Room: ePoster Forums
Purpose: To evaluate a dynamically adaptable planning and delivery technique for Intensity Modulated Proton Therapy (IMPT) of prostate with pelvic lymph nodes to mitigate the dosimetric impact of severe prostate interfraction motion.
Methods: Ten prostate patients treated with pelvic lymph nodes using IMPT were selected for this study. For each patient, two overlapped targets were created: superior target includes common and external iliac nodes; inferior target includes external iliac nodes and prostate with seminal vesicles. Each target was assigned a set of left and right lateral fields during robustness optimization. Real patient interfraction prostate motion data (maximum magnitude were 1.5cm and 0.8cm in the SI and AP directions, respectively) were used to generate a total of 10 prostate motion CT datasets and their corresponding contours. To simulate conventional seed matching treatment, all beam isocenter was shifted based on the direction and magnitude of interfraction prostate motion of each CT dataset and plan dose was recalculated; for dynamically adapted seed matching, superior target fields had a separate isocenter to accommodate lymph nodes due to severe prostate interfraction motion. Dose to the prostate and pelvic node CTVs, rectum, bladder and femoral heads were obtained and presented.
Results: For conventional seed matching, dose degradation of nodal CTV was prominent for interfraction prostate motion beyond nodal PTV margins. For dynamically adapted seed matching, the minimum doses to nodal CTV were much improved compared to conventional seed matching. Dose coverage to prostate CTV were comparable to original treatment plans for both techniques. There were large dose variations to the rectum, bladder and femoral head among different prostate interfraction motion datasets.
Conclusion: Prostate interfraction motion from pelvic lymph node can have significant dosimetric impact on prostate and nodal CTVs coverage. The dynamically adaptable IMPT plans can mitigate the dose degradation due to severe prostate interfraction motion.