Trial Evaluating the Safety and Efficacy Of MR-Linac-Guided Radiotherapy as Salvage Treatment After External Beam Radiotherapy Recurrence (TUMORNATOR II)

Inclusion Criteria: 1. 18 years of age or older male participants 2. Biopsy proven intraprostatic prostate cancer after prior definitive external beam radiotherapy (moderate hypofractionation or Stereotactic Body Radiation Therapy (SBRT) acceptable). a. NOTE: Seminal vesicle involvement allowed 3. Pathology confirming recurrent disease must have evidence of viable cancer as indicated with pathology interpretation where a Gleason score can be assigned 4. Serum testosterone ≥ 50 ng/dL determined within 2 months prior to enrollment a. Prior Androgen Deprivation Therapy (ADT) use acceptable if testosterone level \>50ng/dL 5. At least 4 weeks must have elapsed from major surgery 6. Karnofsky Performance Scale (KPS) ≥ 80% or Eastern Cooperative Oncology Group (ECOG) 0-1 7. Prostate size as determined on MRI to be \< 90 cc. 8. International Prostatism Symptom Score (IPSS) ≤ 20 9. Patient must be available for follow-up. After 2 years of follow-up, upon completion of post-treatment biopsy, telephone, and chart review-based follow-up will be acceptable for up to 5 years 10. Presence of a T2 and/or DWI-visible prostatic lesion on mpMRI a. NOTE: Lesion visible on MRI required for enrollment but PSMA/PET avid lesion not required 11. Satisfy all MRI screening criteria and be willing to fill out the standard MRI screening form Exclusion Criteria: 1. History of prostate brachytherapy (low dose rate or high dose rate) 2. Actively undergoing androgen deprivation therapy and/or anti-androgens at time of enrollment 3. CT or MRI or PET scan evidence of extraprostatic disease 4. Patients with one or more positive lymph nodes considered suspicious as determined by clinical assessment on MRI or CT or PET scan 5. History of another malignancy within the previous 2 years except for the following: adequately treated basal cell or squamous cell skin cancer, superficial bladder cancer, currently in complete remission, or any other cancer that has been in complete remission for at least 3 years 6. Patients with Crohn's disease or ulcerative colitis 7. MRI ineligibility due to: the presence of a cardiac pacemaker, defibrillator, or other implanted metallic or electronic device which is considered MR unsafe; severe claustrophobia; inability to lie flat for the duration of the study; etc. 8. Metallic implant or device in the pelvis that might distort the local magnetic field and compromise quality of mp-MRI 9. Lateral pelvic separation greater than 50 cm and/or anterior-posterior separation greater than 35 cm which are incompatible with MRCAT reconstruction 10. Contra-indications to receiving gadolinium contrast or PSMA radioligand 11. Karnofsky Performance Scale (KPS) \< 80 or ECOG 2+ 12. Grade 3 or higher toxicity experienced after the initial course of External Beam Radiation Therapy (EBRT) 13. Disease free interval \< 2 years 14. Hx of Transurethral Resection Of Prostate (TURP) within the year 15. Prior high-intensity focused ultrasound (HIFU) or Cryotherapy 16. Prior history of urethral stricture 17. Unable to give informed consent 18. Unable to complete quality of life questionnaires