Primary large cell neuroendocrine carcinoma of the skin: An under‐recognized entity and a mimic of metastatic disease

Primary large cell neuroendocrine carcinomas of the skin are exceptionally rare and can be diagnosed only when a metastasis from another organ has been excluded. We report the case of a 62‐year‐old woman with a cutaneous papule on the mid‐chest which generated a differential diagnosis of vascular lesion and basal cell carcinoma. Following excision, microscopic evaluation revealed a dermal large cell undifferentiated carcinoma, with a brisk mitotic rate and focal geographic necrosis. Mucin production was absent. On immunohistochemistry, the lesion expressed CK7, AE1AE3, CK8/18, chromogranin, synaptophysin, CD56, calcitonin (patchy) and TTF‐1 (minimal focal). Stains for neurofilament, CK20, CK5/6, p40, p63, SOX10, MART‐1, EMA, CEA, ER/PR, GATA3, GCDFP, mammoglobin, PAX‐8, CDX2, napsin, ERG and MCPyV proved negative. The histopathological diagnosis was of a large cell neuroendocrine carcinoma, probably metastatic. The patient underwent comprehensive clinical, laboratory and radiographic investigations and no underlying primary carcinoma was detected. During a 20‐month follow‐up period with an oncologist, the patient remains well and free of any apparent carcinoma. This suggests a primary large cell neuroendocrine carcinoma of skin. To date, 3 such cases have been reported in Japanese patients. This is the first in a Caucasian resident of North America.