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Cutaneous metastasis from neuroendocrine carcinoma of visceral origin is rarely described. The primary sites of origin include the lungs, larynx, mediastinum, uterus, and thymus. Histologically, a rosette-like structure is regarded as the best marker for recognition of neuroendocrine differentiation and is described as a small and regular, oval or round lumina, deeply eosinophilic luminal surfaces, and the absence of or rare accumulation of non- mucous material but frequent apoptotic debris in the lumina. To confirm these neuroendocrine features, CD56, synaptophysin, neuron-specific enolase and chromagranin A were used as immuno- histochemical staining and cytokeratin 20 was used to differentiate from a Merkel cell carcinoma. However, tumor cells were stained with cytokeratin 20 in our case. Cytokeratin 20, a low molecular weight cytokeratin, is found in a variety of normal tissues, including intestinal epithelium, gastric epithelium, urothelium, and Merkel cells. Most neuroendocrine carcinomas do not express cyto- keratin 20, with the exception of Merkel cell carcinoma, and most colorectal adenocarcinomas express cytokeratin 20. Kato et al. reported a case of cytokeratin 20-positive large cell neuroendocrine carcinoma of the colon, suggesting a link between colorectal neuroendocrine carcinoma and conventional adenocarcinoma. These considerations also supported the notion that the present case shares common immunophentypes with colorectal adeno- carcinoma.
We also found similar histological and immu- nohistochemical features in surgically biopsied rectal cancer section. Colorectal neuroendocrine carcinoma is an extremely rare neoplasm that exhibits rapid progression showing aggressive behavior biologically, for example fulminant early distant metastasis to liver, lung, bone, distant lymph node, or peritoneum. To our knowledge, this is the first report of cutaneous metastasis of neuroendocrine tumor derived from the rectum.
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