Background: Prostatectomy has become the most common treatment for high-risk prostate cancer. After prostatectomy, risk features for local recurrence include extra-capsular extension and/or seminal vesicle invasion (pT3 disease) and positive surgical margins. Post-operative radiotherapy is recommended for patients with pT3 disease, positive margins and/or Group Grade 4-5 and has been shown to reduce the risk of recurrence. In planning adjuvant radiotherapy, the clinical target volume is the prostate bed which is difficult to delineate once the prostate has been surgically removed and can be overestimated by following the location of surgical clips. As such, controversy exists as to the optimal target volume. Guidelines suggest including a significant portion of the posterior and inferior aspects of the bladder, and anterior aspect of the rectum exposing a significant amount of normal tissue to high dose radiation. Pre-operative radiotherapy potentially offers the following advantages: 1) reduced radiation dose to normal tissues; 2) ability to utilize stereotactic body radiotherapy (SBRT) with real-time MRI guidance 3) patient convenience with less fractions and 4) radiobiologic advantage of high dose per fraction treatment delivery. Methods: This is a modified dose escalation trial for patients with high-risk prostate cancer defined as cT3 on MRI or Group Grade 4-5 disease. Patients will sequentially undergo 1) placement of a rectal SpaceOAR, 2) SBRT on the ViewRay MRIdian linear accelerator and 3) prostatectomy. As a modified dose escalation study, patients will be accrued starting at 5 Gy x 5 (up to 8 Gy x 5) with 3 patients per cohort with Grade 4 or 5 gastrointestinal (GI) and/or genitourinary (GU) adverse event related to preoperative radiotherapy as an unacceptable dose limiting toxicity.The primary endpoint is that a patient can undergo a radical prostatectomy after SBRT without a post-operative GI or GU grade 4 or 5 toxicity within 30 days related to preoperative radiotherapy. Secondary objectives include assessment of acute toxicity and quality of life scores. Exploratory analyses include analysis of tumor and normal biopsied and resected tissue and serum markers and interpretation of pre- and post-SBRT MRIs. Clinical trial information: NCT03663218