all the given set of energies and angles. There occurred a difference of 0·9% between the maximum and minimum mean values of output with the large snout. Beam output at SOBP centre was 12% higher with large snout as compared to that with medium snout for all the given beam energies. Flatness and symmetry were found within ±2·5% tolerance limits with medium and large snouts.ConclusionFlatness and symmetry were found within explicit limits with both medium and large snouts. Large snout produced higher beam output than that of medium snout at the centre of SOBP. This exploration can be extended to the determination of beam output, flatness and symmetry with a small snout.

Luminescence imaging of water during uniform-field irradiation by spot scanning proton beams

Luminescence was found during pencil-beam proton irradiation to water phantom and range could be estimated from the luminescence images. However, it is not yet clear whether the luminescence imaging is applied to the uniform fields made of spot-scanning proton-beam irradiations. For this purpose, imaging was conducted for the uniform fields having spread out Bragg peak (SOBP) made by spot scanning proton beams. We designed six types of the uniform fields with different ranges, SOBP widths and irradiation fields. One of the designed fields was irradiated to water phantom and a cooled charge coupled device camera was used to measure the luminescence image during irradiations. We estimated the ranges, field widths, and luminescence intensities from the luminescence images and compared those with the dose distribution calculated by a treatment planning system. For all types of uniform fields, we could obtain clear images of the luminescence showing the SOBPs. The ranges and field widths evaluated from the luminescence were consistent with those of the dose distribution calculated by a treatment planning system within the differences of −4 mm and −11 mm, respectively. Luminescence intensities were almost proportional to the SOBP widths perpendicular to the beam direction. The luminescence imaging could be applied to uniform fields made of spot scanning proton beam irradiations. Ranges and widths of the uniform fields with SOBP could be estimated from the images. The luminescence imaging is promising for the range and field width estimations in proton therapy.

Determination of the Range and Spread-Out Bragg Peak Width of Proton Beams Using a Large-Volume Liquid Scintillator.

To determine the range, spread-out Bragg peak (SOBP) width, and output of a passive-scattering proton beam with a liquid scintillator detector, without the need for quenching correction. The depth-light profiles of 3 beam energies (140, 160, and 180 MeV) with 6 SOBP widths at each energy, produced in a 20 × 20 × 20-cm3 liquid scintillator tank, were collected by a charge-coupled device camera. By defining landmarks on the light signals, measured ranges and SOBP widths were acquired. A linear dependence was found between nominal and measured properties, and calibration factors were obtained by comparing those properties. The daily output stability and reproducibility of the liquid scintillator detector were studied by conducting 8 repeated measurements over 2 weeks in a 60Co beam. The beam ranges were determined with submillimeter accuracy without the need for any correction. The maximum difference between the measured and nominal range was 1.0 mm. The mean difference between the measured and nominal SOBP widths after correction was 0.1 mm (σ = 1.8 mm), with a maximum difference of 3.5 mm. The light output was reproducible with an SD of 0.14%. The method described here makes it possible to quickly and accurately determine the range and SOBP width of a passive-scattering proton beam in a liquid scintillator, without the need for quenching correction. In addition, the detector proved to be reliable over time, showing good output consistency with a high degree of precision.

TOPAS Simulation of the Mevion S250 compact proton therapy unit.

As proton therapy becomes increasingly popular, so does the need for Monte Carlo simulation studies involving accurate beam line modeling of proton treatment units. In this study, the 24 beam configurations of the Mevion S250 proton therapy system installed recently at our institution were modeled using the TOolkit for PArticle Simulation (TOPAS) code. Pristine Bragg peak, spread out Bragg peak (SOBP), and lateral beam profile dose distributions were simulated and matched to the measurements taken during commissioning of the unit. Differences in the range for all Percent Depth Dose (PDD) curves between measured and simulated data agreed to within 0.1 cm. For SOBP scans, the SOBP widths all agreed to within 0.3 cm. With regards to lateral beam profile comparisons between the measured and simulated data, the penumbras differed by less than 1 mm and the flatness differed by less than 1% in nearly all cases. This study shows that Monte Carlo simulation studies involving the Mevion S250 proton therapy unit can be a viable tool in commissioning and verification of the proton treatment planning system.

Evaluation of the influence of double and triple Gaussian proton kernel models on accuracy of dose calculations for spot scanning technique.

The main purpose in this study was to present the results of beam modeling and how the authors systematically investigated the influence of double and triple Gaussian proton kernel models on the accuracy of dose calculations for spot scanning technique. The accuracy of calculations was important for treatment planning software (TPS) because the energy, spot position, and absolute dose had to be determined by TPS for the spot scanning technique. The dose distribution was calculated by convolving in-air fluence with the dose kernel. The dose kernel was the in-water 3D dose distribution of an infinitesimal pencil beam and consisted of an integral depth dose (IDD) and a lateral distribution. Accurate modeling of the low-dose region was important for spot scanning technique because the dose distribution was formed by cumulating hundreds or thousands of delivered beams. The authors employed a double Gaussian function as the in-air fluence model of an individual beam. Double and triple Gaussian kernel models were also prepared for comparison. The parameters of the kernel lateral model were derived by fitting a simulated in-water lateral dose profile induced by an infinitesimal proton beam, whose emittance was zero, at various depths using Monte Carlo (MC) simulation. The fitted parameters were interpolated as a function of depth in water and stored as a separate look-up table. These stored parameters for each energy and depth in water were acquired from the look-up table when incorporating them into the TPS. The modeling process for the in-air fluence and IDD was based on the method proposed in the literature. These were derived using MC simulation and measured data. The authors compared the measured and calculated absolute doses at the center of the spread-out Bragg peak (SOBP) under various volumetric irradiation conditions to systematically investigate the influence of the two types of kernel models on the dose calculations. The authors investigated the difference between double and triple Gaussian kernel models. The authors found that the difference between the two studied kernel models appeared at mid-depths and the accuracy of predicting the double Gaussian model deteriorated at the low-dose bump that appeared at mid-depths. When the authors employed the double Gaussian kernel model, the accuracy of calculations for the absolute dose at the center of the SOBP varied with irradiation conditions and the maximum difference was 3.4%. In contrast, the results obtained from calculations with the triple Gaussian kernel model indicated good agreement with the measurements within ±1.1%, regardless of the irradiation conditions. The difference between the results obtained with the two types of studied kernel models was distinct in the high energy region. The accuracy of calculations with the double Gaussian kernel model varied with the field size and SOBP width because the accuracy of prediction with the double Gaussian model was insufficient at the low-dose bump. The evaluation was only qualitative under limited volumetric irradiation conditions. Further accumulation of measured data would be needed to quantitatively comprehend what influence the double and triple Gaussian kernel models had on the accuracy of dose calculations.

A comparison study of the ridge filter parameter by using FLUKA and GEANT4 simulation codes

We investigated the parameter optimization of the ridge filter’s thickness for carbon-ion therapy by using a Monte Carlo simulation. For this study, a ridge filter was designed for the spreadout Bragg peak (SOBP) by considering the relative biological effectiveness (RBE). The thickness, height, and width of the ridge filter were designed by using the FLUKA and the GEANT4 codes, and we analyzed and compared the results of the physical dose distributions for the FLUKA and the GEANT4 codes. The results showed that the minimum width of the groove for the ridge filter should be at least 0.5 cm for an appropriate biological dose. The SOBP sections were 8 cm, 9 cm, and 10 cm, respectively, when the heights were 3.5 cm, 4.0 cm, and 4.5 cm. The height of the ridge filter was designed to be associated with the SOBP width. Also, the results for the FLUKA and the GEANT4 codes showed that the average value of the difference was 3% and that the maximum error was 5%; however, the trends were similar. Therefore, the height and the width of the groove for the ridge filter are important parameters for deciding the length and the plateau of the SOBP.

MO‐F‐CAMPUS‐T‐03: Verification of Range, SOBP Width, and Output for Passive‐Scattering Proton Beams Using a Liquid Scintillator Detector

Purpose:Liquid scintillators have been shown to provide fast and high‐resolution measurements of radiation beams. However, their linear energy transfer‐dependent response (quenching) limits their use in proton beams. The purpose of this study was to develop a simple and fast method to verify the range, spread‐out Bragg peak (SOBP) width, and output of a passive‐scattering proton beam with a liquid scintillator detector, without the need for quenching correction.Methods:The light signal from a 20×20×20 cm3 liquid scintillator tank was collected with a CCD camera. Reproducible landmarks on the SOBP depth‐light curve were identified which possessed a linear relationship with the beam range and SOBP width. The depth‐light profiles for three beam energies (140, 160 and 180 MeV) with six SOBP widths at each energy were measured with the detector. Beam range and SOBP width calibration factors were obtained by comparing the depth‐light curve landmarks with the nominal range and SOBP width for each beam setting. The daily output stability of the liquid scintillator detector was also studied by making eight repeated output measurements in a cobalt‐60 beam over the course of two weeks.Results:The mean difference between the measured and nominal beam ranges was 0.6 mm (σ=0.2 mm), with a maximum difference of 0.9 mm. The mean difference between the measured and nominal SOBP widths was 0.1 mm (σ=1.8 mm), with a maximum difference of 4.0 mm. Finally an output variation of 0.14% was observed for 8 measurements performed over 2 weeks.Conclusion:A method has been developed to determine the range and SOBP width of a passive‐scattering proton beam in a liquid scintillator without the need for quenching correction. In addition to providing rapid and accurate beam range and SOBP measurements, the detector is capable of measuring the output consistency with a high degree of precision.This project was supported in part by award number CA182450 from the National Cancer Institute.

TH-CD-BRA-06: Proton Beam Entrance Dosimetry with the Exradin W1 Plastic Scintillation Detector

Purpose: To assess the feasibility of using a commercial plastic scintillation detector, the Exradin W1, for precise, accurate entrance dosimetry in a passively scattered proton beam Methods: Measurements of passively scattered proton beam entrance dose were performed using an Exradin W1 calibrated on a Cobalt-60 unit, and a calibrated parallel plane ion chamber at energies from 140 to 250 MeV. A fixed spread out Bragg peak (SOBP) width of 8 cm was used at all energies except 140 and 225 MeV, where selected SOBP widths spanning all those available were used. Comparisons between the ion chamber and W1 measured dose were used to determine whether and how much the W1 under-responded in the presence of the high linear energy transfer radiation (a phenomenon called ionization quenching, well known to occur at depth in proton beams where LET reaches its maximum). The W1’s precision was also evaluated by taking the coefficient of variation of repeated measurements. Results: From 200 MeV to 250 MeV the W1 under-responded by approximately 7% ±1%, and from 140 MeV to 180 MeV it under-responded by approximately 10% ±1%. The under-response was consistent within these bounds and negligibly affected by the SOBP width. An empirical correction factor with only two discrete values (one corresponding to energies greater than or equal to 200 MeV, and another for lower energies) was sufficient to produce better than 1% agreement between the W1 and ion chamber for all measurements. The W1 exhibited excellent precision, better than 1% at all energies but for the lowest where it was less than 2%. Conclusion: The Exradin W1 was found to be capable of precise, accurate entrance dosimetry with the application of a simple correction factor based on proton beam energy. This project was supported in part by award number CA182450 from the National Cancer Institute.

SU‐E‐T‐08: A Beam Source Model for Monte Carlo Simulations of a Double‐Scattering Proton Beam Delivery System Using Beam Current Modulation

Purpose:To reconstruct phase‐space information upstream of patient specific collimators for Monte Carlo simulations using only radiotherapy planning system data.Methods:The proton energies are calculated based on residual ranges, e.g., sum of prescribed ranges in a patient and SSD. The Kapchinskij and Vladimirskij (KV) distribution was applied to sample proton's x‐y positions and momentum direction and the beam shape was assumed to be a circle. Free parameters, e.g., the initial energy spread and the emittance of KV distribution were estimated from the benchmarking with commissioning data in a commercial treatment planning system for an operational proton therapy center. The number of histories, which defines the height of individual pristine Bragg peaks (BP) of Spread‐out Bragg peak (SOBP), are weighted based on beam current modulation and a correction factor is applied to take into account the fluence reduction as the residual range decreases due to the rotation of the range modulator wheel. The timedependent behaviors, e.g., the changes of the residual range and histories per a pristine BP, are realized by utilizing TOPAS (Tool for Particle Simulation).Results:Benchmarking simulations for selected SOBPs ranging 7.5 cm to 15.5 cm matched within 2 mm in range and up to 5 mm in SOBP width against measurement data in water phantom. We found this model tends to underestimate entrance dose by about 5 % in comparison to measurement. This was attributed to the situation that the energy distribution used in the model was limited in its granularity at the limit of single energy spectrum for the narrow angle modulator steps used in the proximal pull back region of the SOBPs.Conclusion:Within these limitations the source modeling method proved itself an acceptable alternative of a full treatment head simulation when the machine geometry and materials information are not available.

Reformulation of a clinical-dose system for carbon-ion radiotherapy treatment planning at the National Institute of Radiological Sciences, Japan

At the National Institute of Radiological Sciences (NIRS), more than 8,000 patients have been treated for various tumors with carbon-ion (C-ion) radiotherapy in the past 20 years based on a radiobiologically defined clinical-dose system. Through clinical experience, including extensive dose escalation studies, optimum dose-fractionation protocols have been established for respective tumors, which may be considered as the standards in C-ion radiotherapy. Although the therapeutic appropriateness of the clinical-dose system has been widely demonstrated by clinical results, the system incorporates several oversimplifications such as dose-independent relative biological effectiveness (RBE), empirical nuclear fragmentation model, and use of dose-averaged linear energy transfer to represent the spectrum of particles. We took the opportunity to update the clinical-dose system at the time we started clinical treatment with pencil beam scanning, a new beam delivery method, in 2011. The requirements for the updated system were to correct the oversimplifications made in the original system, while harmonizing with the original system to maintain the established dose-fractionation protocols. In the updated system, the radiation quality of the therapeutic C-ion beam was derived with Monte Carlo simulations, and its biological effectiveness was predicted with a theoretical model. We selected the most used C-ion beam with αr = 0.764 Gy−1 and β = 0.0615 Gy−2 as reference radiation for RBE. The C-equivalent biological dose distribution is designed to allow the prescribed survival of tumor cells of the human salivary gland (HSG) in entire spread-out Bragg peak (SOBP) region, with consideration to the dose dependence of the RBE. This C-equivalent biological dose distribution is scaled to a clinical dose distribution to harmonize with our clinical experiences with C-ion radiotherapy. Treatment plans were made with the original and the updated clinical-dose systems, and both physical and clinical dose distributions were compared with regard to the prescribed dose level, beam energy, and SOBP width. Both systems provided uniform clinical dose distributions within the targets consistent with the prescriptions. The mean physical doses delivered to targets by the updated system agreed with the doses by the original system within ±1.5% for all tested conditions. The updated system reflects the physical and biological characteristics of the therapeutic C-ion beam more accurately than the original system, while at the same time allowing the continued use of the dose-fractionation protocols established with the original system at NIRS.