ESTRO 2025 - Abstract Book

S1912

Clinical - Urology

ESTRO 2025

983

Digital Poster Does prostate volume affect SABR-induced toxicity? Ereny Samwel Poles SAAD 1 , Anoja mannarkandy 1 , Peter Hoskin 1,2 , yeepei song 1 , Hitesh Mistry 3 , Andrew Hudson 1 , Ruth Conroy 1 , Hanis Hanafi 1 , Hannah NIGHTINGALE 1 , Ananya Choudhury 1,2 1 Clinical Oncology, The Christie, Manchester, United Kingdom. 2 Division of science, Manchester University, Manchester, United Kingdom. 3 Statestics, Manchester University, Manchester, United Kingdom Purpose/Objective: MR-based planning improves anatomical differentiation and may reduce clinical target volume ( CTV) in localised prostate cancer receiving SABR. We evaluate if prostate volume on the diagnostic MRI affects the rate and grade of SABR-induced genitourinary (GU) and gastrointestinal (GI) toxicities and whether MR- Linac -based treatment has advantage. Material/Methods: A single-centre retrospective study included localized prostate cancer patients received SABR. Data collection included age, TNM staging, Gleason Grade Group (GGG), PSA at diagnosis, prostate volume on the diagnostic MRI, and CT-Linac or MR-Linac-based treatment. The PTV is CTV plus 4-5mm, reduced to 3 -5 mm posteriorly in both cohorts. Toxicity data from the CT and MR Linac based treatment included GU and GI CTCAE v5.0 graded toxicity at 6 weeks and one year after treatment. Statistical analysis was done using the SPSS package v20; Chi-square and Mann-Whitney U tests used with p value <0.05. Results: The primary analysis included 165 patients with T staging (T1c-T3a)N0M0. GGG 2 and 3 represented 66.6% and 31.9%, median PSA at diagnosis was 9.07 ug/L range (1.85-32.00). Median prostate volume was 38 ml (range 14-85). 56.4% of patients were treated on CT-Linac, while 43.6% on MR-Linac. A six-week review was available for 160 patients: 89 on the CT-Linac and 71 on the MR-Linac. At one year of follow up, 63 patients were evaluated; 16 on CT and 47 on MR Linac. GU and GI toxicity are presented in tables (1) and (2).Recorded late GU toxicity was significantly higher in patients treated on MR-Linac. Six-week toxicity review for the whole cohort showed no significant association between the median prostate volume and GU or GI toxicity (p=0.51 and p=0.90). Median prostate volume did not associate with GU or GI toxicities for CT-Linac (p=0.30 and p=0.99 respectively), or for MR-Linac based treatments (p=0.75 and p=0. 85 respectively). At one year, there was no significant association between median prostate volume and late GU or GI toxicity (p=0.85 and p=0.94 respectively) for the whole cohort. There was no association between median prostate volume and late GU toxicity in patients treated on CT -Linac or MR- Linac (p=0.35 and p=0.74 respectively). The rate of GI toxicity in the separate cohorts was too low for statistical analysis. Tables