Not All Cases of Mammary Paget's Disease are Cytokeratin-7 Positive: A Challenging Diagnosis!

Mammary Paget's disease accounts for 1% to 3% of all breast tumors and manifests as a chronic eczematous lesion of the areolar skin. It can occur without any underlying neoplasia or can be present in association with an underlying invasive and/or in situ carcinoma of the breast. The present report describes a challenging nipple punch biopsy showing an infiltration of the lower third to two-thirds of the epidermis by large, ovoid, neoplastic cells. The morphology was consistent with mammary Paget's disease, although immunohistochemistry for cytokeratin-7 (CK7) was repeatedly negative. This resulted in an initial misdiagnosis and, subsequently, a delay in the patient's follow-up. Additional immunohistochemistry for GATA binding protein 3 (GATA3) and human epidermal growth factor receptor 2 (HER2), as well as a second opinion of a breast pathologist, resulted in the diagnosis of mammary Paget's disease. The aim of this article is to raise awareness among pathologists and prevent them from misdiagnosing CK7-negative Paget disease of the breast.