Background: Androgen Receptor Targeted Agents (ARTA) represent one of the main treatment options for metastatic castrate resistant prostate cancer (mCRPC). Addition of stereotactic radiation therapy (SBRT) to ablate metastatic foci may improve clinical outcomes in oligometastatic setting. ARTO trial (NCT03449719) is a randomized phase II trial testing the benefit of upfront SBRT on all sites of metastatic disease in oligo-mCRPC patients undergoing I line therapy with Abiraterone Acetate (AA). In this preliminary analysis, we report results after 6 months of follow up, together with an exploratory analysis of androgen receptor splice variants (ARV7/ARFL) Prostate Specific Antigen (PSA) and Prostate Specific Membrane Antigen (PSMA) expression on Circulating Tumor Cells (CTCs) detected in this cohort of patients. Methods: 31 patients affected by oligo-mCRPC (defined as < 3 non-visceral metastatic lesions) were randomized to receive I line AA therapy with or without SBRT on all metastatic sites. Baseline blood samples to detect CTCs and evaluate their ARV7, ARFL, PSA and PSMA expression were taken before AA treatment start. Assessments comprehensive of clinical examination and serum PSA were performed every three months. Toxicity was assessed by the Common Terminology Criteria for Adverse Events toxicity scale (CTCAE v.4.03). Results: Thirteen and 18 patients were enrolled in the treatment and control arm, respectively. Nineteen metastatic lesions were treated with SBRT in the treatment arm. At 6 months, complete response (defined as a serum PSA level < 0.2 ng/dl) and biochemical response (defined as a PSA reduction > 50% if compared to baseline) were achieved in 6 vs 4 and in 10 vs 8 patients in the treatment vs control arm, respectively. One patient in the treatment arm died for other causes, 1 biochemical progression occurred in the control arm. No adverse events occurred in both arms of treatment. CTCs analysis was available for 10 patients, out of whom 4 were found positive for CTCs (1 and 3 from the treatment and control arm, respectively). ARV7 and ARFL were expressed in 1 patient from the control arm, PSMA was expressed in all CTC positive patients, PSA was expressed in 2 patients from the control and one from the treatment arm. Conclusions: SBRT+AA in oligo-mCRPC patients was safe and yielded promising biochemical results. CTCs detection in this selected cohort of oligo-mCRPC was lower if compared to historical data of unselected mCRPC patients. Data about ARV7, ARFL, PSA and PSMA expression represent an interesting snapshot of biomarker arrangement in this setting. Clinical trial information: NCT03449719