Carcinoids arise from Neuroendocrine cells of endodermal derivation. The pathogenesis of intestinal carcinoids is unknown. No other sex, racial or geographic preference has been described. These tumors are fairly well circumscribed but not encapsulated, white or pale yellow tan, and very firm due to a marked desmoplastic reaction. The overlying mucosa is usually intact, and the tumor is seen as a submucosal smooth nodule.
Large lesions can ulcerate the mucosa or penetrate the serosa.
Virtually all symptomatic carcinoids have metastasized when diagnosed, and these lesions gnerally over 2cm in size. The nomenclature of Carcinoid tumors is in flux, with some restricting the term to those tumors showing the classical histologic patterns described for Carcinoids, while other prefer to use the term neuroendocrine tumor for all tumors showing a predominant neuroendocrine expressiopn. The most striking feature is the bland monotous uniform appearance of the tumor without pleomorphism or mitotic activity. As with other endocrine tumors, prediction of aggressive biologic behavior is not possible on histologic grounds with the only absolute critieria being the presence of metastases. Carcinoids are slow growing low grade malignant tumors most of which do not metastasize and are asymptomatic at death.