Well-differentiated Neuroendocrine Tumor : CD56
Neuroendocrine tumors arising at various body sites are a heterogenous group of tumors with diverse clinical, biological, and pathologic features. The common underlying thread that connects them is characteristic histology, the presence of neurosecretory granules by electron microscopy, and demonstration of neuroendocrine markers by immunohistochemistry or in-situ hybridization. Immunohistochemical markers useful in the diagnosis of gastrointestinal neuroendocrine tumors include: synaptophysin (most sensitive), chromogranin (most specific), CD56/NCAM1, Leu7, PGP9.5, and neuron specific enolase. Glucagon, somatostatin, gastrin, and serotonin are variably positive. Most gastrointestinal NETs also express CDX-2 and CEA. Transcription factors such as pancreatic and duodenal homeobox 1 (PDX1), islet 1 (ISL-1), and PAX8 are considered to be pancreas specific. Metastatic NETs with an unknown primary site should be evaluated with a panel of CDX-2 (+ in gastrointestinal), ISL-1 or PDX1 (pancreas specific), and thyroid transcription factor 1 (TTF-1; favors lung origin). Rectal WDNETs frequently express prostate-specific acid phosphatase. The image shows immunoreactivity for CD56 in an incidentally discovered well-differentiated neuroendocrine tumor (carcinoid) of rectum. Image courtesy of: Jian-Hua Qiao, MD, Los Angeles, California, USA. Used with permission.