S1150
Clinical - Urology
ESTRO 2026
Digital Poster Highlight 683
MCID.
Simultaneous MR-Guided SBRT to the Prostate Primary, Pelvis and Intraprostatic Lesion Boost in High-Risk Prostate Cancer: A Prospective Study Darren MC Poon 1 , JING YUAN 2 , Oi Lei Wong 2 , Bin Yang 3 , Sin Ting Chiu 4 , George Chiu 4 , Ben Yu 3 1 Comprehensive Oncology Centre, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong. 2 Research Department, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong. 3 Medical Physics Department, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong. 4 Department of Radiotherapy, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong
Figure 1: Changes in EPIC-26 incontinence scores based on MCID from baseline, to EOT, 12, 36 and 60 M after completion of radiotherapy. Deterioration is denoted in red while improvement is in green.
Purpose/Objective: High-risk prostate cancer (PCa) carries substantial risks
of intraprostatic and pelvic recurrence. While stereotactic body radiotherapy (SBRT), focal
intraprostatic lesion (IPL) boost, and elective pelvic lymphatic irradiation (WPRT) have each been reported separately, a single five-fraction regimen that simultaneously treats the prostate, pelvic lymphatics, and IPL under magnetic resonance (MR) guidance has not been described. This study reports on the feasibility, safety, and early outcomes of MR-guided SBRT (MRgSBRT) that concurrently irradiates the prostate, pelvic lymphatics, and intraprostatic lesion (IPL). Material/Methods: Between July 2021 and March 2024, PSMA-PET–staged patients with high- or very-high-risk PCa were prospectively enrolled. Exclusion criteria included MRI contraindication, prior pelvic radiotherapy, or evidence of metastasis. MRgSBRT was delivered on a 1.5T MR- LINAC (Elekta Unity): 25Gy in 5 fractions to pelvic lymphatics (CTV_L), 35–40 Gy in 5 fractions to the prostate±seminal vesicles, and 40–42.5Gy in 5 fractions to IPLs identified by mpMRI (PI-RADS 4–5) and/or PSMA-PET. Daily online adaptive planning, either adapt-to-position (ATP) or adapt-to-shape (ATS), was performed. Androgen-deprivation therapy was prescribed for 18–36 months. Primary endpoints were acute ( ≤ 3 months) and late (>3 months) genitourinary (GU) and gastrointestinal (GI) toxicities graded by CTCAE v5.0. Secondary endpoints included PSA kinetics and patient-reported outcomes (EPIC). Results: Thirty-four patients (median age 75 years; range 51– 87) completed MRgSBRT with pelvic lymphatic and IPL irradiation. Baseline characteristics are summarized in Table 1. A total of 49 IPLs were boosted. Across 170 fractions, ATS adaptation occurred in 18.8% (32/170). Median follow-up was 25.4months (range 1.4–48.5). Acute grade ≥ 2 GU and GI toxicities were 5.8% (2/34; frequency/dysuria) and 0%, respectively; no grade ≥ 3
Figure 2: Changes in EPIC-26 irritative scores based on MCID Conclusion: Our findings confirm subsets of men who experience genitourinary toxicity in the acute, subacute and late settings after prostate SBRT. Furthermore, those who suffer from irritative symptoms at baseline or those who develop subacute MCID at 12 M are more likely to experience longer-term irritative symptoms. Future research should focus on methods to reduce radiation dose to genitourinary organs at risk, as well as optimal case selection for SBRT versus longer-term hypofractionation. References: 1. Nikitas J, Jamshidian P, Tree AC, Hall E, Dearnaley D, Michalski JM, et al. The interplay between acute and late toxicity among patients receiving prostate radiotherapy: an individual patient data meta-analysis of six randomised trials. Lancet Oncol. 2025;26(3):378– 86. 2. Wegener E, Sidhom M, Pryor D, Bucci J, Yeoh K, Richardson M, et al. Prostate virtual high-dose-rate brachytherapy boost: 5-year results from the PROMETHEUS prospective multicentre trial. Eur Urol Oncol. 2024;7(5):1042–50. 3. Skolarus TA, Dunn RL, Sanda MG, Chang P, Greenfield TK, Litwin MS, et al. Minimally Important Difference for the Expanded Prostate Cancer Index Composite Short Form. Urology. 2015 Jan;85(1):101–6. Keywords: vHDR boost, urinary toxicity, prostate cancer
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