Poorly differentiated squamous cell carcinoma of the nipple: a unique case for marked exophytic growth, but little invasion with neuroendocrine differentiation.

A 73-year-old woman showed marked exophytic growth of a tumor (25 × 23 × 14 mm) of the nipple over a period of 2 months. Histologically, numerous tumor nodules with no apparent keratinization were observed in the exophytic lesion. The tumor cells also showed little invasion to the dermis and no metastasis to the axillary lymph nodes (LN). The tumor cells were immunohistochemically positive for cytokeratins (CKs; AE1/AE3 and 34βE12), epithelial membrane antigen (EMA), and p53, but negative for Ber-EP4 and human papilloma virus (HPV). The MIB-1 index was 56%. Some tumor cells were also positive for some neuroendocrine markers, and showed some tonofilaments and neurosecretory granules in the cytoplasm under electron microscopy. We made the differential diagnosis of mammary ductal carcinoma, basal cell carcinoma (BCC), Paget's disease, and neuroendocrine carcinoma including Merkel cell carcinoma. The final diagnosis was poorly differentiated squamous cell carcinoma (SCC) showing exophytic growth with neuroendocrine differentiation (ND) in the nipple. To our knowledge, although only five cases of Bowen's disease have been reported in the nipple, such a unique SCC has not been reported previously.