Background: Metastasis directed therapy (MDT) is able to prolong progression free survival (PFS) and forestall initiation of androgen deprivation therapy (ADT) in men with hormone-sensitive, oligometastatic prostate cancer (HSOPCa) compared to observation. While MDT appears to be effective in HSOPCa, a large percentage of men will have disease recurrence. Patterns of failure demonstrate patients tend to recur in the bone following MDT, raising the question of sub-clinically-apparent osseous disease. Radium-223 dichloride is a radiopharmaceutical with structural similarity to calcium, allowing it to be taken up by bone where it emits alpha particles, and therefore might have utility in the treatment of micrometastatic osseous disease. Therefore, the primary goal of the phase II RAVENS trial is to evaluate the efficacy of Stereotactic ablative radiation (SABR) + radium-223 dichloride in prolonging PFS in men with HSOPCa. Methods: Patients with HSOPCa and 3 or less metastases with at least 1 bone metastasis (by conventional imaging) will be randomized 1:1 to SABR alone vs. SABR + radium-223 dichloride. Eligibility criteria include PSA doubling time of < 15 months and ECOG performance status of < 2. Patients cannot be on ADT and must have normal testosterone levels at the time of randomization. Patients randomized to the combination arm will receive six doses of Radium-223 dichloride at four week intervals. A sample size using a 1:1 randomization scheme of 30 patients per arm will provide 80% power to detect an increase of median PFS from 10 months to 20 months with type I error = 0.1, using a one-sided log-rank test. To account for 5% early drop out, we will randomize a total of 64 patients (32 per arm). The primary end point is PFS with a primary hypothesis that SABR + radium-223 dichloride will increase median PFS from 10 months in the SABR arm to 20 months in the SABR + radium-223 dichloride arm. Progression is a composite endpoint including PSA progression per Prostate Cancer Working Group 2 (PCWG2), symptomatic progression, radiologic progression per RECIST 1.1 criteria, initiation of ADT, or death due to any cause. Secondary clinical endpoints include toxicity and quality of life assessments, local control at 12 months, locoregional progression, time to distant progression, time to new metastasis, and duration of response. Biological correlates will be evaluated including changes in circulating tumor cells following therapy, deep sequencing of circulating tumor DNA, and T-cell repertoire profiling before and after therapy. Clinical trial information: NCT04037358.