S1143
Clinical - Urology
ESTRO 2026
metastasis-free survival (DMFS, first imaging confirmed distant metastasis after sRT). BRFS and DMFS was estimated using Kaplan–Meier and predictors assessed by Cox regression. Results:
Oncology, Saitama Medical University International Medical Center, Hidaka, Japan. 6 Division of Integrative Medical Oncology, Saiseikai Kumamoto Hospital, Kumamoto- Shi, Japan. 7 Department of Medical Physics, National Cancer Center Hospital, Chuo-ku, Japan. 8 Department of Radiation Oncology, National Cancer Center Hospital East, Kashiwa, Japan. 9 Department of Supportive and Palliative Care Research Support Office, National Cancer Center Hospital East, Kashiwa, Japan Purpose/Objective: This all - Japan, multicentre, single - arm confirmatory study evaluated the safety and efficacy of two - fraction MR - guided adaptive radiotherapy. Superiority for the incidence of grade ≥ 2 acute genitourinary (GU) adverse events was tested against a historical control from the 2STAR trial, a CT - LINAC clinical trial that adopted the same dose-fractionation.In this report, we present the incidence of grade ≥ 2 acute GU adverse events as the primary endpoint, together with other acute toxicities and longitudinal changes in the International Prostate Symptom Score (IPSS). Material/Methods: Eligible patients had low to intermediate risk prostate cancer. Treatment was delivered on MR - LINAC (Unity or MRIdian LINAC) with adaptive planning and intrafraction imaging: 26 Gy in two fractions, 7–14 days apart. The safety population included all patients who initiated protocol radiotherapy. The prespecified analysis used an 80% CI (normal approximation) and two historical CT - based benchmarks: (i) non - inferiority versus 36.25 Gy in five fractions from the PACE-B trial (reference 23.1%, non - inferiority margin 30.6%), and conditional on non - inferiority, (ii) superiority versus 26 Gy in two fractions from the 2STAR trial (historical rate 40%). Results: Sixty patients were analysed; 13 experienced grade ≥ 2 acute GU toxicities, yielding an incidence of 21.7% (13/60; 80% CI, 14.9–28.5). The upper CI bound was below 30.6%, meeting the protocol - defined criterion for non - inferiority versus the CT - based five-fraction benchmark. The same bound was also below 40%, demonstrating superiority to the CT - based two - fraction historical control. Median IPSSs at baseline, end of radiotherapy, and at one and three months after initiation of radiotherapy were 6.5, 8.0, 12.5, and 7.0, respectively. Treatment-related grade ≥ 2 acute gastrointestinal toxicity occurred in only one patient (grade 2 radiation enteritis), and no other acute adverse events were observed. Conclusion: In this all - Japan multicentre setting, two - fraction MR - guided adaptive radiotherapy achieved a 21.7% incidence of grade ≥ 2 acute GU toxicities, meeting non - inferiority compared with the CT-LINAC five-
Of 128 patients, 79 (62%) were PET-negative and 49 (38%) had LR. In ADT-free patients at event or last follow up, 3- and 5-year BRFS rates were 71% and 57%, while DMFS rates were 85% and 72%. BRFS did not differ significantly between PET-negative and LR patients (p = 0.096), even when differentiating between their postoperative stadium (BCR vs. BCP, p = 0.396). On multivariable analysis, PSA nadir after sRT <0.1 ng/ml (p < 0.001) and ISUP score (p = 0.010) independently predicted BRFS; PSA nadir <0.1 ng/ml was the only significant predictor of DMFS (p < 0.001). Conclusion: PSMA PET/CT–guided sRT achieved durable long-term control in PET-positive and PET-negative patients, independent of postoperative status (BCR vs. BCP). Achieving an ADT-free PSA nadir <0.1 ng/ml after sRT was the predominant predictor of outcome. These results support PSMA PET/CT–guided sRT as an effective standard in the salvage setting. Keywords: PSMA PET/CT, salvage radiotherapy, prostate cancer Two-fractionated stereotactic MR-guided adaptive radiotherapy for patients with prostate cancer: a nationwide, multi-institutional prospective study Tairo Kashihara 1 , Noriyoshi Takahashi 2 , Haruo Inokuchi 3 , Makoto Saito 4 , Shin-ei Noda 5 , Tetsuo Saito 6 , Takahito Chiba 7 , Noriyuk Kadoya 2 , Nobutaka Mukumoto 3 , Masato Tsuneda 4 , Yu Kumazaki 5 , Sadamoto Zenda 8,9 , Keiko Shibuya 3 , Shingo Kato 5 , Keiichi Jingu 2 , Takashi Uno 4 , Hiroyuki Okamoto 7 , Hiroshi Igaki 1 1 Department of Radiation Oncology, National Cancer Center Hospital, Chuo-ku, Japan. 2 Department of Radiation Oncology, Tohoku University Graduate School of Medicine, Sendai, Japan. 3 Department of Radiation Oncology, Osaka Metropolitan University Graduate School of Medicine School of Medicine, Osaka, Japan. 4 Diagnostic Radiology and Radiation Oncology, Graduate School of Medicine, Chiba University, Chiba, Japan. 5 Department of Radiation Mini-Oral 481
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