This single arm, prospective study will determine the prostate-specific quality of life (QOL) of patients undergoing undergoing a 2 fraction MRI-guided stereotactic ablative body radiation (SABR) protocol. We propose prescribing a prostate dose of 26 Gy in 2 fractions and a dose of up to 32 Gy to the dominant intraprostatic lesion (DIL) in 2 fractions over one week.

Stereotactic Ablative Body Radiation Two weekly fractions of:

13 Gy to whole prostate +/- seminal vesicles AND Up to 16 Gy simultaneous boost to dominant MRI nodule

Treatment Delivery and Quality Assurance Treatment will be delivered on treatment units equipped with with volumetric arc therapy and hexapod couches. Only photons with megavoltage energies of 6 MV will be used. Cone-beam CT imaging will be performed using the implanted fiducials to set up each treatment. Imaging after each fraction of radiation will be performed and the shifts in prostate positioning (as measured by displacement of gold seed fiducials) will be measured and recorded. Also, the total treatment time for each fraction of radiotherapy will be recorded.

Patient Assessments / Follow-up Time zero will be the start of radiotherapy. Baseline and follow-up parameters are as listed in Appendix A. Baseline rectal and urinary function will be recorded. Acute toxicities will be assessed at weeks 1, 4, and 13 and late toxicities will be assessed at month 6 and every 6 months until year 5 using the Common Terminology Criteria Adverse Events, version 4.0. Bloodwork (PSA and testosterone), will be performed at months 3, 6 and every 6 months until year 5. QOL using the Expanded Prostate Cancer Index Composite (EPIC) will be obtained at baseline, weeks 1, 4, 13, month 6 and every 6 months until year 5.

Inclusion Criteria:

1. Informed consent obtained
2. Men >18 years
3. Histologically confirmed prostate adenocarcinoma (centrally reviewed)
4. Favorable risk prostate cancer, defined as clinical stage T1-2c, Gleason Sum less than or equal to 7, and PSA less than or equal to 20 ng/mL (patients can have only one of T2c, Gleason Sum 7 and PSA 10-20 ng/ml)

Exclusion Criteria:

1. Androgen deprivation therapy (LHRH-agonists or antiandrogens) > 6 mo
2. Prior pelvic radiotherapy
3. Anticoagulation medication (if unsafe to discontinue for gold seed insertion)
4. Diagnosis of bleeding diathesis
5. Large prostate (>90cm3) on imaging
6. Immunosuppressive medications
7. Inflammatory bowel disease
8. Presence of a hip prosthesis
9. Contraindications to having MRI