SU‐E‐T‐445: Prostate Motion Effect Evaluation in Proton Pencil Beam Scanning Delivery

Abstract

Purpose: To quantitatively assess the impact of interplay effect associated with the intra‐and inter‐fraction prostate motion and the delivery of the scanning proton beam. Methods: A cohort of ten prostate patients with weekly verification CTs was treated by proton pencil beam scanning with bi‐lateral single field uniform dose modality. A typical field has 10–15 energy layers and 500–1000 spots. According to patient treatment logs, each layer delivery time was less than 1 second with average time to change layers of about 8 seconds. Real‐time intrafraction prostate motions were determined from a previously reported prospective study using the Calypso beacon transponder. The worst‐case‐scenario patient prostate motion was adopted in this study. The prostate motion and the beam delivering sequence were synchronized to demonstrate the interplay effect. Results: CTV coverage was altered due to intrafraction motion. In each individual fraction, the CTV D_99% can vary more than 10% relative to the initial plan. However, over the entire course of treatment the total dose degradation of D_99% had a mean of −1.5% and standard deviation of 1.2%. The large variation generally happens where one beam spot is significantly heavier weighted relative to the surrounding spots. The magnitude of deviation also depends on individual patient. The consideration of both intrafraction and interfraction motion further reduced the CTV coverage D_99%, with mean of ‐ 1.7% and standard deviation of 1.3%. The mean dose to the anterior rectal wall varied about 10% due to either intrafraction or interfraction motion. Conclusion: The intrafraction and interfraction prostate motion have comparable dosimetric impact. The dose over the course of treatment can reduce ∼2% for CTV coverage and increase about 10% for anterior rectal wall.