Safety of online MR-based adaptive ultra-hypofractionated radiotherapy for prostate cancer in China: Preliminary analysis of data from a phase II trial

Background This study aimed to evaluate the feasibility and safety of online adapt-to-shape (ATS) workflow for prostate cancer patients on 1.5-T MR linac in China. Methods This prospective phase II study enrolled patients with localized or oligometastatic prostate cancer. Ultra-hypofractionated radiotherapy (UHF-RT) with dose of 36.25-40 Gy in five fractions was delivered every other day. After each fraction, feasibility and tolerability of the treatment were assessed. The primary endpoints were acute grade 2 or above genitourinary (GU) and gastrointestinal (GI) toxicities after up to 12 weeks follow-up.Results From March 2021 to November 2021, 26 patients were enrolled (23 with localized prostate cancer, 3 with oligometastatic prostate cancer). For all fractions, the online ATS plans met the dose criteria for both the target volume and normal tissues. The median on-couch time was 55 (34-95) minutes and 39 (24-50) minutes with T2WI 6-minute sequence and 2-minute sequence scans, respectively. For 98.4% fractions, treatment was well tolerated. Twenty-four patients completed treatment and were followed-up for at least 2 weeks. Grade 2 or above GU and GI toxicities occurred in 33.3% and 8.3% patients, respectively; two patients had RTOG grade 3 GU toxicity (hourly nocturia). IPSS remained unchanged during UHF-RT, increased from week 2 (mean, 9.1) to week 4 (mean, 12.4), and then gradually decreased at week 6. Patient-reported urinary and bowel scores were consistent with IPSS.Conclusions UHF-RT with ATS workflow is well tolerated by patients with localized and oligometastatic prostate cancer, with only moderate GU and mild GI toxicities. Trial Registration: NCT05183074, ChiCTR2000033382