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Title: SU-F-T-137: Out-Of-Beam Dose for a Compact Double-Scattering Proton Beam Therapy System

Abstract

Purpose: The out-of-beam dose is important for understanding the peripheral dose in radiation therapy. In proton radiotherapy, the study of out-of-beam dose is scarce and the treatment planning system (TPS) based on pencil beam algorithm cannot accurately predict the out-of-beam dose. This study investigates the out-of-beam dose for the single-room Mevion S250 double scattering proton therapy system using experimentally measured and treatment planning software generated data. The results are compared with those reported for conventional photon beam therapy. However, this study does not incorporate the neutron contribution in the scattered dose. Methods: A total of seven proton treatment plans were generated using Varian Eclipse TPS for three different sites (brain, lung, and pelvis) in an anthropomorphic phantom. Three field sizes of 5×5, 10×10, and 20×20 cm{sup 2} (lung only) with typical clinical range (13.3–22.8 g/cm{sup 2}) and modulation widths (5.3–14.0 g/cm{sup 2}) were used. A single beam was employed in each treatment plan to deliver a dose of 181.8 cGy (200.0 cGy (RBE)) to the selected target. The out-of-beam dose was measured at 2.0, 5.0, 10.0, and 15.0 cm from the beam edge in the phantom using a thimble chamber (PTW TN31010). Results: The out-of-beam dose generally increased with fieldmore » size, range, and volume irradiated. For all the plans, the scattered dose sharply fell off with distance. At 2.0 cm, the out-of-beam dose ranged from 0.35% to 2.16% of the delivered dose; however, the dose was clinically negligible (<0.3%) at a distance of 5.0 cm and greater. In photon therapy, the slightly greater out-of-beam dose was reported (TG36; 4%, 2%, and 1% for 2.0, 5.0, and 10.0 cm, respectively, using 6 MV beam). Conclusion: The measured out-of-beam dose in proton therapy excluding neutron contribution was observed higher than the TPS calculated dose and comparable to that of photon beam therapy.« less

Authors:
; ;  [1]
  1. University of Oklahoma Health Sciences Center, Oklahoma City, OK (United States)
Publication Date:
OSTI Identifier:
22642378
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; 61 RADIATION PROTECTION AND DOSIMETRY; COMPUTER CODES; PHOTON BEAMS; PLANNING; PROTON BEAMS; RADIATION DOSES; RADIOTHERAPY; SCATTERING

Citation Formats

Islam, M, Ahmad, S, and Jin, H. SU-F-T-137: Out-Of-Beam Dose for a Compact Double-Scattering Proton Beam Therapy System. United States: N. p., 2016. Web. doi:10.1118/1.4956273.
Islam, M, Ahmad, S, & Jin, H. SU-F-T-137: Out-Of-Beam Dose for a Compact Double-Scattering Proton Beam Therapy System. United States. doi:10.1118/1.4956273.
Islam, M, Ahmad, S, and Jin, H. 2016. "SU-F-T-137: Out-Of-Beam Dose for a Compact Double-Scattering Proton Beam Therapy System". United States. doi:10.1118/1.4956273.
@article{osti_22642378,
title = {SU-F-T-137: Out-Of-Beam Dose for a Compact Double-Scattering Proton Beam Therapy System},
author = {Islam, M and Ahmad, S and Jin, H},
abstractNote = {Purpose: The out-of-beam dose is important for understanding the peripheral dose in radiation therapy. In proton radiotherapy, the study of out-of-beam dose is scarce and the treatment planning system (TPS) based on pencil beam algorithm cannot accurately predict the out-of-beam dose. This study investigates the out-of-beam dose for the single-room Mevion S250 double scattering proton therapy system using experimentally measured and treatment planning software generated data. The results are compared with those reported for conventional photon beam therapy. However, this study does not incorporate the neutron contribution in the scattered dose. Methods: A total of seven proton treatment plans were generated using Varian Eclipse TPS for three different sites (brain, lung, and pelvis) in an anthropomorphic phantom. Three field sizes of 5×5, 10×10, and 20×20 cm{sup 2} (lung only) with typical clinical range (13.3–22.8 g/cm{sup 2}) and modulation widths (5.3–14.0 g/cm{sup 2}) were used. A single beam was employed in each treatment plan to deliver a dose of 181.8 cGy (200.0 cGy (RBE)) to the selected target. The out-of-beam dose was measured at 2.0, 5.0, 10.0, and 15.0 cm from the beam edge in the phantom using a thimble chamber (PTW TN31010). Results: The out-of-beam dose generally increased with field size, range, and volume irradiated. For all the plans, the scattered dose sharply fell off with distance. At 2.0 cm, the out-of-beam dose ranged from 0.35% to 2.16% of the delivered dose; however, the dose was clinically negligible (<0.3%) at a distance of 5.0 cm and greater. In photon therapy, the slightly greater out-of-beam dose was reported (TG36; 4%, 2%, and 1% for 2.0, 5.0, and 10.0 cm, respectively, using 6 MV beam). Conclusion: The measured out-of-beam dose in proton therapy excluding neutron contribution was observed higher than the TPS calculated dose and comparable to that of photon beam therapy.},
doi = {10.1118/1.4956273},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • Purpose: The purpose of this investigation is to determine if a single set of beam data, described by a minimal set of equations and fitting variables, can be used to commission different installations of a proton double-scattering system in a commercial pencil-beam dose calculation algorithm. Methods: The beam model parameters required to commission the pencil-beam dose calculation algorithm (virtual and effective SAD, effective source size, and pristine-peak energy spread) are determined for a commercial double-scattering system. These parameters are measured in a first room and parameterized as function of proton energy and nozzle settings by fitting four analytical equations tomore » the measured data. The combination of these equations and fitting values constitutes the golden beam data (GBD). To determine the variation in dose delivery between installations, the same dosimetric properties are measured in two additional rooms at the same facility, as well as in a single room at another facility. The difference between the room-specific measurements and the GBD is evaluated against tolerances that guarantee the 3D dose distribution in each of the rooms matches the GBD-based dose distribution within clinically reasonable limits. The pencil-beam treatment-planning algorithm is commissioned with the GBD. The three-dimensional dose distribution in water is evaluated in the four treatment rooms and compared to the treatment-planning calculated dose distribution. Results: The virtual and effective SAD measurements fall between 226 and 257 cm. The effective source size varies between 2.4 and 6.2 cm for the large-field options, and 1.0 and 2.0 cm for the small-field options. The pristine-peak energy spread decreases from 1.05% at the lowest range to 0.6% at the highest. The virtual SAD as well as the effective source size can be accurately described by a linear relationship as function of the inverse of the residual energy. An additional linear correction term as function of RM-step thickness is required for accurate parameterization of the effective SAD. The GBD energy spread is given by a linear function of the exponential of the beam energy. Except for a few outliers, the measured parameters match the GBD within the specified tolerances in all of the four rooms investigated. For a SOBP field with a range of 15 g/cm{sup 2} and an air gap of 25 cm, the maximum difference in the 80%–20% lateral penumbra between the GBD-commissioned treatment-planning system and measurements in any of the four rooms is 0.5 mm. Conclusions: The beam model parameters of the double-scattering system can be parameterized with a limited set of equations and parameters. This GBD closely matches the measured dosimetric properties in four different rooms.« less
  • Proton radiation therapy can deliver high radiation doses to tumors while sparing normal tissue. However, protons yield secondary neutron and gamma radiation that is difficult to detect, small in comparison to the prescribed dose, and not accounted for in most treatment planning systems. The risk for secondary malignancies after proton therapy may be dependent on the quality of this dose. Consequently, there is interest in characterizing the secondary radiation. Previously, we used the dual ionization chamber method to measure the separate absorbed dose from gamma-rays and neutrons secondary to the proton beam1, relying on characterization of ionization chamber response inmore » the unknown neutron spectrum from Monte Carlo simulation. We developed a procedure to use Shieldwerx activation foils, with neutron activation energies ranging from 0.025 eV to 13.5 MeV, to measure the neutron energy spectrum from double scattering (DS) and pencil beam scanning (PBS) protons outside of the treatment volume in a water tank. The activated foils are transferred to a NaI well chamber for gamma-ray spectroscopy and activity measurement. Since PBS treats in layers, the switching time between layers is used to correct for the decay of the activated foils and the relative dose per layer is assumed to be proportional to the neutron fluence per layer. MATLAB code was developed to incorporate the layer delivery and switching time into a calculation of foil activity, which is then used to determine the neutron energy fluence from tabulated foil activation energy thresholds.1. Diffenderfer et. al., Med. Phys., 38(11) 2011.« less
  • Hokkaido University and Hitachi Ltd. have started joint development of the Gated Spot Scanning Proton Therapy with Real-Time Tumor-Tracking System by integrating real-time tumor tracking technology (RTRT) and the proton therapy system dedicated to discrete spot scanning techniques under the {sup F}unding Program for World-Leading Innovative R and D on Science and Technology (FIRST Program){sup .} In this development, we have designed the synchrotron-based accelerator system by using the advantages of the spot scanning technique in order to realize a more compact and lower cost proton therapy system than the conventional system. In the gated irradiation, we have focused onmore » the issues to maximize irradiation efficiency and minimize the dose errors caused by organ motion. In order to understand the interplay effect between scanning beam delivery and target motion, we conducted a simulation study. The newly designed system consists of the synchrotron, beam transport system, one compact rotating gantry treatment room with robotic couch, and one experimental room for future research. To improve the irradiation efficiency, the new control function which enables multiple gated irradiations per synchrotron cycle has been applied and its efficacy was confirmed by the irradiation time estimation. As for the interplay effect, we confirmed that the selection of a strict gating width and scan direction enables formation of the uniform dose distribution.« less
  • Purpose: The purpose of the present study is to develop a calculation method of dose-calibration-factor using Clarkson integration for proton therapy employing the wobbling system and to evaluate accuracy of the calculation by comparison between calculations and measurements. Methods: CF and CALF stand for a dose-calibration-factor and a dose per monitor unit (MU), respectively. A measured dose-calibration-factor CFmeas is defined as a ratio of the measured dose per monitor unit in a patient-specific condition CALFpat to the measured dose per MU in a reference beam condition CALFref. The CFcalc is a product of three factors: CF1, CF2 and CF3. Themore » CF1 and CF2 are a factor reflecting the effect of common beam delivery devices and that of patient specific devices and parameter (an aperture collimator, a range compensator and an air gap), respectively. The CF1 was obtained by interpolation using measured data. The CF2 was calculated using the Simplified Monte Carlo (SMC) method. The SMC method calculates a dose distribution by tracing individual protons and by using a measured Bragg curve in water. The CF3 representing the correction factor of field size effect was obtained by using the Clarkson integration. We compared the calculated and measured CF values for 20 prostate cases. Results: Field size correction was found to be important. The calculations reproduce the measurement results within an error of ±2.0%, except for a few cases. The error was about –3.1% for the small field area of less than19 square centimeters. Conclusion: We have developed a calculation method of dose-calibration-factor. Calculations agreed with measurements within ±2.0% for 90% of 20 prostate cases. Except for a small field size cases, the calculation method can be applied to determine the dose-calibration–factor for majority cases of prostate cancer.« less
  • 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. Themore » 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.« less