Multiple carcinoid tumors of the rectum: report of two cases suggesting the origin of carcinoid tumors.

Two cases of multiple carcinoid tumors of the rectum with numerous micronests of carcinoid tumors are reported. The patients were 51- and 58-year-old males. Many carcinoid tumors and numerous carcinoid micronests were found in the resected rectum; the total number of carcinoid tumors, groups of micronests, and solitary micronests was 69 in the first case and 62 in the second case. The micronests, consisting of a few to many endocrine cells, were observed in the lamina propria, muscularis mucosa, and/or submucosa. Micronests increased in number, gathered and formed carcinoid tumors, which were up to 8 mm in diameter. It was found that a nest of the carcinoid tumors in the lamina propria showed continuity with the endocrine cells of a crypt in the different carcinoid tumors in both cases. The carcinoid tumor and micronest infiltrated the nerves and ganglions in the muscularis mucosa and submucosa. Nests of the carcinoid tumors and micronests were surrounded by S-100-positive cells. Lymph node metastases of the carcinoid tumor were found in both cases. Rectal carcinoid tumors may originate from endocrine cells of the crypts, and multiple carcinoid tumors may occur heterogeneously.