Background: Radiation therapy (RT) to the prostate bed and pelvic nodes with short-term androgen deprivation therapy (STAD) is considered a standard of care (SOC) salvage therapy (ST) paradigm for prostate cancer (PC) patients (pts) with post-prostatectomy (RP) biochemical recurrence (BCR). Fluciclovine-PET/CT imaging is FDA-approved in this setting, with improved accuracy for detection of metastases (mets) not identified with conventional imaging (CIM). Given greater sensitivity and specificity of PET, its findings are being increasingly but variably applied to justify modification or omission of SOC therapies without high-level evidence of clinical benefit. PET may help identify candidates for local or systemic treatment intensification of otherwise non-tailored SOC. Earlier detection of mets with molecular imaging has led to increasing use of focally ablative met-directed RT, to delay or enhance systemic therapy through better local control. There is also interest in earlier use of advanced systemic therapy; apalutamide (Apa) is a nonsteroidal antiandrogen with established efficacy in improving overall and radiographic progression-free survival (PFS) for non-metastatic castrate-resistant and metastatic castration-sensitive PC, and potential activity for low-volume mets. This study will evaluate whether pts with PET-detected lesions benefit from such local or systemic treatment intensification approaches. Methods: PC pts with post-RP BCR (PSA>0.5ng/mL; >0.2 if RP within 12 mos), and negative CIM who are candidates for SOC ST (RT to prostate bed and pelvic nodes + STAD) and undergo SOC baseline PET are eligible. The study will initially use ¹⁸F-fluciclovine but permit additional radiotracers based on FDA approval and availability. Based on institutional clinical interpretation of the SOC PET, pts will be placed in Cohort 1 (PET-negative) or 2 (PET-positive for extra-pelvic mets). Cohort 1 will be randomized to SOC ST +/- Apa for 6 months and Cohort 2 will be randomized to SOC ST and Apa +/- met-directed RT to PET-positive lesions. The primary endpoint is PFS, defined as time from randomization to radiographic progression on CIM, symptomatic disease or death. Primary objectives are to evaluate whether addition of Apa to SOC ST and addition of met-directed RT to SOC ST and Apa could prolong PFS in Cohorts 1 and 2, respectively. For Cohort 1, 480 pts will be randomized with 85% power to distinguish 5-year PFS rate of 90% (Apa arm) vs. 80% (SOC arm) using one-sided stratified logrank test with type I error of 0.025. For Cohort 2, 324 pts will be randomized with 85% power to distinguish 5-year PFS rate of 76.5% in experimental arm from 61.5% in control arm. Secondary endpoints include overall and event-free survival, toxicity, and PET progression. Trial was activated on October 8, 2020; NCT04423211. Acknowledgement: This study was conducted by the ECOG-ACRIN Cancer Research Group (Peter J. O'Dwyer, MD and Mitchell D. Schnall, MD, PhD, Group Co-Chairs) and supported by the National Cancer Institute of the National Institutes of Health under the following award numbers: U10CA180794, U10CA180820, U10CA180868, U10CA180888, U10CA180821, UG1CA233196, UG1CA233253, UG1CA233277, UG1CA233328, and UG1CA233330. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. Mention of trade names, commercial products, or organizations does not imply endorsement by the U.S. government. Clinical trial information: NCT 04423211.