# Dosimetric effect of internal error correction on pubic bones in image-guided passive scattering proton therapy for prostate cancer

## TL;DR

This study examines how correcting internal errors in proton therapy for prostate cancer can increase radiation doses to the pubic bones, potentially increasing fracture risks.

## Contribution

The study quantifies the dosimetric impact of anterior and inferior isocenter shifts on pubic bone dose in proton therapy for prostate cancer.

## Key findings

- Anterior isocenter shifts cause a 14.2 cc increase in pubic bone V80% dose on average.
- Dose elevation correlates strongly with proximity between the target and pubic bones (r > 0.66, P < 0.001).
- Anterior shifts have a more pronounced effect than inferior shifts on pubic bone dose.

## 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.