Stereotactic Body Radiation Therapy for Prostate Cancer: An Institutional Experience Using MRI-guided Treatment Planning

With 222,500 new cases estimated for 2017, prostate cancer makes up approximately 10% of all new cancer diagnoses in the United States and is the third most common cancer after breast and lung cancer. In 2013, the American Society of Radiation Oncology (ASTRO) policy model recognized that stereotactic body radiation therapy (SBRT) may be used as an alternative to standard treatment modalities, i.e. intensity modulated radiation therapy (IMRT), to treat prostate cancer. In this study, we report outcomes for a cohort of 30 patients with prostate cancer treated with SBRT at our institution. We also describe, in detail, the technical aspects of SBRT planning and delivery for these patients, specifically the use of MRI in determining treatment volumes and detecting gross lesions. After institutional review board (IRB) approval, a retrospective analysis was done of 30 males with the diagnosis of prostate cancer treated in the Department of Radiation Oncology at the Baylor College of Medicine between January 2011 and June 2016. All patients received image-guided SBRT. Treatment planning was performed using a non-contrast computed tomography (CT) scan as well as a contrast thin-slice open MRI with the patient in the treatment position. Patient comparisons were done using the Mann-Whitney U, Fishers Exact, and Kaplan-Meier tests. Thirty patients were treated between January 2011 and June 2016. Twenty-six had follow-up data available and were included in the analysis. Median follow-up was 32 months (range 2-72 months). Mean and median ages at diagnosis were both 68.5 years. A total of 64% of the patients had foci on magnetic resonance imaging (MRI) or a palpable nodule on an exam. The median prostate-specific antigen (PSA) at diagnosis was 7.35 ng/mL (range 2.8-13), and the median PSA nadir after treatment was 0.4 ng/mL (range 0.01-4.5). The biochemical disease-free recurrence rate per Phoenix definition was 96%, with only one patient experiencing a biochemical recurrence four years after treatment. The patient with a recurrence was T2c, high-intermediate risk with a Gleason score of 7(3+4). He had a focus visible on MRI. Overall survival was 96%, with the only patient death unrelated to his prostate cancer. There was no statistical significance associated with recurrence and nodule on MRI (p=0.318), T-stage (p=0.222), Gleason score (p=0.890), risk group (p=0.654), age (p=0.692), or race (p=0.509). There were no grade three or four acute or long-term toxicities. SBRT of the prostate is an effective method for treating prostate cancer. We saw excellent PSA control and minimal acute or long-term toxicities after a median of three years of follow-up.