This segment of jejunum shows a nodular lesion located in the submucosa. Upon histological examination, the tumor was composed of uniform round cells arranged in trabeculae, with a "salt-and-pepper" chromatin pattern. Electron microscopic studies revealed secretory granules, and immunohistochemical stains were positive for serotonin. Which of the following parameters correlates best with the metastatic potential of this tumor?

The tumor in the picture is a carcinoid. This neoplasm derives from the neuroendocrine cells of the intestinal epithelium and often retains the capacity to produce hormonal substances, such as serotonin, gastrin, somatostatin, etc. The architectural arrangement is variable, although trabecular and insular patterns are the most common. Carcinoid tumors may be found throughout the intestinal canal, as well as in the bronchial tree and other organs. Location and size are the most impoant predictor of their metastatic potential. Carcinoids of the appendix and cecum (the most frequent) only rarely metastasize to the liver. Carcinoids of the stomach, ileum and colon have a high metastatic propensity, especially if they are larger than 2 cm in main diameter. Neither architectural pattern nor cell pleomorphism is a good predictor of the malignant potential of carcinoid tumors. In general, this rule applies to most of endocrine tumors, in which histologic features and biologic behavior do not often correlate with one another. In other words, a tumor that shows relatively bland cells may undergo metastatic spread, while tumors with more anaplastic features may behave in a benign fashion. Hormonal production is common in carcinoids, but does not often lead to a clinical syndrome nor does it correlate with metastatic potential.