In Silico Dosimetric Comparison of Intensity-Modulated Photon Versus Proton Therapy for Pelvic and Para-Aortic Nodal Recurrence of Prostate Cancer Following Prior Irradiation

Background/Objectives: Limited data exists comparing photon-based intensity-modulated (IMRT) or volumetric modulated arc therapy (VMAT) against intensity-modulated proton therapy (IMPT) when treating pelvic or para-aortic nodal recurrences of prostate cancer following prior irradiation. The aim of this study was to evaluate dosimetric differences between VMAT and IMPT when treating nodal recurrences in the re-irradiation setting. Methods: VMAT and IMPT plans for 15 patients with node-recurrent disease were evaluated. Prescription doses to the whole pelvis and prostate bed planning target volumes (PTVs) from the initial VMAT plans were 45–50.4 Gy and 66.6–72 Gy, respectively. IMPT re-irradiation prescription doses were 45–53.75 Gy radiobiological equivalent (GyE) (proton radiobiological effective dose 1.1) to at-risk pelvic and para-aortic nodal chains and 60–64 GyE to gross nodal disease. For re-planning and comparative study purposes, clinical target volume (CTV) prescription doses were standardized to 50.4 Gy for the regional pelvic and para-aortic nodes, and 63 Gy for the grossly involved nodal disease. Organ-at-risk (OAR) contours were standardized and re-irradiation plans were generated on Raystation. Student’s t-test was used to compare CTV coverage and dose to OARs. Results: CTV coverage was met for all plans, with 99% of CTVs receiving ≥99% of prescription doses. Composite analysis demonstrated IMPT not only sustained dosimetric benefits in the re-irradiation setting for all OARs except sigmoid but also conferred significant advantages. Select endpoints with significant differences on composite analysis included bowel cavity V45Gy, left kidney V18Gy and mean dose, right kidney V18Gy and mean dose, and integral dose to ‘Body-CTV’. Conclusions: Composite analysis of the initial treatment and re-irradiation plans showed that IMPT delivered and maintained significant OAR dose reductions in the nodal re-irradiation setting, potentially decreasing the risk of long-term toxicities. This provides a rationale for the utilization of IMPT in this clinical setting.