PRIMARY OBJECTIVE:

I. To determine whether (MRI)-guided stereotactic body radiotherapy (SBRT) improves acute physician-scored genitourinary (GU) toxicity when compared with standard computed tomography (CT)-guided SBRT for prostate cancer (PCa).

SECONDARY OBJECTIVES:

I. To determine whether there are differences in acute grade >= 2 gastrointestinal (GI) toxicity as assessed by the Common Terminology Criteria for Adverse Events (CTCAE) version 4.03 scale, following MRI-guided SBRT versus CT-guided SBRT.

II. To determine whether there are differences in 5-year cumulative incidences of late grade >= 2 GU and GI physician-reported toxicity, following MRI-guided SBRT versus CT-guided SBRT.

III. To quantify the temporal changes in patient-reported quality of life (QOL) outcomes, as assessed by the Expanded Prostate Cancer Index-26 (EPIC-26), International Prostate Symptom Scores (IPSS), and Sexual Health Inventory for Men (SHIM) QOL indices, following MRI-guided SBRT.

IV. To determine whether there are differences in 5-year biochemical recurrence-free survival (BCRFS) following MRI-guided SBRT.

V. To observe the proportion of SBRT fractions for which on-line adaptive radiotherapy is required due to changes in organ-at-risk anatomy.

OUTLINE: Patients are randomized to 1 of 2 groups.

GROUP I: Patients undergo 5 fractions of CT-guided SBRT over 14 days in the absence of disease progression or unacceptable toxicity.

GROUP II: Patients undergo 5 fractions of MRI-guided SBRT over 14 days in the absence of disease progression or unacceptable toxicity.

After completion of study treatment, patients are followed up every 3 months for 1 year, every 6 months for 4 years, and then yearly thereafter.