Neuroendocrine carcinomas (NEC) of the large bowel are not graded as they are considered high-grade by definition. NEC includes large cell neuroendocrine carcinoma with a more organoid appearance (as shown in this image) as well as small cell carcinoma with typical histology resembling those seen in the lung. Foci of glandular or squamous differentiation may be present. Despite sharing histologic, immunophenotypic, and ultrastructural neuroendocrine features, neuroendocrine carcinomas are not biologically closely related to well-differentiated neuroendocrine tumors.
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