Case of low-grade neuroendocrine carcinoma of the skin presenting metastases to lymph nodes and peritoneum.

A 61-year-old Japanese man had a gradually growing, red-colored nodule in his umbilicus from 3 years ago. He had no symptoms such as hot flush, diarrhea or wheezing. Computed tomography detected a 3.0 cm × 3.0 cm enhanced nodule on the umbilical portion, inguinal and axillary lymph node swelling, and peritoneal nodules. Upper and lower gastrointestinal endoscopy and cystoscopy did not show any other tumor. We resected the umbilical nodule and subjected the inguinal lymph node and peritoneal nodule to biopsy. Histopathological findings indicated that the cutaneous lesion was composed of variously sized nests that included small, monomorphic, round to polygonal cells, mainly in the dermis to the peritoneum. Mitotic figures were scant. The inguinal lymph node and peritoneal nodule were positive for metastasis. Immunohistochemistry was diffusely positive for cytokeratin (CK)-7, CD56, chromogranin A, synaptophysin, estrogen receptor-α, progesterone receptor, GATA3 and carcinoembryonic antigen, and focally positive for mammaglobin and gross cystic disease fluid protein 15. The Ki-67 labeling index was 1.5%. The patient was diagnosed with a case of low-grade neuroendocrine carcinoma of the skin (LGNECS) occurring on the umbilicus. This case exhibited distant peritoneal metastasis, as well as inguinal and axillary lymph node metastases; however, the patient is alive without chemotherapy at 23 months after the first visit. LGNECS is a newly proposed, extremely rare entity that has been reported under various names, including primary cutaneous carcinoid tumor. In the present case, this tumor shows a slow-growing nature and favorable prognosis, even though it harbors metastatic potential.