Comparative analysis of radiation therapy plans before and after biodegradable hydrogel (SpaceOAR) injection for reducing rectal toxicity in patients with prostate cancer undergoing carbon ion radiotherapy

Background: Carbon ion radiotherapy (CIRT) offers superior dose conformity and biological effectiveness, making it an effective modality for treating prostate cancer. However, dose escalation is limited by the anatomical proximity of the rectum. Although the SpaceOAR™ hydrogel spacer has demonstrated efficacy in photon-based radiotherapy by reducing rectal dose, its application in CIRT remains insufficiently studied. This study aimed to assess the impact of SpaceOAR on dose distribution and plan robustness in patients with prostate cancer receiving CIRT. Methods: Twenty-five patients with clinically localized prostate cancer who received SpaceOAR insertion between the prostate and rectum were included. Treatment plans were generated using the RayStation treatment planning system and compared across conventional, rectum-protection, and post-SpaceOAR insertion plans in terms of dosimetry, including dose-volume histograms (DVHs). Robustness evaluation assessed plan stability against patient setup uncertainties, and ProKnow scoring was used to evaluate treatment plan quality. Results: Compared with conventional CIRT and rectum-protection plans without SpaceOAR, plans with SpaceOAR insertion resulted in a substantial reduction in rectal radiation dose across all 25 patients (p < 0.001). SpaceOAR insertion also improved overall plan quality, as reflected in DVH metrics and ProKnow scores (p < 0.001). Robust evaluation confirmed that SpaceOAR enhanced the safety and reliability of the treatment plans. Conclusions: Integrating SpaceOAR into CIRT for prostate cancer appears to reduce rectal radiation exposure, improves plan robustness, and enables potential dose escalation to the target volume without compromising safety.