Stereotactic Body Radiation Therapy For Low-Intermediate Risk Prostate Cancer-A Single-Institution Initial Analysis Of Toxicity

Utilizing radiation for effective cancer treatment carries risks for both early and late effects, often compounded with larger fractional doses. Recent studies of stereotactic body radiation therapy (SBRT) for prostate cancer demonstrate excellent biochemical disease free survival with low toxicity. Few detailed toxicity reports, however, have been published. The purpose of this study is to present a detailed toxicity profile analysis of patients treated at a single institution with robotic SBRT. An IRB approved retrospective study of 70 patients treated with robotic SBRT for low-intermediate risk prostate adenocarcinoma was performed. Five fractions of 7.25 Gy was prescribed to a planning target volume (PTV) comprising the prostate, and often, 2 cm of the proximal seminal vesicles, for a total dose of 36.25 Gy (BED = 62.5 Gy). The mean (range) prostate volume and PTV were 42.1 cc (13.5-108.0 cc) and 154.4 cc (41.1-448.1 cc), respectively. Univariate and multivariate logistic regression analyses were performed to assess for factors associated with acute/late toxicity and late complications. Overall, RTOG severe (grade ≥ 3) acute and late toxicities were limited, with 11.4% and 9.8% reported respectively. On univariate analysis, max PTV dose predicted acute ≥ 3 toxicity (P = 0.048), while late toxicity was associated with PTV size (P = 0.038). A high proportion of patients, however, had late complications necessitating urologic intervention: Grade III Clavien-Dindo complications occurred in 22.9%, 9/16 requiring general anesthesia (IIIb). Three patients (4.3%) had grade IV complications, two suffered multi organ dysfunction (IVb). On univariate analysis, prostate volume ≥ 60 cc, max prostate dose, PTV ≥ 131.8 cc, mean PTV dose, max bladder dose and mean penile bulb dose all predicted Clavien toxicity grade ≥ III (P = 0.038; P = 0.008; P = 0.005; P = 0.017; P = 0.010; P = 0.038). Odds of severe toxicity increased by 1% per 1 cc increase in PTV. Ten (14.9%) patients had late urethral stricture and one developed rectourethral fistula. On multivariate analysis, PTV and max bladder dose remained significant for grade ≥ III complications (P = 0.0013; P = 0.0098). SBRT can be an effective treatment modality for low-intermediate risk prostate adenocarcinoma. While acute and late RTOG toxicities were low, a number of patients experienced late complications requiring intervention. Prostate volumes for this cohort were typically large, which, along with PTV size, max bladder dose and mean dose to the penile bulb, predicted late toxicity. These data provide insight into specific individualized risk factors that can help guide appropriate patient selection. A prospective randomized trial comparing SBRT vs. IMRT should be supported to determine the efficacy of SBRT in this context, and to confirm factors predictive for severe toxicity.