Types There are no meaningful cell type (histologic) variations that help predict the clinical course of these tumors. Carcinoids are broadly classified as neuroendocrine or APUD (amine precursor uptake and decarboxylation) tumors. The APUD type is associated with the MEN (multiple endocrine neoplasia) type 1 syndrome (pancreas islet cell carcinoma). Malignant carcinoids may produce at least two hormones , serotonin and substance P, but many carcinoid tumors are nonfunctional and do not release any substances.
How It Spreads Carcinoid tumors spread by direct invasion of underlying layers of tissue (submucosally). They can also spread via lymphatics to regional lymph nodes and through the bloodstream to the liver, lungs, bone or other organs.
The site of origin and size of the primary tumor are important in determining whether the carcinoid tumor is likely to spread. A tumor less than 1 cm (½ in.) rarely metastasizes. But 88 percent of patients with tumors larger than 2 cm (¾ in.) have metastases, and 18 percent of these develop the carcinoid syndrome ( See "Common Signs and Symptoms" ).
