Dose-volume analysis and the temporal nature of toxicity with stereotactic body radiation therapy for prostate cancer

Purpose: 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. Methods and materials: 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). Results: 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. Conclusions: 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.