SU‐E‐T‐22: Is the Residual Range a Universal Quantity to Specify the Quality of Modulated Proton Beams?

Purpose: To investigate the validity of using the residual range as a universal quantity to specify the quality of modulated proton beams. Methods: We used TOPAS (Tool for Particle Simulation), an application of the Geant4 toolkit, to simulate absorbed dose and stopping‐power distributions from a commercial passive scattering nozzle. We used the standard physics lists from Geant4 in the simulations. All particles were included, as well as physics models for nuclear interactions. No variance reduction techniques were used. Dose and averaged stopping‐power as functions of depth were scored in a water box with 320 scoring volumes of 15 × 15 × 0.1 cm³. Stopping‐power spectra were scored in a15 × 15 × 0.1 cm³ volume located in the middle of SOBPs. All particles were considered in the dose scoring. Only protons (primary and secondary) were considered in the scoring of stopping‐power. Results: For the same residual range, differences in averaged stopping‐power values of up to 13% were observed for a 200 MeV beam with modulations of 4 cm and 8 cm, respectively. Simulations of four modulated proton energies with the same SOBP of 8 cm showed differences of up to 13% in the averaged stopping‐power values even in the SOBP region. We also simulated stopping power spectra in the middle of 8 cm SOBPs for four modulated proton energies. The averaged stopping‐power values calculated from the spectra were within 3%, however, their distributions were very different with full width at half‐maximum 150% larger for the 250 MeV beam compared to that of the 140 MeV beam. Conclusion: Large differences in the averaged stopping‐power values and stopping‐power spectra were observed for the same residual range. Determining whether these differences have a significant effect on the response of radiation detectors exposed to proton beams requires further investigation. Natural Sciences and Engineering Research Council of Canada and Ontario Graduate Scholarship Program, Ontario Ministry of Training, Colleges and Universities.