PSMA PET Response Guided SabR in High Risk Pca

Inclusion Criteria: -Pathologically confirmed adenocarcinoma of the prostate (within 180 days of registration) of high risk by national comprehensive cancer network (NCCN) criteria as determined by >=cT3a stage (AJCC 8th edition) OR PSA>20ng/mL OR ISUP Grade Group 4-5 (Gleason Grade 8-10). Age ≥ 18 years. Planned for definitive intent stereotactic ablative radiotherapy (SabR) with integrated dose boost to intra-prostatic tumor and androgen deprivation therapy (ADT) with baseline AUA IPSS <=18 and prostate size <=100cc Staging 68Ga PMSA-11 PET -CT or -MRI performed within 90 days of registration and before initiation of anti-androgen or androgen deprivation therapy and demonstrating no evidence of distant metastases by (PMSA avid or non-avid nodes <=1.5cm short axis allowed). Conventional imaging (CT, bone scan, MRI) may also be used in addition to PMSA-PET, and definitive findings of distant extra-pelvic metastases on these scans are not allowed for enrollment. Staging 68Ga PSMA-11 PET -CT or -MRI demonstrating a PSMA-avid primary intra-prostatic target lesion amenable at investigator discretion to dose boost All men must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry, for the duration of standard of care SabR and for a period of time of 6 months thereafter as per standard guidelines. Should a man's partner become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately. Ability to understand and the willingness to sign a written informed consent. Exclusion Criteria: Prior curative intent local therapy (e.g. prostatectomy, radiotherapy, focal ablative therapy) for prostate cancer is not allowed, with following exceptions regarding androgen deprivation therapy (ADT)/anti-androgen therapy (AAT): Prior androgen deprivation therapy (ADT) allowed if <3 month total duration and stopped >=3 months prior to registration with demonstration of non-castrate testosterone recovery (>50ng/dL) and meeting all other inclusion criteria. Ongoing androgen deprivation therapy (ADT) is allowed if <=60 days total duration AND meeting following criteria: If GnRH agonist used (e.g. leuprolide), bicalutamide must have been used for at least 30 days +/-14 days from start of GnRH agonist. All other inclusion criteria. Subjects may not be receiving any other investigational agents for the treatment of the cancer under study. History of allergic reactions to PMSA-11 68Ga imaging agent. Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that, in the opinion of the investigator, would limit compliance with study requirements. Prior pelvic radiotherapy other than cutaneous/superficial treatments.