Molecular pathology provides significant insights into the diagnosis and treatment of cancer of unknown primary origin: a case report of an extremely rare male occult breast apocrine cancer.

We present a case of occult apocrine breast cancer in a man presenting with back pain and multiple bone and large liver metastases. The primary tumour was not identified, and the patient was initially considered to have cancer of unknown primary origin. During referral to our hospital, 2 months had passed since the initial consultation, and the prognosis was considered poor. Empirical treatment with carboplatin and paclitaxel was then administered. Histological examination of the liver metastases revealed large cells with eosinophilic cytoplasm, morphologically suggestive of apocrine breast cancer, but no tumour was found in the breast. The diagnosis of occult apocrine breast cancer was confirmed by concurrent immunohistochemical examination of the biopsy tissue sample, revealing androgen receptor positivity, oestrogen receptor negativity, gross cystic disease fluid protein 15 positivity, human epidermal growth factor receptor 2 overexpression, and gene amplification. Carboplatin and paclitaxel treatment resulted in the near-complete remission of liver metastases and reduction of bone metastases. The reduction has still been maintained at 16 months since the symptom onset. Molecular targeted therapy, including trastuzumab, is expected to further extend survival. Male occult apocrine breast cancer is extremely rare, with only one other case reported in the literature. We report the diagnosis and treatment response of this extremely rare case.