SU‐E‐T‐352: Comparison of Dose Distributions Between Two Arrangements of a Range Compensator and of An Aperture Collimator in a Passive Scattering Method for Proton Therapy
Purpose: Two arrangements of a range compensator (RC) and an aperture collimator (AC), RC placed in front of AC (ARGT‐1) or AC placed in front of RC (ARGT‐2), are currently used in proton therapy. We studied difference of the dose distributions for two arrangements.Methods: We calculated dose distributions in water formed by a 190‐MeV modulated proton beam (SOBP width 80 mm) passing through two arrangements of a simple geometrical RC and an AC using a Simplified Monte Carlo method. The air gap between the downstream device and water surface was set at 195 mm. Results: Calculated dose distributions were normalized in the same uniform‐dose region. We compared sharpness of the 80%–20% dose falloffs at a depth of 150 mm where protons passing through the thinnest part of RC mainly contribute the dose. The value for the ARGT‐1 is written first and that for the ARGT‐2 next. Dose falloffs were 8.5±0.2 mm / 7.9±0.2 mm near an inner RC edge. Dose falloffs were 7. 1±0.2 mm / 8.0±0.2 near the AC edge region. In addition, dose contributions of edge‐scattered protonsscattered in the AC at water surface were 19% / 10% Conclusions: We verified that ARGT‐1 is superior to the other in dose conformation near the lateral target boundary and that ARGT‐2 is superior to the other in that near the distal target boundary. We also noticed that the RC serves to absorb a part of scatteredprotons in the AC for ARGT‐2.