Carcinoid tumor: The duodenum is the most common site of these well-differentiated neuroendocrine tumors. These can be functional or nonfunctional in the production of hormones. Serotonin production is common in ileal carcinoids.
• Gastrin production is common in duodenal carcinoids. Immunohistochemical stains cannot be used to predict the functional status of the tumor. All carcinoids are considered to have metastatic potential. Histologic architecture varies from nested, trabecular, cords, or glandular morphologic characteristics and consists of cells with scant amphophilic cytoplasm that show a salt-and-pepper chromatin pattern in the round or ovoid nuclei with inconspicuous nucleoli. Mitotic figures are rare. Gastrin-producing, somatostatin cell, and serotonin-producing tumors have aggressive behavior and metastasize.
• Gangliocytic paragangliomas are usually benign infiltrative lesions and consist of ganglion cells, spindle cells (neural), and epithelial cells forming trabeculae, nests, and pseudoglandular architecture. Occasional large tumors (more than 2 cm) may spread to the lymph nodes.
• Small cell carcinoma is the other end of spectrum of neuroendocrine tumors. These are poorly differentiated neuroendocrine carcinomas with small cell morphologic characteristics, necrosis, and increased mitotic activity.
Small Intestinal Lymphomas
• Small intestinal lymphomas are less common than gastric lymphomas and include extranodal marginal zone lymphoma (low-grade mucosa-associated lymphoid tissue [MALT], MALToma, or MALT lymphoma), mantle cell lymphoma, Burkitt lymphoma, immunoproliferative small intestinal disease (IPSID), and enteropathy-like T-cell lymphoma (rare).
• IPSID is seen exclusively in Mediterranean and Middle Eastern regions. This is a variant of MALT lymphoma that secretes defective alpha heavy chains. The infiltrate consists of plasma cells with small lymphocytes, and monoclonal alpha heavy chain can be demonstrated in the cytoplasm of neoplastic cells. Transformation to large B-cell lymphoma is frequent in late stages.