Passive Scattering Proton Beam Research Articles

An Assessment of the Secondary Neutron Dose in the Passive Scattering Proton Beam Facility of the National Cancer Center

Abstract The purpose of this study is to assess the additional neutron effective dose during passive scattering proton therapy. Monte Carlo code (Monte Carlo N-Particle 6) simulation was conducted based on a precise modeling of the National Cancer Center's proton therapy facility. A three-dimensional neutron effective dose profile of the interior of the treatment room was acquired via a computer simulation of the 217.8-MeV proton beam. Measurements were taken with a 3 He neutron detector to support the simulation results, which were lower than the simulation results by 16% on average. The secondary photon dose was about 0.8% of the neutron dose. The dominant neutron source was deduced based on flux calculation. The secondary neutron effective dose per proton absorbed dose ranged from 4.942 ± 0.031 mSv/Gy at the end of the field to 0.324 ± 0.006 mSv/Gy at 150 cm in axial distance.

Evaluation of the relative biological effectiveness of spot-scanning proton irradiation in vitro.

Variations in relative biological effectiveness (RBE) from a fixed value of 1.1 are critical in proton beam therapy. To date, studies estimating RBE at multiple positions relative to the spread-out Bragg peak (SOBP) have been predominantly performed using passive scattering methods, and limited data are available for spot-scanning beams. Thus, to investigate the RBE of spot-scanning beams, Chinese hamster fibroblast V79 cells were irradiated using the beam line at the Hokkaido University Hospital Proton Therapy Center. Cells were placed at six different depths, including the entrance of the proton beam and the proximal and distal part of the SOBP. Surviving cell fractions were analyzed using colony formation assay, and cell survival curves were obtained by the curve fitted using a linear–quadratic model. RBE10 and RBE37 were 1.15 and 1.21 at the center of the SOBP, respectively. In contrast, the distal region showed higher RBE values (1.50 for RBE10 and 1.85 for RBE37). These results are in line with those of previous studies conducted using passive scattering proton beams. Taken together, these data strongly suggest that variations in RBE should be considered during treatment planning for spot-scanning beams as well as for passive scattering proton beams.

Spot Scanning and Passive Scattering Proton Therapy: Relative Biological Effectiveness and Oxygen Enhancement Ratio in Cultured Cells

To determine the relative biological effectiveness (RBE), oxygen enhancement ratio (OER), and contribution of the indirect effect of spot scanning proton beams, passive scattering proton beams, or both in cultured cells in comparison with clinically used photons. The RBE of passive scattering proton beams at the center of the spread-out Bragg peak (SOBP) was determined from dose-survival curves in 4 cell lines using 6-MV X rays as controls. Survival of 2 cell lines after spot scanning and passive scattering proton irradiation was then compared. Biological effects at the distal end region of the SOBP were also investigated. The OER of passive scattering proton beams and 6 MX X rays were investigated in 2 cell lines. The RBE and OER values were estimated at a 10% cell survival level. The maximum degree of protection of radiation effects by dimethyl sulfoxide was determined to estimate the contribution of the indirect effect against DNA damage. All experiments comparing protons and X rays were made under the same biological conditions. The RBE values of passive scattering proton beams in the 4 cell lines examined were 1.01 to 1.22 (average, 1.14) and were almost identical to those of spot scanning beams. Biological effects increased at the distal end of the SOBP. In the 2 cell lines examined, the OER was 2.74 (95% confidence interval, 2.56-2.80) and 3.08 (2.84-3.11), respectively, for X rays, and 2.39 (2.38-2.43) and 2.72 (2.69-2.75), respectively, for protons (P<.05 for both cells between X rays and protons). The maximum degree of protection was significantly higher for X rays than for proton beams (P<.05). The RBE values of spot scanning and passive scattering proton beams were almost identical. The OER was lower for protons than for X rays. The lower contribution of the indirect effect may partly account for the lower OER of protons.

Dosimetric Characteristics of a Two-Dimensional Diode Array Detector Irradiated with Passively Scattered Proton Beams

Purpose: To evaluate the dosimetric characteristics of a two-dimensional (2D) diode array detector irradiated with passively scattered proton beams. Materials and Methods: A diode array detector, MapCHECK (Model 1175, Sun Nuclear, Melbourne, FL, USA) was characterized in passive-scattered proton beams. The relative sensitivity of the diodes and absolute dose calibration were determined using a 250 MeV beam. The pristine Bragg curves (PBCs) measured by MapCHECK diodes were compared with those of an ion chamber using a range shift method. The water-equivalent thickness (WET) of the diode array detector’s intrinsic buildup also was determined. The inverse square dependence, linearity, and other proton dosimetric quantities measured by MapCHECK were also compared with those of the ion chambers. The change in the absolute dose response of the MapCHECK as a function of accumulated radiation dose was used as an indicator of radiation damage to the diodes. 2D dose distribution with and without the compensator were measured and compared with the treatment planning system (TPS) calculations. Results: The WET of the MapCHECK diode’s buildup was determined to be 1.7 cm. The MapCHECK-measured PBC were virtually identical to those measured by a parallel-plate ion chamber for 160, 180, and 250 MeV proton beams. The inverse square results of the MapCHECK were within ±0.4% of the ion chamber results. The linearity of MapCHECK results was within 1% of those from the ion chamber as measured in the range between 10 and 300 MU. All other dosimetric quantities were within 1.3% of the ion chamber results. The 2D dose distributions for non-clinical fields without compensator and the patient treatment fields with the compensator were consistent with the TPS results. The absolute dose response of the MapCHECK was changed by 7.4% after an accumulated dose increased by 170 Gy. Conclusions: The MapCHECK is a convenient and useful tool for 2D dose distribution measurements using passively scattered proton beams. Variations in MapCHECK’s dose response with increasing levels of total accumulated radiation dose should be carefully monitored.

A Recommendation on How to Analyze In-Room PET for In Vivo Proton Range Verification Using a Distal PET Surface Method

We describe the rationale and implementation of a method for analyzing in-room positron emission tomography (PET) data to verify the proton beam range. The method is based on analyzing distal PET surfaces after passive scattering proton beam delivery. Typically in vivo range verification is done by comparing measured and predicted PET distribution for a single activity level at a selected activity line along the beam passage. In the method presented here, we suggest using a middle point method based on dual PET activity levels to minimize the uncertainty due to local variations in the PET activity. Furthermore, we introduce 2-dimensional (2D) PET activity level surfaces based on 3-dimensional maps of the PET activities along the beam passage. This allows determining not only average range differences but also range difference distributions as well as root mean square deviations (RMSDs) for a more comprehensive range analysis. The method is demonstrated using data from 8 patients who were scanned with an in-room PET scanner. For each of the 8 patients, the average range difference was less than 5 mm and the RMSD was 4 to 11 mm between the measured and simulated PET activity level surfaces for single-field treatments. An ongoing protocol at our institution allows the use of a single field for patients being imaged for the PET range verification study at 1 fraction during their treatment course. Visualizing the range difference distributions using the PET surfaces offers a convenient visual verification of range uncertainties in 2D. Using the distal activity level surfaces of simulated and measured PET distributions at the middle of 25% and 50% activity level is a robust method for in vivo range verification.

WE-F-16A-03: 3D Printer Application in Proton Therapy: A Novel Method to Deliver Passive-Scattering Proton Beams with a Fixed Range and Modulation for SRS and SRT

Purpose: To present a novel technique to deliver passive-scattering proton beam with fixed range and modulation using a 3D printed patient-specific bolus for proton stereotactic radiosurgery and radiotherapy. Methods: A CIRS head phantom was used to simulate a patient with a small brain lesion. A custom bolus was created in the Eclipse Treatment Planning System (TPS) to compensate for the different water equivalent depths from the patient surface to the target from multiple beam directions. To simulate arc therapy, a plan was created on the initial CT using three passive-scattering proton beams with a fixed range and modulations irradiating from different angles. The DICOM-RT structure file of the bolus was exported from the TPS and converted to STL format for 3D printing. The phantom was rescanned with the printed custom bolus and head cup to verify the dose distribution comparing to the initial plan. EBT3 films were placed in the sagital plane of the target to verify the delivered dose distribution. The relative stopping power of the printing material(ABSplus-P430) was measured using the Zebra multi-plate ion chamber. Results: The relative stopping power of the 3D printing material, ABSplus-P430 was 1.05 which is almost water equivalent. The dose difference between verificationmore » CT and Initial CT is almost negligible. Film measurement also confirmed the accuracy for this new proton delivery technique. Conclusion: Our method using 3D printed range modifiers simplify the treatment delivery of multiple passive-scattering beams in treatment of small lesion in brain. This technique makes delivery of multiple beam more efficient and can be extended to allow arc therapy with proton beams. The ability to create and construct complex patient specific bolus structures provides a new dimension in creating optimized quality treatment plans not only for proton therapy but also for electron and photon therapy.« less

SU‐E‐T‐493: Analysis of the Impact of Range and Setup Uncertainties On the Dose to Brain Stem and Whole Brain in the Passively Scattered Proton Therapy Plans

Purpose:To quantify the impact of range and setup uncertainties on various dosimetric indices that are used to assess normal tissue toxicities of patients receiving passive scattering proton beam therapy (PSPBT).Methods:Robust analysis of sample treatment plans of six brain cancer patients treated with PSPBT at our facility for whom the maximum brain stem dose exceeded 5800 CcGE were performed. The DVH of each plan was calculated in an Eclipse treatment planning system (TPS) version 11 applying ±3.5% range uncertainty and ±3 mm shift of the isocenter in x, y and z directions to account for setup uncertainties. Worst‐case dose indices for brain stem and whole brain were compared to their values in the nominal plan to determine the average change in their values. For the brain stem, maximum dose to 1 cc of volume, dose to 10%, 50%, 90% of volume (D10, D50, D90) and volume receiving 6000, 5400, 5000, 4500, 4000 CcGE (V60, V54, V50, V45, V40) were evaluated. For the whole brain, maximum dose to 1 cc of volume, and volume receiving 5400, 5000, 4500, 4000, 3000 CcGE (V54, V50, V45, V40 and V30) were assessed.Results:The average change in the values of these indices in the worst scenario cases from the nominal plan were as follows. Brain stem; Maximum dose to 1 cc of volume: 1.1%, D10: 1.4%, D50: 8.0%, D90:73.3%, V60:116.9%, V54:27.7%, V50: 21.2%, V45:16.2%, V40:13.6%,Whole brain; Maximum dose to 1 cc of volume: 0.3%, V54:11.4%, V50: 13.0%, V45:13.6%, V40:14.1%, V30:13.5%.Conclusion:Large to modest changes in the dosiemtric indices for brain stem and whole brain compared to nominal plan due to range and set up uncertainties were observed. Such potential changes should be taken into account while using any dosimetric parameters for outcome evaluation of patients receiving proton therapy.

The effects of titanium mesh on passive-scattering proton dose

High-density metallic implants can introduce considerable uncertainties in proton therapy treatment planning. These uncertainties eventually translate into proton range errors, which may cause significant underdosing to the target volume or overdosing to normal tissue beyond the target. This study investigated the dosimetric impact of a 0.6 mm titanium (Ti) mesh implant in passive-scattering proton beam therapy through the study of the depth dose and output in water, and the dose profiles in solid water at various depths. The measurements were performed for a beam with a range of 8.5 cm and a modulation of 7.5 cm. The titanium mesh was placed at a depth of 1 cm below the surface of the phantom for all measurements. A range reduction of 0.5 ± 0.1 mm was observed for a beam perpendicular to the mesh, with no further reductions when the incident angle increased to 60°. We conclude that the dosimetric effect of a 0.6 mm titanium mesh implant is small for a passive scattering proton beam. With proper correction applied to metal artifacts, consistent results were observed in the phantom study in the treatment planning system.

Evaluation of neutron dose equivalent from the Mevion S250 proton accelerator: measurements and calculations

Neutron production is of concern for proton therapy, especially for passive scattering proton beam delivery methods. The levels of neutron dose equivalent vary significantly with system design and treatment parameters. The purpose of this study was to examine neutron dose equivalent per therapeutic dose (H/D) around the Mevion S250 proton therapy system, a novel design of proton therapy systems. The benchmark comparisons between measurement and simulation were found to be within a factor of 2 for most cases. The H/D values were evaluated as a function of various parameters. The results showed that, at a standard reference condition (10 × 10 cm2 field size, distance 1 m detector-to-isocenter lateral to the primary proton beam direction), the H/D values range from 0.72 to 3.37 mSv Gy−1 for all configurations studied. The H/D values generally (1) decreased as the neutron detectors moved away from the isocenter, (2) decreased with increasing aperture field sizes, (3) increased with increasing angle from the initial beam axis and (4) were independent of treatment nozzle position. The H/D trends were consistent with other existing passive scattering proton accelerators reported in the literature.

Quality of Life and Toxicity From Passively Scattered and Spot-Scanning Proton Beam Therapy for Localized Prostate Cancer

To report quality of life (QOL)/toxicity in men treated with proton beam therapy for localized prostate cancer and to compare outcomes between passively scattered proton therapy (PSPT) and spot-scanning proton therapy (SSPT). Men with localized prostate cancer enrolled on a prospective QOL protocol with a minimum of 2 years' follow-up were reviewed. Comparative groups were defined by technique (PSPT vs SSPT). Patients completed Expanded Prostate Cancer Index Composite questionnaires at baseline and every 3-6 months after proton beam therapy. Clinically meaningful differences in QOL were defined as ≥0.5 × baseline standard deviation. The cumulative incidence of modified Radiation Therapy Oncology Group grade ≥2 gastrointestinal (GI) or genitourinary (GU) toxicity and argon plasma coagulation were determined by the Kaplan-Meier method. A total of 226 men received PSPT, and 65 received SSPT. Both PSPT and SSPT resulted in statistically significant changes in sexual, urinary, and bowel Expanded Prostate Cancer Index Composite summary scores. Only bowel summary, function, and bother resulted in clinically meaningful decrements beyond treatment completion. The decrement in bowel QOL persisted through 24-month follow-up. Cumulative grade ≥2 GU and GI toxicity at 24 months were 13.4% and 9.6%, respectively. There was 1 grade 3 GI toxicity (PSPT group) and no other grade ≥3 GI or GU toxicity. Argon plasma coagulation application was infrequent (PSPT 4.4% vs SSPT 1.5%; P=.21). No statistically significant differences were appreciated between PSPT and SSPT regarding toxicity or QOL. Both PSPT and SSPT confer low rates of grade ≥2 GI or GU toxicity, with preservation of meaningful sexual and urinary QOL at 24 months. A modest, yet clinically meaningful, decrement in bowel QOL was seen throughout follow-up. No toxicity or QOL differences between PSPT and SSPT were identified. Long-term comparative results in a larger patient cohort are warranted.

SU-E-T-04: Application of a Semi-Empirical Formula to Determine Spread Out Bragg Peak Factors and Range Shifter Factors From Measured Depth Dose Curves for Passively Scattered Proton Beams

Purpose: To apply two semi-empirical formulas to calculate the spread out Bragg peak ( SOBP ) factors ( SOBPFs ) and range shifter ( RS ) factors (RSFs) used for monitor unit calculation1 of passively scattered proton beams (PSPB). Methods: Following two recently developed2 and refined formulas were used to calculate the SOBPFs and RSFs from measured depth dose curves for various options in our PSPB. For a specific option,SOBPF(W)=PDDWr(0)/PDDW(0).((SSD+dcW)/(SSD+dcWr))2 .NSF ( W).For the same SOBP width and option, but for different RS thickness, t,RSF (W, t) = (( SSD+dcWt) /)( SSD+dcW)) 2.( PDDW( 0) /PDDW(t)). NSF(W,t),W is the width of the SOBP under consideration, Wr is the reference SOBP width relative to which other SOBPF are determined, SSD is the source to surface distance for the percentage depth dose (PDD) measurement, PDDWr(0) and PDDW(0) are PDDs at the surface, and dcW and dcWr are the depths of the centers of SOBP under consideration and reference SOBP respectively, t is the RS thickness, dcWt and dcW are the depths of the centers of SOBP under consideration with RS of thickness t and without any RS respectively, and PDDw(t) and PDDw(0) are the PDD at depth t and surface respectively for the SOBP width W for which the RSFs are being determined. NSF(W) and NSF (W,t) accounts for the contribution of the nozzle elements including the aperture to the back scatter dose at the dose monitor. Results: The SOBPFs and RSFs calculated using above formulas agreed within 2 percent with their respective measured values using an ionization chamber in a water tank for all of our clinically used options for PSPB. Conclusion: The proposed formulas were found to be valid to calculate the required SOBPFs and RSFs for monitor unit calculation of PSPB fields from measured PDD curves. 1N. Sahoo et. al, Med. Phys. 35, 5088, (2008).2N. Sahoo et. al, IJROBP, 84(3S) S849, (2012).

SU‐C‐105‐04: Measurement of Absorbed Dose Due to Secondary Radiation Produced by Passively Scattered and Uniformly Scanned Proton Beams

Purpose: Using the dual ionization chamber (IC) method we compare the neutron and combined gamma + proton (γ+p) components of absorbed dose in water due to secondary radiation produced by passively scattered and uniformly scanned proton beams collimated by a tungsten MLC. Methods: We adapt the dual IC method of mixed field dosimetry for use in measuring the components of absorbed dose due to secondary radiation produced by proton beams. Measurements are performed in water 5 cm beyond the distal edge and 15 cm lateral to the beam isocenter for passively scattered/uniformly scanned proton beams. Beams with ranges of 15 cm or 25 cm and modulation 10 cm (R15M10 and R25M10) are employed, collimated to 10 × 10 cm field size at isocenter with the MLC. Results: Measured neutron dose per primary proton dose ranges from 0.2–0.4 mGy/Gy for R15M10 and 0.3–0.8 mGy/Gy for R25M10 beams. The secondary γ+p dose is 0.3–0.7 mGy/Gy for R15M10 and 0.5–2.0 mGy/Gy for R25M10. Doses are reported here as physical dose without application of a radiation quality factor. Conclusion: The largest difference in secondary radiation dose for the two modalities is seen at 5 cm beyond the distal edge for R15M10, with increases of 40–50% for neutron dose and 20% for γ+p for scattered over scanned beams. For the R25M10 beam the difference is smaller, about 10% higher neutron and γ+p doses for scattered over scanned beams. Outside the beam at 15 cm lateral to isocenter, the γ+p dose dominates the neutron dose and smaller differences are observed between the two beam delivery modalities. This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17‐W81XWH‐07‐2‐0121. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the US Army. This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17‐W81XWH‐07‐2‐0121. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the US Army.

Determination of the quenching correction factors for plastic scintillation detectors in therapeutic high-energy proton beams

Plastic scintillation detectors (PSDs) have many advantages over other detectors in small field dosimetry due to their high spatial resolution, excellent water equivalence and instantaneous readout. However, in proton beams, the PSDs undergo a quenching effect which makes the signal level reduced significantly when the detector is close to the Bragg peak where the linear energy transfer (LET) for protons is very high. This study measures the quenching correction factor (QCF) for a PSD in clinical passive-scattering proton beams and investigates the feasibility of using PSDs in depth-dose measurements in proton beams. A polystyrene-based PSD (BCF-12, ϕ0.5 mm × 4 mm) was used to measure the depth-dose curves in a water phantom for monoenergetic unmodulated proton beams of nominal energies 100, 180 and 250 MeV. A Markus plane-parallel ion chamber was also used to get the dose distributions for the same proton beams. From these results, the QCF as a function of depth was derived for these proton beams. Next, the LET depth distributions for these proton beams were calculated by using the MCNPX Monte Carlo code, based on the experimentally validated nozzle models for these passive-scattering proton beams. Then the relationship between the QCF and the proton LET could be derived as an empirical formula. Finally, the obtained empirical formula was applied to the PSD measurements to get the corrected depth-dose curves and they were compared to the ion chamber measurements. A linear relationship between the QCF and LET, i.e. Birks' formula, was obtained for the proton beams studied. The result is in agreement with the literature. The PSD measurements after the quenching corrections agree with ion chamber measurements within 5%. PSDs are good dosimeters for proton beam measurement if the quenching effect is corrected appropriately.

Determination of Spread Out Bragg Peak Factors and Range Shifter Factors From Measured Depth Dose Curves for Passively Scattered Proton Beams

Monitor units of treatment fields of passively scattered proton beam (PSPB) can be determined from three major dosimetry factors, namely, relative output factors, spread out Bragg peak factors (SOBPFs) and range shifter factors (RSFs). As the SOBP widths and range shifter thickness of a PSPB field change, the percentage depth dose at the surface increases, but the incident proton fluence decreases due to the introduction of the additional material to the beam path leading to a decrease in the dose per monitor units.

SU-D-BRCD-01: Evaluation of Zebra Multi-Layer Ionization Chamber System for Patient Treatment Field and Machine QA for Spot Scanning and Passive Scattering Proton Beams.

To evaluate Zebra multi-layer ionization chamber system for patient treatment field and machine QA for spot scanning proton beams (SSPB) and passive scattering proton beams (PSPB). Zebra dose measurement system (IBA Dosimetry), consisting of 180 parallel platechambers with 2 mm detector spacing, was used for measuring proton beamdepth dose curves (DDC) for spread out Bragg peaks (SOBP) and single spot pristine Bragg peaks (PBP). The measurements were performed for 100 to 250 MeV PSPB and 89.2 to 221.8 MeV SSPB using the Hitachi ProBeat synchrotron based delivery system. An in-house Matlab based analysis software was used to compare the Zebra measured DDC with those measured by the Markus chamber in a PTW water tank (MC-WT). Several verification plans in the water phantom were created for patient treatment fields using the Eclipse treatment planning system (TPS). The DDC for individual verification fields were measured using the Zebra andcomparisons were made with the TPS calculations. The dosedifferences between the Zebra and MC-WT measurements in the plateau regions of the DDC are within 2% for various energies of PSPB, but are larger than 2% at the sharp dose distal gradient regions. The values for distal penumbra widths, range and SOBP widths from Zebra and MC-WT measurements agree within 0.5 mm, 1.5 mm, and 2 mm, respectively. The Zebra measured values of the range of the single spots also agreed within 1 mm with their established values from other measurements. The Zebra measured DDC of verification plan of patient treatment fields showed goodagreement with those from the TPS. Our investigation shows that Zebra can be useful for fast and reasonably accurate measurements of the DDC of pristine and spread-out Bragg peaks of both spot scanning and passive scattering proton beams.

SU-E-T-103: Three-Dimensional Measurements of Dose and LET from a Proton Beam via Polymer Gel Dosimetry.

To obtain and assess the accuracy of 3D dose distributions and LET maps from a passively-scattered proton beam through the use of LET-dependent and LET-independent polymer gel formulations. Dose measurements were performed with BANG3-Pro2 (MGS Research, Inc, Madison, CT) polymer gel dosimeters. The dosimeters were mixed from kits and were poured into 14.5 cm high by 15 cm diameter acrylic cylinders. For LET measurements, a new BANG3-Pro variant intended for use as an 'LET- meter' was employed. Initial irradiations were performed using a 200 MeV passively scattered proton beam with a 4 cm spread out Bragg peak (SOBP), delivering a physical dose of 3 Gy to the center of the SOBP. Dosimeters were read out using the OCTOPUS-IQ (MGS Research, Inc, Madison, CT) optical CT scanner. The optical densities measured in the gel were compared against ion chamber data to assess the accuracy of the dosimeter. Initial analysis indicates 80% of points measured along the central axis agreed with ion chamber data at the ±5%/±3mm level in the sensitive region of the gel. An artifact in the reconstructed image produced inaccurate readings beyond the depth of 50% dose on the distal edge of the SOBP, resulting in the majority of the agreement failures. BANG3-Pro2 polymer gel dosimeters demonstrate promise as a 3D dosimeter for use in proton therapy. The dosimetric data obtained to date will be used as the baseline measurement against which the LET-sensitive BANG3-Pro formulation will be compared for the measurement of proton LET.

Characteristics of optically stimulated luminescence dosimeters in the spread‐out Bragg peak region of clinical proton beams

Optically stimulated luminescent detectors (OSLDs) have a number of advantages in radiation dosimetry making them excellent dosimeters for quality assurance and patient dose verification. Although the dosimeters have been investigated in several modalities, relatively little work has been done in examining the dosimeters for use in clinical proton beams. This study examined a number of characteristics of the response of the dosimeters in the spread-out Bragg peak (SOBP) region of clinical proton beams. Optically stimulated luminescence (OSL) dosimeters from Landauer, Inc., specifically the nanoDot dosimeter, were investigated. These dosimeters were placed in a special phantom with a recess to fit the dosimeters without an air gap. Beams with nominal energies of 160, 200, and 250 MeV were used in the passively-scattered proton beam at the MD Anderson Cancer Center Proton Therapy Center. Dosimetric properties including linearity, field size dependence, energy dependence, residual signal as a function of cumulative dose, and postirradiation fading were investigated by taking measurements at the center of SOBPs. The dosimeters showed 1% supralinearity at 200 cGy and 5% supralinearity at 1000 cGy. No noticeable field size dependence of the detector was found for field sizes from 2 × 2 cm(2) to 18 × 18 cm(2). Residual signal as a function of cumulative dose showed a small increase for measurements up to 1000 cGy. Readout signal depletion of the dosimeters after consecutive readings showed a slightly larger depletion in protons for doses up to 500 cGy but not by a clinically significant amount. Within the center of various SOBP widths and proton energies the variation in response was less than 2%. An average beam quality factor of 1.089 with experimental standard deviation of 0.007 was determined and applied to the data such that the results were within 1.2% of ion chamber data. The nanoDot OSL dosimeter characteristics were studied in the SOBP region of clinical proton beams. To achieve accurate dosimetric readings, corrections to the dosimeter response were applied. Corrections tended to be minimal or broadly consistent. The nanoDot OSLD was found to be an acceptable dosimeter for measurement in the SOBP region for a range of clinical proton beams.

SU-E-T-360: A Novel Procedure to Determine the Dose Output Change with Spread Out Bragg Peak Width and Range Shift in Passively Scattered Proton Beams

Purpose: Two of the major factors affecting the change in output of a passively scattered proton beam are the spread‐out Bragg peak width factor (SOBPF) and range shifter factor (RSF), both of which are usually measured during clinical commissioning. The purpose of this study is to determine these two factors by numerically constructing the SOBP from experimentally measured pristine Bragg peak (PBP), which can serve as an independent monitor unit (MU) calculation procedure. Methods: SOBPs were constructed by superposition of PBPs that are shifted by the water‐equivalent thicknesses (WET) of the corresponding steps of the range modulation wheel. A least‐squares fit was used to determine the weightings of the shifted PBPs to form the required SOBPs. With a simplification in modeling that MU monitor in the nozzle receives dose proportional to the surface dose of the SOBP (higher by an inverse‐square factor), we can relate the MU to the dose at the center of SOBP and compute the SOBP factors for different SOBP widths. When the range shifter was added, the SOBP was shifted toward the surface by the WET of the range shifter and the RSF can also be calculated from the change in the surface dose.Results: The SOBP and range shifter factors were calculated for SOBPs with modulation widths between 4 and 16 cm of 250 and 200 MeV proton beams. The calculated SOBPF and RSF agreed within 2%, and 1.5% with the respective measured values. Conclusions: Starting with measured pristine Bragg peaks, SOBPs can be numerically constructed by simulating the range modulation process, and important dosimetry factors dependent on the modulation width and range shifters can be calculated within 2% accuracy.

TU‐G‐BRB‐08: 3D Proton Dose Distributions in an Anthropomorphic Lung Phantom, Comparing Monte Carlo, Film, and TLD Measurements

Purpose: Evaluate and determine the feasibility of using thoracic phantoms in the credentialing of proton therapy facilities, by using irradiations, treatment planning calculations and Monte Carlo simulations.Methods: The Radiological Physics Center (RPC, Houston, TX) has developed an anthropomorphic lung phantom to be used for credentialing of lung trials in proton therapy. The phantom has structures like heart, spinal cord, right and left lungs and a cylindrical target located in the middle of the left lung. EBT2 films were placed cutting the target on the axial, coronal and sagittal planes for relative dosimetry. The absolute dose was measured with LiF TLDs located in the center of the target. The proton stopping power ratio of each material used in the phantom as well the respective CT number was also evaluated for this work. Based on a CT image of the phantom we created a plan in ECLIPSE with two fields delivering 6 Gy RBE to the target. A customized table was used to translate the phantom CT numbers into stopping power ratio for the ECLIPSE plan. The treatment plan was also simulated in Monte Carlo using a customized version of MCNPX. The phantom was irradiated at University of Texas MD Anderson Proton Therapy Center, Houston, Texas. Profiles and the Gamma tool were used to compare the film measurements to ECLIPSE and MCNPX calculations. Results: The gamma agreement of the Monte Carlo and ECLIPSE calculations was above 85% in the GTV region for all planes. The profiles of the Monte Carlo showed a better agreement with film measurements than ECLIPSE calculations. We also identified LET dependence on film measurements. Conclusion: We concluded that this method can give accurate results for passive scattering proton beams for the GTV areas. We are conducting further studies to correct for the film LET response.This project is supported in part by P01CA021239 from the National Cancer Institute.

External-Beam Accelerated Partial Breast Irradiation Using Multiple Proton Beam Configurations

To explore multiple proton beam configurations for optimizing dosimetry and minimizing uncertainties for accelerated partial breast irradiation (APBI) and to compare the dosimetry of proton with that of photon radiotherapy for treatment of the same clinical volumes. Proton treatment plans were created for 11 sequential patients treated with three-dimensional radiotherapy (3DCRT) photon APBI using passive scattering proton beams (PSPB) and were compared with clinically treated 3DCRT photon plans. Monte Carlo calculations were used to verify the accuracy of the proton dose calculation from the treatment planning system. The impact of range, motion, and setup uncertainty was evaluated with tangential vs. en face beams. Compared with 3DCRT photons, the absolute reduction of the mean of V100 (the volume receiving 100% of prescription dose), V90, V75, V50, and V20 for normal breast using protons are 3.4%, 8.6%, 11.8%, 17.9%, and 23.6%, respectively. For breast skin, with the similar V90 as 3DCRT photons, the proton plan significantly reduced V75, V50, V30, and V10. The proton plan also significantly reduced the dose to the lung and heart. Dose distributions from Monte Carlo simulations demonstrated minimal deviation from the treatment planning system. The tangential beam configuration showed significantly less dose fluctuation in the chest wall region but was more vulnerable to respiratory motion than that for the en face beams. Worst-case analysis demonstrated the robustness of designed proton beams with range and patient setup uncertainties. APBI using multiple proton beams spares significantly more normal tissue, including nontarget breast and breast skin, than 3DCRT using photons. It is robust, considering the range and patient setup uncertainties.