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

Abstract

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.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.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.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.