Answer: Carcinoid tumor

Histology: The tumor has a nested, tubular, and trabecular architecture. It infiltrates the mucosa and permeates the muscularis propria. The tumor cells are uniform and polygonal. They feature nuclei with fine “salt and pepper” chromatin. Nucleoli are not prominent. The cytoplasm is finely granular, with accentuation of dark eosinophilic granules at the basal aspect of the cells, facing the stroma. The nests of tumor frequently have retraction artifact separating them from the stroma.

Discussion: Paraganglioma features the similar nested architecture as carcinoid tumor. However, paragangliomas do not usually present as primary gastrointestinal tract tumors. Paragangliomas feature amphophilic cytoplasm and lack the retraction artifact and basal cytoplasmic granules typical of carcinoid tumor. Metastatic prostatic adenocarcinoma is a consideration in a male with a nested tumor involving the abdomen. However, this patient lacked a history of prostatic carcinoma, and tumor cells lack the prominent nucleoli characteristic of prostatic adenocarcinoma. Carcinoid tumors may show tubular architecture like an adenocarcinoma, but lack the cytologic atypism, mitotic activity, and intercellular mucin seen in intestinal type adenocarcinomas.

Carcinoid tumors of the ileum tend to be more aggressive than those of the appendix. This patient had regional lymph node metastases, which diminishes the 5-year survival to approximately 85%. Approximately 7% of ileal carcinoids present with a carcinoid syndrome due to liver metastasis in the face of serotonin production. Occasionally, carcinoid tumors may induce dense fibrosis in the subserosa, leading to kinking of the bowel wall and resulting ischemia. Approximately 30% of ileal carcinoids are multifocal; these are associated with a worse prognosis in multivariate analysis, suggesting that these are intramucosal metastases, not synchronous primary tumors.