Alopecia neoplastica due to metastatic breast carcinoma vs. extramammary Paget's disease: mimicry in epidermotropic carcinoma.

Clinically similar skin diseases can exhibit phenotypic mimicry as demonstrated here for alopecia neoplastica. An 89-year-old female patient had been treated for scalp eczema for several years until biopsy revealed pagetoid carcinoma. Her clinical history showed a mastectomy 20 years ago. For that reason the differential diagnosis was extramammary Paget's disease or epidermotropic metastasis of the breast carcinoma. Immunohistochemistry revealed positivity of MUC1, indicating that the latter is the case.