The purpose of this study was to evaluate the dose-volume relationships of genitourinary toxicity after stereotactic body radiation therapy (SBRT) monotherapy for prostate cancer.

Fifty-six patients diagnosed with low- to intermediate-risk prostate cancer treated with SBRT alone were reviewed retrospectively. All patients received a total dose of 38 Gy in 4 fractions with a planning target volume expansion of 2 mm. Overall, acute, and late genitourinary toxicity were documented according to the Common Terminology Criteria for Adverse Events (version 4) and International Prostate Symptom Scores (IPSS).

The median age at treatment was 68 years, and the median prostate volume was 45.5 mL, with a median baseline IPSS of 9.95. The median prescription isodose line was 68%. The median clinical follow-up was 35.49 months. Acute grade 1, 2, and 3 genitourinary toxicities occurred in 41.1%, 35.7%, and 0% of patients. All acute genitourinary toxicities resolved except 1 patient with grade 2 toxicity that progressed to grade 3 late toxicity. No dose-volume relationships were associated with acute genitourinary grade 2+ toxicity. Late grade 1, 2, and 3 genitourinary toxicity occurred in 19.6%, 19.6%, and 3.6% of cases, respectively. Of the cases with late toxicities, 16.7% were persistent. Late grade 2+ genitourinary toxicity was associated with prostate volume ≥50 mL, lower homogeneity index, and urethral maximum point dose ≥47 Gy. The overall risk of any grade 2+ genitourinary toxicity was associated with baseline IPSS >7, prostate volume ≥50 mL, urethral volume receiving 44 Gy, and bladder volume receiving 19 Gy.

SBRT for prostate cancer appears well tolerated, with mostly transient low-grade toxicity. Urethral sparing should be used with a maximum point dose <47 Gy, volume receiving 120 Gy <50% of the prostate, and bladder volume receiving 19 Gy <15 mL in 4 fraction treatments. Patients with prostate volumes ≥50 mL should be counseled regarding the increased risk of moderate-grade genitourinary toxicity.