How are neuroendocrine tumors classified?
The spectrum ranges from well-differentiated neuroendocrine tumors (carcinoid tumors) to poorly differentiated (small cell carcinomas) and large cell neuroendocrine carcinomas.
The common site of involvement is the rectum, followed by the cecum and sigmoid colon.
The histologic characteristics are similar to those described in the small intestine section. These are sporadic tumors. The malignancy rate of 11% to 14% has been calculated for rectal carcinoids.
The malignancy criteria include size greater than 2 cm, invasion into muscularis propria, and increased mitoses.