Abstract
Image-guided passive scattering proton therapy (PSPT) has been widely adopted in Japan and worldwide, with substantial long-term clinical data supporting its efficacy in treating prostate cancer. However, as hypofractionated protocols become increasingly common, the impact of internal anatomical shifts on surrounding organs at risk (OARs) warrants renewed attention. The pubic bones, situated near the prostate, are often exposed to unintended high doses, especially during internal error correction based on fiducial marker alignment. This study retrospectively analyzed 30 patients with localized prostate cancer treated with PSPT using lateral opposed fields. Simulated isocenter shifts were applied anteriorly and inferiorly in 2-mm increments up to 10 mm to assess dose changes to the pubic bones. Dose-volume histogram metrics including V80%, V90% and V95% were evaluated. Pubic bones dose increased in both shift directions, with a more pronounced effect for anterior shifts, with a 10-mm anterior shift increasing V80% by 14.2 cc on average-2.4 times greater than inferior shifts. Dose elevation correlated strongly with the anatomical proximity between the clinical target volume and pubic bones (r > 0.66, P < 0.001). These results suggest that anterior correction in PSPT can cause substantial dose escalation to the pubic bones, potentially increasing the risk of insufficiency fractures. As extreme hypofractionation becomes more common, careful evaluation of pubic bones dose should be incorporated into treatment planning, alongside traditional OARs such as the rectum and bladder. Early replanning should be considered when persistent anterior displacement is observed to maintain patient safety and quality of life.