Primary squamous neuroendocrine carcinoma of the skin: A case report with immunohistochemical study

Primary neuroendocrine carcinoma (NEC) except for Merkel cell carcinoma has rarely been reported in the skin. Herein reported is a unique case of a primary cutaneous NEC with histological features of squamous cell carcinoma (SCC) with high cellular anaplasia; the tumor is called “squamous NEC” in this report. A 77-year-old man presented with a skin tumor measuring 0.7 cm × 0.8 cm × 0.7 cm of the right second finger. A biopsy showed invasive malignant epithelial cells with keratinization. The carcinoma cells were continuous with epidermis, and showed high cellular anaplasia. No apparent lympho-vascular permeations were seen. The pathological diagnosis was poorly differentiated SCC. Because the histological features were unusual, an immunohistochemical study was performed. The carcinoma cells were positive for cytokeratin (CK) AE1/AE3, CK34BE12, CK5/6, neuron-specific enolase (NSE) (positive percentage (PP) = 70%), chromogranin (PP = 95%), synaptophysin (PP = 95%), NCAM (PP = 20%), p63, p40, S100 protein (Langerhans cells), and Ki-67 (labeling index (LI) = 98%). The carcinoma cells were negative for CK7, CK8, CK9, CK14, CK18, CK19, CK20, CK CAM5.2, smooth muscle actin, vimentin, desmin, HMB45, p53, CD45, TTF-1, HCG, and surfactant apo-protein. Because the skin SCC showed neuroendocrine features and the neuroendocrine cells exceeded 50% of the total tumor cell population, a diagnosis of squamous NEC was made. The histological features, NEC features, high anaplasia, and high Ki-67 labeling index (LI = 98%) suggested poor prognosis. Systemic investigations using CT, MRI, PET, and upper and lower gastrointestinal endoscopy revealed no tumors except for the finger tumor. No lympho-adenopathy was seen. The outcome seemed not good. The tumor of the finger was difficult for operation. Wide excision or finger amputation is considered.