Mammary Paget's Disease of the Male Breast: A Rare Case With an Unusual Immunohistochemical Profile.

Mammary Paget's disease is rare and comprises about 0.62% of all breast cancer cases, only 1.65% of which occur in male patients. This case report involves a 76-year-old man who presented to his primary care physician with an itching, scaly, unilateral lesion involving the nipple skin. He underwent wide local excision of the lesion for a diagnosis of Bowen's disease (squamous cell carcinoma in situ). Histologic examination of the specimen revealed mammary Paget's disease with ductal carcinoma in situ in the underlying breast tissue. A panel of immunohistochemical stains revealed the Paget cells to be positive for cytokeratin 7, MUC1, GATA3, and androgen receptor and negative for cytokeratins 5/6, p63, SOX10, and MART-1/Melan-A. Paget cells were also negative for estrogen receptor and progesterone receptor, and positive for HER2/neu. However, the underlying ductal carcinoma in situ was positive for both estrogen receptor and progesterone receptor and negative for HER2/neu. This discordance, supported by the current literature, suggests an alternative etiology for Paget's disease in certain cases that cannot be explained by the well-established epidermotropic and transformative theories of Paget's disease evolution.