To investigate the influence of diabetes mellitus (DM) on late genitourinary (GU) and gastrointestinal (GI) toxicity in patients treated with external beam radiotherapy (RT) for prostate cancer.

A total of 626 men were treated with curative-intent RT for prostate cancer from 1988 to 2008. Using the National Comprehensive Cancer Network risk category, the patients were considered to have low-risk (30%), intermediate-risk (42%), or high-risk (28%) prostate cancer. The median radiation dose was 74 Gy; 45% received androgen deprivation therapy for a median of 4 months. Late GU and GI Radiation Therapy Oncology Group toxicity was recorded prospectively at each visit after external beam RT. The median follow-up period was 55 months.

Of the 626 men, 102 (16%) had DM that was controlled by diet (8%), oral medications (52%), or insulin (39%). The patients with DM were more likely to receive intensity-modulated RT and androgen deprivation therapy and to have a shorter follow-up duration (P ≤.05 for all). Univariate analyses demonstrated that greater radiation dose, baseline urinary dysfunction, intensity-modulated RT, and DM were associated with grade 2 or greater GU toxicity, and transurethral resection of the prostate and DM were associated with grade 3 or greater GU toxicity. In addition, androgen deprivation therapy use, age ≥70 years, and anticoagulation were associated with grade 2 or greater GI toxicity, and age ≥70 years and anticoagulation were associated with grade 3 or greater GI toxicity. The multivariate analyses for late toxicity demonstrated a greater risk of grade 2 or greater (relative risk 1.36, P = .10) and grade 3 or greater GU toxicity (relative risk 2.74, P = .04) with DM.

A greater incidence of late GU toxicity was seen in patients with DM treated for prostate cancer. This relationship might be useful when considering the treatment of patients with DM, especially those receiving dose-escalated RT or with a history of transurethral resection of the prostate.