The objectives of the study are to analyze the results obtained in survival, safety and quality of life after the combination of SBRT plus second-generation antiandrogen in patients diagnosed with metastatic castration-resistant prostate cancer in oligometastasis (≤5).

Hence, the investigators evaluate the results of the combination of two widely used treatments in prostate cancer: SBRT plus the 'standard of care' for patients with mCRPC, second-generation antiandrogens (abiraterone and enzalutamide).

Primary objective: Determine the PRFS measured from the time of initiation of second-generation antiandrogen to the time of radiological progression by choline PET/CT or 68Ga-PSMA PET/CT, in patients with mCRPC treated with the combination of second-generation antiandrogen plus SBRT.

Secondary objectives:

• Overall survival (time from initiation of second-generation antiandrogen to exitus) in patients with mCRPC treated with the combination of second-generation antiandrogen plus SBRT.
• Quality of life of these patients using the European Organization for Research and Treatment of Cancer (EORTC) quality of life scale, EORTC QLQ-C30, validated for oncology patients.
• Acute and chronic toxicity according to the "Common Terminology Criteria for Adverse Events" (CTCAEv5.0) scale.

This is a phase II, multicenter, prospective, single group study in patients with castrate-resistant prostate cancer (CRPC).

Patients will be recruited at the participant sites, after study approval and hospital authorization.

51 patients will be included in the study. Eligible patients will require:

• Choline PET/CT or 68Ga-PSMA PET/CT, dated no more than 8 weeks prior to inclusion.
• Clinical history, physical examination and functional evaluation scale (ECOG).
• Geriatric assessment scale for patients > 75 years (modified G8 scale)
• Toxicity scale assessment (CTCAEv5.0)
• Baseline quality of life questionnaire (EORTC QLQ-C30) (47)
• Blood tests with complete blood count, coagulation, liver function, renal function, ions, alkaline phosphatase, PSA and total and free testosterone, dated no more than 4 weeks prior inclusion.
• Inclusion/exclusion criteria assessment
• Second generation antiandrogen prescription, chosen by the responsible physician according to clinical practice, in each case.

Each patient will be included in the study once the second generation antiandrogen is initiated (which will be maintained until radiological progression, suspension due to poor tolerance and/or exitus). SBRT will be performed at least 2 weeks after initiating treatment with second generation antiandrogen, without exceeding 8 weeks from the start of treatment, and follow-up will continue up to a maximum of 36 months after completion of SBRT and/or exitus.

Recruitment time is estimated at 36 months and a total follow-up time of 36 months. The total duration of the trial is therefore expected to be 6 years, to allow complete follow-up of the last patients included.