SU‐E‐T‐351: Dosimetric Impact of Periodic and Random Motion on Proton Double‐Scattering and Uniform‐Scanning Deliveries

Abstract

Purpose: To experimentally investigate target motion impact on proton dosimetry and the spatiotemporal interplay in double scattering (DS) and uniform scanning (US) deliveries using 3D polymer gel dosimeters and a programmable motion platform. Methods: A one beam proton plan with 16cm range and 6cm modulation (total 13 energy layers) in both DS and US modes was generated in the Eclipse treatment planning system (TPS) to be used for gel irradiation in static, periodic and random motion. The periodic motion was a single sinusoidal trace along the SI direction with 2‐cm peak‐to‐peak amplitude and 0.25Hz frequency. The random motion trace was an actual patient prostate trajectory from a treatment fraction. All irradiated gel dosimeters were setup on the platform in an identical fashion. The dose distribution of static gel dosimeter was compared to those of treatment plan. Dose distributions were compared between DS and US in both periodic and prostate motion conditions. 3D gamma analysis (4%, 4mm criterion) was calculated against planned dose distribution for all the gel dosimeters. Results: There was an overall good agreement in dose distributions between stationary DS delivery and TPS plan. 93% of voxels has gamma less than 1. 3D gamma failure maps were significantly different between DS and US periodic motion gels. There were 88% and 91% of voxels has gamma less than 1 for DS and US, respectively. In prostate motion cases, even though about 70% voxels has gamma larger than 1 for both DS and US deliveries, significant gamma failure map difference was still observed. Conclusion: Good dose agreement between TPS plan and stationary gel dosimeter demonstrated the validity of using gel dosimeter study target motion effect and its spatiotemporal interplay with proton irradiation. Isodose and 3D gamma comparisons between DS and US for both periodic and prostate motion confirmed their dosimetric impact.