ESTRO 2025 - Abstract Book

S2069

Clinical - Urology

ESTRO 2025

3801

Digital Poster Single-Center RETROspective Study of Five-Fraction DOminant-TArgeted Boost with MR-Guided Adaptive Radiotherapy in Prostate Cancer (RETRO-DOTA-5F) Pittaya Dankulchai 1 , Akrapol Suppasedtanon 2 , Ratchapas Romrattaphan 1 , Tissana Prasartseree 1 , Wiwatchai Sittiwong 1 , Wajana Thaweerat 1 1 Division of Radiation Oncology, Department of Radiology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand. 2 Medical Student, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand Purpose/Objective: This single-center retrospective study aimed to evaluate the efficacy and safety, in terms of acute and late toxicities and oncologic outcomes, of a five-fraction stereotactic body radiotherapy (SBRT) with dominant intraprostatic lesion-targeted (DIL) boost using MR-guided adaptive radiotherapy (MRgART) by the Unity MR-Linac system in prostate cancer patients. The study was named the “RETRO -DOTA- 5F” study. Material/Methods: The “RETRO -DOTA- 5F” study reviewed patient data from 2020 to 2023. Patients who underwent a five fraction MRgART SBRT using the Unity MR-Linac system delivered over 2 weeks, were included in the analysis. The DIL was identified using multiparametric MRI and targeted with a radiation boost. The prescribed dose was 36.25 Gy to the prostate and 40 Gy to the DIL. Patient and tumor characteristics were collected and summarized with descriptive statistics. Primary outcomes of acute and late toxicities were graded according to the Common Terminology Criteria for Adverse Events (CTCAE) version 5.0. Biochemical recurrence-free survival defined by Phoenix criteria was analyzed as an oncologic outcome. Kaplan-Meier survival analysis was employed to evaluate late toxicities and oncologic outcomes. Results: Between October 2020 and December 2023, 55 patients were treated with five-fraction MRgART SBRT with DIL boost using the Unity MR-Linac system. The median age was 72 years, and the median baseline PSA was 8.29 ng/ml (IQR: 5.81-11.86). Seventy-one percent of patients were intermediate risk, 16% low risk, and 13% high risk. Androgen deprivation therapy (ADT) was used in 51% of patients. Most patients (78%) had only one DIL as a boost target. The median follow-up time was 13 months (IQR: 11-20). Acute grade ≥2 genitourinary (GU) toxicity was observed in 13% of patients (7/55), and grade ≥2 gastrointestinal (GI) toxicity in 2% (1/55) at 6 months (Figure 1). One patient experienced acute grade 3 GU toxicity, but no acute grade 3 GI toxicity was observed. The cumulative incidence of late grade ≥2 GU/GI toxicity was 10.29% (95% CI, 4.41 -23.01)/ 1.96% (95% CI, 0.28-13.11) at 1 year and 16.27% (95% CI, 7.96-31.62)/ 1.96% (95% CI, 0.28-13.11) at 2 years, respectively. No late grade ≥3 GU or GI toxicities were identified. No biochemical recurrence was observed at the 1 year follow-up.