Inclusion Criteria:  Male A history of proven ER+ (>10% of cells), AR+ (>10% of cells), and HER2- metastatic BC Tumor progression after at least one line of conventional endocrine therapy (tamoxifen, AI, fulvestrant, CDK4/6, ±LHRH analogue). Age ≥ 18 years  Adequate hematological, renal and liver function as follows:  Absolute neutrophil count > 1.5 x 109/L Platelet count >100 x 109/L White blood cell count >3 x 109/L AST and ALT <2.5 or <5.0 in case of liver metastases x upper limit of normal (ULN) Creatinine clearance >50mL/min Prothrombin time, partial thromboplastin time and INR <1.5 x ULN Written informed consent  Exclusion Criteria:  History of prostate, testicular or liver cancer Patients already using testosterone supplements Patients using medication with anti-androgenic effects (e.g. spironolactone) Elevated PSA (>4μg/L) or severe urinary tract problems (as defined with a Prostate Symptom Score >19). Patients with known BRCA mutation and PSA >3 μg/L will be referred to the urologist for prostate cancer screening, and can participate if they have no signs of prostate cancer. Hematocrit >50% Patients with uncontrolled hypertension, diabetes mellitus or other significant cardiovascular morbidity. Patients with recent history of coronary artery disease or trombo-embolic events within 6 months prior to screening Severe concurrent disease, infection, co morbid condition that, in the judgment of the investigator would make the patient inappropriate for enrollment Visceral crisis and/or rapid progression necessitating chemotherapy Previous allergic reaction to androgen agonists Contra-indication for PET imaging Tamoxifen or fulvestrant treatment <5 weeks prior to FES-PET.