Five patients were included; it did not succeed in recruiting the target number of 6 patients. This is as a result of the rarity of this disease in men.

This is a concise single arm, feasibility study, which will be executed in the University Medical Center Groningen, The Netherlands. Male patients with metastatic BC (n=6) are eligible for this study after at least 1 line of conventional endocrine therapy.

Inclusion Criteria:

1. Male

2. A history of proven ER+ (>10% of cells), AR+ (>10% of cells), and HER2- metastatic BC

3. Tumor progression after at least one line of conventional endocrine therapy (tamoxifen, AI, fulvestrant, CDK4/6, ±LHRH analogue).

4. Age ≥ 18 years

5. 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

6. Written informed consent

Exclusion Criteria:

1. History of prostate, testicular or liver cancer
2. Patients already using testosterone supplements
3. Patients using medication with anti-androgenic effects (e.g. spironolactone)
4. 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.
5. Hematocrit >50%
6. Patients with uncontrolled hypertension, diabetes mellitus or other significant cardiovascular morbidity.
7. Patients with recent history of coronary artery disease or trombo-embolic events within 6 months prior to screening
8. Severe concurrent disease, infection, co morbid condition that, in the judgment of the investigator would make the patient inappropriate for enrollment
9. Visceral crisis and/or rapid progression necessitating chemotherapy
10. Previous allergic reaction to androgen agonists
11. Contra-indication for PET imaging
12. Tamoxifen or fulvestrant treatment <5 weeks prior to FES-PET.

male breast cancer patients