S1311
Clinical - Urology
ESTRO 2026
Digital Poster 4914 Rectal V60 predicts acute toxicity after prostate radiotherapy: real-world data with EQD2 translation to hypofractionation Milos Grujic 1,2 , Neda Milosavljevic 1,2 , Jelica Grcak 2 , Ivana Vujinovic 2,1 , Emilija Subanovic 1,2 , Aleksandra Dugalic 2 , Katarina Krasic 2 , Katarina Jankovic 2 , Marija Zivkovic Radojevic 1,2 1 Department of Clinical Oncology, Faculty of Medical Sciences, Kragujevac, Serbia. 2 Center for Radiation Oncology, University Clinical Center Kragujevac, Kragujevac, Serbia Purpose/Objective: Although QUANTEC rectal dose–volume constraints remain foundational for prostate radiotherapy, practice has shifted toward moderate and ultrahypofractionation [1]. Translating conventional- fractionation data to modern schedules is essential for safe implementation in resource-limited settings. We aimed to identify rectal DVH parameters associated with acute toxicity after conventional prostate RT and to map the findings to biologically equivalent constraints for 20- and 5-fraction regimens, in alignment with ESTRO Vision 2030 [2]. Material/Methods: A retrospective cohort of 162 men received curative- intent external-beam RT (76–78 Gy in 2 Gy fractions). The primary endpoint was any acute gastrointestinal or genitourinary toxicity (CTCAE v5.0) during or ≤ 3 months after RT[3]. Candidate predictors were rectal V50 Gy, V60 Gy, bladder V65 Gy, and age. Logistic regression (backward conditional entry p < 0.05, removal p > 0.10) identified independent predictors. Equivalent doses in 2 Gy fractions (EQD2, α / β = 3 Gy) were used to project comparable thresholds for moderate-hypofractionation and SBRT. Results: Median rectal V60 Gy was 32 % (IQR 27–38). Acute toxicity occurred in 24 (14.8 %) patients. Rectal V60 Gy was an independent predictor of acute toxicity (OR 1.07 per 1 % increase; 95 % CI 1.01–1.14; p = 0.017), whereas rectal V50 Gy, bladder V65 Gy, and age were not significant (p > 0.10). A 5 % absolute rise in V60 Gy increased the odds of toxicity ≈ 1.4-fold. EQD2 conversion indicated that 60 Gy delivered in 2 Gy fractions corresponds to approximately 53 Gy in 3 Gy fractions (20 fractions) or 32 Gy in 7.25 Gy fractions (5 fractions), consistent with the rectal-dose levels reported in CHHiP and PACE-B [4,5].
baseline within three months post-treatment in almost all cases. Functional and global health status scales showed some temporary acute decline at the end of re-RT, but most patients recovered to baseline or near- baseline levels by three months post-treatment. Symptom scales maintained a mean score of 0.0 at all time points, except for fatigue, constipation, and dyspnea which showed mean mild scores at the end of re-RT.
Conclusion: Proton-based re-RT appears to be a feasible and well- tolerated salvage strategy for locally recurrent PCa. While early oncologic and toxicity outcomes are promising, longer follow-up is essential to determine response durability and to optimize dose constraints to OARs. Keywords: reirradiation, proton therapy, prostate cancer
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