Diabetes mellitus as a potential risk factor for severe late toxicity after stereotactic body radiotherapy for prostate cancer: a large single-institution cohort study.

This study aimed to evaluate the incidence of severe late gastrointestinal and genitourinary toxicities after prostate stereotactic body radiotherapy (SBRT) and to explore potential clinical risk factors, with a particular focus on diabetes mellitus. We retrospectively analyzed patients with clinically non-nodal and non-metastatic prostate cancer treated with SBRT between 2016 and 2024. A total of 940 patients were included. The prescribed doses ranged from 36.25 to 47.5 Gy in five fractions. A total of 29 grade ≥ 3 composite toxicity events (20 gastrointestinal and 14 genitourinary events) were observed. The cumulative incidence of late grade ≥ 3 composite toxicity was 2.5% at 2 years (95% CI, 1.6-3.9%) and 4.6% at 5 years (95% CI, 3.1-6.8%). In univariate analysis, diabetes mellitus was significantly associated with an increased risk of severe toxicity (HR 2.6, 95% CI 1.2-5.4, P = 0.014). Subgroup analyses demonstrated generally consistent associations across clinically relevant subgroups. Although severe late toxicities were infrequent overall, clinically meaningful and potentially devastating events were observed. Exploratory analyses suggested that patient-related clinical factors, including diabetes mellitus, may contribute to increased vulnerability to severe late radiation injury following prostate SBRT and should be considered when interpreting toxicity risk.