Colonic polyp presenting as a tubulovillous adenoma and harbinger of high-grade neuroendocrine carcinoma: a unique presentation

Abstract

High-grade neuroendocrine carcinomas (HGNEC) of the colon are rare. However, when they occur, there is evidence to support a strong association with tubulovillous/villous adenomas (TVA). We present a unique case of a colonic TVA associated with HGNEC in a 64-year-old woman with a large polypoid mass in the rectosigmoid colon diagnosed as TVA with high-grade dysplasia (HGD) on biopsy. Pathologic examination revealed a 4.8 cm friable polypoid mass composed of two morphologically distinct elements. The luminal portion of the tumor consisted of a TVA with areas of HGD. A distinct component was identified deeper to the adenoma, consisting of nested small-to-medium sized cells with increased nuclear-to-cytoplasmic ratio, nuclei exhibiting finely dispersed chromatin, inconspicuous nucleoli, patchy areas of necrosis, and increased mitotic figures. The two tumor components were closely apposed with focal areas of intermixing at their interface. Immunohistochemistry demonstrated diffuse positive staining of the deeper, nested neoplastic cells with synaptophysin, chromogranin, CD56 and CAM 5.2 and a perinuclear-dotlike pattern of pancytokeratin AE1/AE3 staining. The proliferation index was up to 90% (Ki-67). Given the overall findings, a diagnosis of HGNEC arising in a background of TVA was rendered. This unique case is one of the few reported cases of mixed adenoma HGNEC involving the colon. This case demonstrates possible pitfalls in diagnosis. Superficial biopsies often do not provide adequate representation of underlying neuroendocrine neoplasms. In this case, metastatic HGNEC was identified although initial biopsy revealed only TVA, suggesting that the two lesions were unrelated.