Pigmented Paget's disease of nipple: A diagnostic challenge on cytology

Paget's disease is a rare form of breast cancer often associated with an underlying ductal carcinoma in situ or invasive cancer. A 47-year-old female patient presented with bleeding from the left nipple since 4 months. Imprint smears from the lesion showed pleomorphic malignant epithelial cells in singles and in small clusters, many of them containing dark brown cytoplasmic pigment granules. There was no palpable breast lump or axillary lymph node enlargement. Based on the physical examination and cytological features, a diagnosis of malignant melanoma with a differential of Paget's disease of the nipple was made. Biopsy showed features of Paget's disease and immunohistochemistry was positive for HER-2/neu, negative for HMB-45 and S-100, thus confirming the diagnosis. This case has been reported to emphasize the fact that Paget's cells can contain imbibed brown cytoplasmic pigment and should not be mistaken for melanoma cells on cytology smears.