An Unexpected Presentation of Male Breast Cancer in Ectopic Breast Tissue

Abstract

Male breast cancer remains rare, only comprising about 1 per cent of all breast cancers diagnosed in the United States. Primary ectopic breast cancer (PEBC) is even more unusual. A suspicious axillary mass can be ectopic breast tissue (EBT) that develops anywhere along the embryologic “milk line,” which extends from the axilla to the groin.1 The ectopic tissue develops when there is incomplete embryologic regression of the ectodermal tissue which develops at the sixth week of gestation and gives rise to breast tissue.2 This tissue can display the same physiologic changes as normal breast tissue. Ectopic presentation of primary breast cancer is extremely rare, but has been described in case reports. Here, we describe such a case. A 54-year-old black male presented with a left superficial axillary mass. The mass was present for several months, but did not change in size. It was approximately 1 cm in size and was mildly tender to palpation. An excisional biopsy was performed in the office under local anesthesia. Surprisingly, pathology revealed a Grade 2 adenocarcinoma consistent with metastatic invasive mammary ductal carcinoma with focal mucinous differentiation and positive margins. On immunohistochemical staining, the cells were positive for mammoglobin, estrogen, and progesterone. Her2Neu was equivocal on immunohistochemical but negative by FISH. The excision site was in his mid-axilla and fell within the embryologic milk line. Because of concern for a metastatic implant, systemic staging was performed with CT and nuclear medicine bone scan, which revealed no evidence of disease. A bilateral breast ultrasound (US) and mammogram did not identify a primary tumor. His past medical history included prostate cancer status/post-radical prostatectomy three years prior, sickle cell trait, and hyperlipidemia. His family history was significant for a sister diagnosed with breast cancer in her fifties and prostate cancer in two brothers, his father, and his uncle. Given his history and negative imaging findings, the invasive ductal carcinoma was determined to be a primary breast cancer located along the embryologic mild duct line. The patient was then counseled on the need for re-excision, given his positive margins and need for sentinel lymph node biopsy (SLNB). Genetic testing was negative for any pathogenic mutations. Preoperatively, the patient was injected with a radionucleotide tracer and blue dye at the EBT and