ESTRO 2025 - Abstract Book

S1910

Clinical - Urology

ESTRO 2025

951

Digital Poster 1.5 Tesla MR-guided stereotactic body radiotherapy (SBRT) with a focal boost to intraprostatic lesions: Preliminary clinical outcomes and toxicities Darren MC Poon 1,2 , Jing Yuan 3 , Bin Yang 4 , Oi Lei Wong 3 , Sin Ting Chiu 5 , George Chiu 5 , Siu Ki Yu 4 1 Comprehensive Oncology Center, Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong. 2 Department of Clinical Oncology, The Chinese University of Hong Kong, Hong Kong, Hong Kong. 3 Research Department, Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong. 4 Medical Physics Department, Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong. 5 Department of Radiotherapy, Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong Purpose/Objective: MR-guided SBRT (MRgSBRT) is superior to CT-guided SBRT for the treatment of localized prostate cancer (PC) with regards to toxicity and quality of life [1]. It also facilitates precise targeting of intraprostatic lesions (IPLs) through superior MR image contrast and online plan adaptation [2, 3]. Despite the potential benefits of MRgSBRT, there are limited data on the clinical outcomes and toxicities in patients who receive this treatment for localized PC using an MR-integrated linear accelerator (MR-LINAC) with a focal boost to IPLs. This study is aimed to explore such data using 1.5 Tesla (T) MRgSBRT. Material/Methods: This study prospectively enrolled patients with biopsy-confirmed localized PC. Patients with prior pelvic radiotherapy, distant metastases, other cancer history, or a follow- up ≤6 months were excluded. All patients underwent 5-fractionated MRgSBRT (clinical target volume [CTV] for the whole prostate, 36.25-40 Gy twice weekly) with a concomitant IPL boost (gross tumor volume [GTV], 40 – 42.5 Gy) using a 1.5T MR-LINAC. Planning margins of 3 – 5 mm and 0 – 3 mm were used for CTV and GTV, respectively. The IPLs were defined as PI-RADS 4/5 lesions based on both diagnostic multiparametric MRI and 1.5T planning MRI. High-risk patients received concomitant 25 Gy for pelvic lymphatics. At the physician’s discretion, androgen deprivation therapy could be used, often for 0– 6 months in low/intermediate-risk patients and for 18 – 36 months in high-risk patients. Toxicity was assessed per the Common Terminology Criteria for Adverse Events v5.0. Clinical outcomes, including prostate-specific antigen (PSA) responses and biochemical failure-free survival (bFFS), were analyzed. Results: A total of 63 patients were included, with 92 IPLs irradiated (Table). The median follow-up was 19.1 months (range, 6.9 – 47.7 months). Up to 12 weeks post-radiotherapy, 12.7% (8/63) of patients experienced grade (G) 2 acute genitourinary (GU) toxicities, mainly cystitis noninfective (nocturia or dysuria). No patients experienced G2 acute gastrointestinal (GI) toxicities. Late G2 GU (cystitis noninfective) and GI (rectal hemorrhage) toxicities were observed in 1.6% (1/63) and 4.8% (3/63) of patients, respectively. No G≥3 toxicities were observed. There was a median PSA decline of 98% within 3 months post-treatment. No patients had biochemical recurrence (PSA nadir + 2 ng/mL; bFFS, 100%).