Gastric Carcinoid Tumours

Carcinoid tumors are rare, slow-growing neuroendocrine tumors arising from the enterochromaffin cells disseminated throughout the gastrointestinal and bronchopulmonary systems. Though they have been traditionally classified based upon the embryologic site of origin, morphologic pattern, and silver affinity, newer classification systems have been developed to emphasize the considerable clinical and histopathologic variability of carcinoid tumors found within each embryologic site of origin. These neoplasms pose a diagnostic challenge because they are often innocuous at the time of presentation, emphasizing the need for a multidisciplinary diagnostic approach utilizing biochemical analysis, standard cross-sectional imaging, and newer advances in nuclear medicine. Similarly, treatment of both primary and disseminated carcinoid disease reflects the need for a multidisciplinary approach, with surgery remaining the only curative modality. The prognosis for patients with these tumors is generally favorable, however can be quite variable and is related to the location of the primary tumor, extent of metastatic disease at initial presentation, and the time of diagnosis.

Originating from the histamine-containing enterochromaffin-like (ECL) cells of the embryologic foregut, gastric carcinoid tumors represent approximately 1.8% of all gastric neoplasms and approximately 7% of all carcinoids. Because they are most often discovered incidentally during endoscopy, the incidence of gastric carcinoid tumors has increased in recent years as endoscopic technology continues to improve both technologically and diagnostically. Carcinoids of the stomach are generally divided into three distinct groups based on their clinical and histological characteristics: carcinoid tumors associated with chronic atrophic gastritis type A (CAG-A), carcinoid tumors associated with Zollinger-Ellison syndrome (ZES) or MEN-1, and carcinoid tumors which occur sporadically.

Type I gastric carcinoids account for 70 to 80% of all gastric carcinoids. These tumors are small, benign tumors associated with chronic atrophic gastritis and chronic hypergastrinemia. The relative importance of gastrin in the development of type I gastric carcinoids in lent support by animal studies in which lifelong inhibition of acid secretion in rats induced by potent inhibitors of acid secretion or subtotal fundectomy was found to be associated with the development of carcinoid tumors of ECL cells in the gastric corpus. To date, however, no cases of carcinoid tumor have been attributed to the use of proton pump inhibitor therapy in humans, suggesting the importance of other factors in the development of type I gastric carcinoids. These tumors often exhibit an indolent disease course and tend to be nonfunctional and asymptomatic. Likewise, metastases are rare, occurring in fewer than 10 percent of tumors less than 2 cm in greatest dimension.

Accounting for approximately 5 percent of gastric carcinoids, type II tumors arise in the setting of MEN-1 and ZES. Like type I gastric carcinoids, they are thought to arise from ECL cells under the influence of hypergastrinemia, resulting in low grade malignant behavior and associated hyperplasia of surrounding ECL cells. Allelic loss of the MEN1 gene locus, a tumor-suppressor gene located on chromosome 11q13, is thought to be involved in the pathogenesis of many of these tumors. Though type II gastric carcinoids are generally indolent and behave similar to type I lesions, these tumors may be large and polypoid, unlike most small type I carcinoids.

Type III gastric carcinoids account for nearly 15% to 25% of tumors. They are usually large, solitary, sporadic tumors unassociated with hypergastrinemic states. Sporadic carcinoid tumors may be aggressive, with a high incidence of metastases and a 5-year survival rate of less than 75%. Unlike type I and II lesions which produce serotonin (5-HT), these tumors are prone to the development of an atypical carcinoid syndrome related to the production of 5-hydroxytryptophan (5-HTP), characterized by flushing, hypotension, excessive lacrimation, edema, and bronchoconstriction. Excessive production of 5-HTP is thought to be related to the absence of the enzyme aromatic acid decarboxylase, which converts 5-HTP to 5-HT.

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