Initial Experience with MR-Guided or CT-Guided Stereotactic Body Radiotherapy for Prostate Cancer.

We report early toxicity outcomes after institutional implementation of prostate SBRT using 42.7 Gy in seven fractions delivered with either MR-guided adaptive SBRT (MRgSBRT) or CT-guided non-adaptive SBRT (CTgSBRT). Between July 2019 and May 2023, 216 patients received this regimen. Genitourinary (GU) and gastrointestinal (GI) toxicities were retrospectively assessed using the Common Terminology Criteria for Adverse Events (CTCAE) version 5.0. Most patients had intermediate-risk disease (86%), and 14% had high-risk disease. Median prostate volume was 40 mL, and median baseline International Prostate Symptom Score (IPSS) was 8 among patients with available data. MRgSBRT and CTgSBRT were used in 100 (46%) and 116 (54%) patients, respectively. At a median follow-up of 19 months, 12- and 24-month grade ≥ 2 GU toxicity rates were 20.0% and 18.2%, while grade ≥ 2 GI toxicity rates were 3.5% and 3.6%, respectively. No statistically significant platform-related differences were detected; however, these comparisons were exploratory and potentially confounded by nonrandomized platform selection, treatment era, rectal-spacer use, adaptive workflow and differing PTV margins. On univariable analysis, baseline GU symptoms were associated with higher 12-month grade ≥ 2 GU toxicity (OR, 3.68; 95% CI, 1.52-10.37; p = 0.007), while prostate-volume category was not significantly associated with this endpoint.