The SUGAR Study; SBRT and Relugolix) for Prostate Cancer

Inclusion Criteria: 1. Provision of signed and dated informed consent form 2. Stated willingness to comply with all study procedures and availability for the duration of the study 3. Has histologically or cytologically confirmed diagnosis of adenocarcinoma of the prostate 4. Born assigned to Male gender, age 18 and above. Female identifying transgender or gender fluid are allowed on study provided they have not undergone testosterone suppressing therapy and were born with a prostate. 5. Has a serum testosterone at the Screening visit of \>100 ng/dL 6. Has a serum PSA concentration at the Screening visit of \> 0.2 ng/mL 7. Able to swallow pills and take medication orally (no documented inability to eat solids and swallow pills) and be willing to adhere to the daily regimen of medication (if assigned to the experimental arm). 8. For patients of reproductive potential: use of condoms or other methods (including abstinence) to ensure effective contraception with partner during radiotherapy and through 4 months after the last dose of the study drug or radiotherapy 9. Agreement to adhere to Lifestyle Considerations (see section 5.3) throughout study duration 10. Clinical favorable intermediate risk prostate cancer: Gleason score must be Gleason 3+4 with a PSA \< 20 ng/mL, or Gleason 6 (3+3) and PSA \> 10 ng/mL and \< 20 ng mL. 11. Decipher GC score of 0.6 or higher or higher indicating high genomic risk for most recent biopsy 12. Documented prostate volume (by MRI or ultrasound within 120 days or physician attestation within 120 days) \<= 80 cc Exclusion Criteria: 1. Current use of medications that cause QT prolongation 2. Known allergic reactions to relugolix 3. Treatment with another investigational drug or other intervention for prostate cancer within 30 days of enrollment 4. Ulcerative colitis or other inflammatory bowel disease history 5. Connective tissue disease such as lupus, scleroderma, or dermatomyositis 6. GNRH antagonist therapy or SBRT to the prostate are medically contraindicated or not tolerated 7. History of long QT syndrome documented in the medical record 8. The following ECG abnormalities are excluded: 1. Q-wave infarction unless identified 6 or more months before the Screening Visit 2. QT interval corrected for heart rate (QTc) \> 470 msec. If the QTc is prolonged in a patient with a pacemaker, the patient may be enrolled in the study upon discussion with the study PI 3. Congenital long QT syndromeQ 9. History of surgical castration 10. Prior treatment for prostate cancer with surgery or prostate directed radiotherapy