Paget disease of the nipple is a rare manifestation of mammary carcinoma. The concept of Paget disease is that malignant cells within the ductal system extend proximally all the way up to the nipple skin without crossing the basement membrane.

This case is a classic presentation of this entity. The patient presented with a history of nipple scaling, oozing, and erythema. This clinical presentation occurs because the malignant Paget cells disrupt the normally tight squamous epithelial barrier. In this case, a skin biopsy lead to a diagnosis of Paget disease and further work up lead to the discovery of a retroareolar invasive ductal carcinoma and positive nodes. The patient underwent neoadjuvant chemotherapy and subsequently had a mastectomy (this specimen). There was no invasive carcinoma remaining in the treated breast. Also, of note, the patient was male.

The Paget cells are generally large and atypical with light cytoplasm and prominent nucleoli. These cells tend to form small groups near the basal epidermis, as opposed to single cells seen more superficially. The microscopic differential diagnosis of Paget disease includes Bowen's disease, melanoma, and Toker cell hyperplasia.

Paget cells stain with CK7 and HER2; see Related Content section.