The purpose of this study is to characterize the reduction in PSMA-avid tumor volume and metastasis-directed radiotherapy treatment intensity facilitated by PET PSMA-response adapted SABR for poly-metastatic castration sensitive prostate cancer.

This study will enroll patients with poly-metastatic hormone-sensitive prostate cancer, as identified by baseline PSMA PET imaging. All participants will undergo standard of care systemic therapy, including ADT and androgen receptor signaling inhibitors intensification as clinically indicated, for up to three months before protocol radiotherapy. The treating radiation oncologist will pre-plan SABR to a total of 35-40 Gy in 5 fractions. The study will consist of initially delivering three qWeekly SABR "pulses" to all identified metastatic sites. The initial pulse should start within 60 (+/-30 days) of initiation of ADT. Following the third pulse, interim PSMA PET imaging will be repeated at 6 months from initiation of ADT (-30 days allowed) to evaluate the response on a per lesion basis (See Section 6.1.2.1). In the absence of a complete response (CR) for a given lesion, two additional qWeekly pulses may be delivered to these sites within 60 days from interim PSMA PET. PSMA avid lesions identified at baseline will be monitored on interim and follow up scan, and new PSMA avid lesions will require confirmation through conventional imaging. If patients exhibit progression on PSA or conventional imaging, they will exit the trial and undergo systemic therapy intensification. More detailed specifications on systemic therapy and SABR and their parameters are provided in the protocol.

Inclusion Criteria:

1. History of pathologically confirmed prostate cancer.
2. Age >=18 years.
3. Performance status ECOG 0-2.
4. Staging 68Ga PMSA-11 PET/CT showing 4-20 sites of metastasis from prostate cancer within <=90 days prior to registration. This scan ideally should be performed before initiation of androgen deprivation therapy (ADT).
5. At the discretion of the treating investigator, it is believed that it is safe to treat all sites of disease using SABR.
6. 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 patient's partner become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately.
7. Ability to understand and the willingness to sign a written informed consent.
8. Planned for standard systemic therapy for metastatic prostate cancer to include androgen deprivation therapy (ADT). Upfront Docetaxel should not be planned (See section 4.1.2).

Exclusion Criteria:

1. Prior radiotherapy to any metastases currently targeted for therapy or overlapping regions.
2. Patient with metastatic lesions involving the gastrointestinal tract, specifically, invading esophagus, stomach, or intestines will be excluded. Patients with ultra-central metastatic lesions defined as 1 cm from the trachea and main bronchi will be excluded.
3. Serious medical co-morbidities precluding safe delivering of radiotherapy to poly-metastatic sites. This includes interstitial lung disease for patients undergoing SABR to thoracic sites and ulcerative colitis or Crohn's disease requiring systemic immunosuppressive therapy for patients undergoing SABR to GI sites.
4. Subjects may not be receiving any other PSMA-directed investigational agents for the treatment of the cancer under study.
5. History of allergic reactions to PMSA-11 68Ga imaging agent.
6. Uncontrolled intercurrent illness or psychiatric illness/social situations that, in the opinion of the investigator, would limit compliance with study requirements.

This study is enrolling men with 4-20 sites of metastasis from prostate cancer