Comparative In Silico Analysis of Intensity Modulated Scanning Beam Proton Therapy (IMPT) and Volumetric Modulated Arc Photon Radiotherapy (VMAT) for the Post-Operative Treatment of Prostate Cancer.

PURPOSE/OBJECTIVE(S) Limited data exist regarding the dosimetric techniques and potential advantages of modern scanning beam proton therapy relative to conventional photon therapy modalities for prostate cancer patients in the post-operative setting. Our purpose was to evaluate the dosimetric differences in IMPT versus IMRT for the post-operative treatment of prostate cancer. MATERIALS/METHODS The 3DCT data of 7 consecutive, post-prostatectomy patients treated at our institution with adjuvant or salvage IMPT in 2020 were used to generate VMAT and IMPT plans using 3 different beam arrangements: 2-field (opposed laterals), 3-field (opposed laterals inferiorly matched to posterior beam superiorly), and 4-field (opposed laterals inferiorly matched superiorly to 2 posterior oblique beams). Prescription was 50 Gy in 2 Gy fractions delivered to elective pelvic nodal regions (i.e., whole pelvis) and 20 Gy in 2 Gy fractions delivered to the prostate bed for a total dose of 70 Gy in 35 fractions. PT doses are reported in Gy (RBE) = 1.1 Gy. Dose goals for target and relevant organs at risk (OAR), as well as dose-volume histogram parameters were assessed. The paired 2-sided Wilcoxon signed-rank test was used to compare the 4-field IMPT versus VMAT plans, with P < .05 indicating statistical significance. RESULTS CTV coverage met pre-specified dose goals for all plans with 99% of CTVs receiving ≥ 100% of the prescription doses. The 2-, 3-, and 4-field IMPT plans showed similar doses to the bladder and bladder minus CTV (bladderless-CTV), while the 4-field IMPT plan showed the lowest mean and low to intermediate doses to the bowel cavity and rectum. For example, mean bowel cavity V15 and V45 for the 2-, 3-, and 4-field IMPT plans were respectively 549.2 ± 282.6 cc and 110.7 ± 39.8 cc, 303.0 ± 102.0 cc and 92.7 ± 32.6 cc, and 263.4 ± 97.3 cc and 90.6 ± 40.6 cc. Mean rectum V50 and V60 for the 2-, 3-, and 4-field IMPT plans were 26.8 ± 8.9% and 18.7 ± 7.4%, 28.9 ± 10.5% and 19.5 ± 8.0%, 24.3 ± 4.8% and 16.7 ± 4.1% respectively. When comparing the 4-field IMPT and VMAT plans, the rectal, bowel, and bladder parameters in Table 1 differed significantly. CONCLUSION The 4-field IMPT beam arrangement showed the greatest reductions in dose to the bowel cavity and rectum compared to VMAT and the 2- and 3-field IMPT arrangements. These data can inform the future clinical management and delivery of proton therapy for prostate cancer in the post-prostatectomy setting.