SU‐E‐T‐189: Commission Range Shifter On a Spot Scanning Proton System Using Raystation Treatment Planning System

Abstract

Purpose:To treat superficial target e.g. chest wall, head&neck or cranial cases, we commissioned two range shifter(RS) in Raystation4.0 with 7.37cm(RS1) and 4.1cm(RS2) Water Equivalent Thickness(WET) respectively. However, current beam model has limitations due to the secondary scattered proton. This study provides a detailed and critical commission data and provides suggestions for using RS in clinic.Methods:RS’ WET was verified by Multi‐Layer Ionization Chamber from 120MeV to 226.7MeV before TPS modeling. Spot characteristics were measured using 2D scintillate detector at ISO with different air gap. A 8×8×10cm3 cube is created in 8cm depth of water to verify the absolute dose accuracy. Plans were created with different air gap using both RS. Absolute dose verification was measured along the central axis from distal end to surface using PPC05. 10 clinical RS2 plans were measured using MatriXXPT in 3 planes (proximal, distal and midSOBP).Results:RS material's proton stopping power is energy dependent(from 70MeV to 226.7MeV) ranging from 7.42 to 7.31cm and from 4.10 to 4.03cm respectively. We chose 7.37cm (RS1) and 4.10cm (RS2) to favor the low and median proton energy. With different air gap(3cm to 32cm), spot size expands from 3.2mm to 5.5mm(RS1) and from 3.1mm to 4.1mm(RS2) respectively(226.7MeV in air, 1‐sigma). For the absolute dose verification, the larger air gap and shallower depth causes larger discrepancy between TPS and measurements. All 10 clinical plans with 5–10cm air gap passed gamma index 95% with 3%/3mm criteria and outputs differences were within 3%.Conclusion:We strongly recommend each institution to verify the WET independently and choose the value to fit the clinical needs. To minimize the output difference in Raystation4.0 while avoid potential collision to the patient, we recommend to use 5–10cm air gap to minimize the output difference within 2% and preferably use RS with smaller WET if possible.