In silico comparison of whole pelvis intensity-modulated photon versus proton therapy for the postoperative management of prostate cancer

Background: Limited data exist comparing intensity-modulated photon (IMRT) and proton (IMPT) radiation therapy when treating the prostate bed and pelvic lymph nodes in the postoperative setting for prostate cancer. The aim of this study was to evaluate dosimetric differences between IMRT and IMPT when treating with whole pelvis radiation therapy (WPRT) postoperatively. Materials and methods: IMRT and IMPT plans were generated for 10 post-prostatectomy patients treated between July and August 2020. The prescription was 50 Gy radiobiologic equivalent (GyE) (proton radiobiological effective dose 1.1) to the pelvis and 70 GyE to the prostate bed in 2 GyE per fraction. Paired 2-sided Wilcoxon signed-rank tests were used to compare clinical target volume (CTV) coverage and dose to organs at risk (OARs). Results: CTV coverage was met for all plans with 99% of CTVs receiving ≥99% of prescription doses. Dose to OARs was significantly higher with IMRT than IMPT for the following endpoints: bladder V5-V65; bowel V5-V45; sigmoid V5-V50; rectum V5-V70; femoral head V40 and maximum dose; bone V5-V65. Select endpoints with significant differences included bladder V30 (63.5 vs. 44.4%, p <.001), bowel V15 (949 vs. 191 cc, p =.001) and V30 (386 vs. 121 cc, p <.001), rectum V40 (81.8 vs. 32.1%, p <.001) and V50 (47.6 vs. 24.9%, p <.001), femoral head maximum doses (46.4–47.1 vs. 38.3–38.6GyE, p <.001), and bone V10 (93.3 vs. 85.4%, p <. 001). Mean doses for all OARs were significantly higher with IMRT, including bladder (41.9 vs. 29.7GyE, p <.001), bowel (21.2 vs. 5.5GyE, p <.001), and rectum (50.8 vs. 27.3GyE, p <.001). Integral dose to ‘Body–CTV’ was significantly higher with IMRT (32.8 vs. 18.4 J, p <.001). Conclusion: IMPT provides comparable target coverage to IMRT when treating prostate cancer with WPRT in the postoperative setting while significantly reducing dose to OARs. These data can inform the future clinical management and delivery of post-prostatectomy irradiation for prostate cancer.