A case report of pigmented mammary Paget’s disease mimicking nevus of the nipple

A 43-year-old Japanese woman consulted our hospital for a pigmented lesion on her right nipple. Two years later, the lesion became enlarged, measuring 5 × 5 mm. It was dark brown, had an irregular shape and relatively clear borders. Incisional biopsy yielded a pathological diagnosis of junctional nevus of the skin. An additional 2 years later, a small mass developed under the right nipple area and core needle biopsy yielded a pathologic diagnosis of invasive ductal carcinoma. Partial resection of the right EC areas included the skin of the nipple and sentinel lymph node biopsy was performed. Histologically, the skin of the nipple demonstrated small clusters of pigmented carcinoma cells that were low molecular weight cytokeratin (CAM5.2) positive. Most of the carcinoma cells were small and did not have abundant cytoplasm, but nuclear enlargement and prominent nucleoli indicated malignancy, and the cytoplasm was pale compared with that of the surrounding squamous epithelial cells. Scattered dendritic melanocytes were identified by S-100 protein and HMB-45 immunohistochemically. In the upper dermis, carcinoma cells also involved the lactiferous ducts. A small focus of carcinoma cells that invaded the fat tissues did not contain melanin pigment. The final diagnosis was pigmented mammary Paget’s disease. Pigmented lesions on the nipple should be carefully examined, because pigmented mammary Paget’s disease sometimes mimics malignant melanoma or junctional nevus.