What are gastric carcinoids?
Gastric carcinoids constitute 4% of all gastrointestinal neuroendocrine tumors (GI-neuroendocrine tumors) and 1% of gastric neoplasms. They can be divided into four types. The first three originate from the enterochromaffin-like cells (ELC cells) in the gastric mucosa. Types 1 and 2 are gastrin-dependent, meaning they develop as a result of a high gastrin level in the blood stream. Gastrin is a hormone, produced in the stomach, and is responsible for the stimulation of acid production in the stomach. Gastric carcinoids have been shown to be increasing. Today the incidence of gastric carcinoids is approaching 6%. It has also been shown that the percentage of gastric carcinoids in relation to all gastric neoplasms both benign and malignant growths, has also increased from 0.4% to 1.8%. Whether this represents a true biological increase in the disease or reflects a change in awareness and/or increased in reporting of these tumors remains unclear. The true incidence of gastric carcinoids was not really appreciated prior to the increased application of upper endoscopy, a common test to look into the lining of the stomach.
Figure 1: Type 1 gastric carcinoid
Type 1 Gastric Carcinoids. Type 1 gastric carcinoids account for 70 - 80% of all gastric carcinoids and develop as a result of the trophic effect of gastrin on the ECL cells. These tumors are associated with the condition called autoimmune chronic atrophic gastritis. The loss of hydrochloric acid producing parietal cells leads to achlorhydria (lack of acid in the stomach), which in turn stimulates the G-cells of the antrum to produce gastrin. The hypergastrinemia promotes the growth of the ECL cells first resulting in hyperplasia out of which multiple ELC tumors arise. Type 1 gastric carcinoids are typically multiple, usually small (< 2cm) and because of their association with autoimmune atrophic gastritis occur more frequently in females aged 50 years or greater, commonly associated with B12 malabsorption and pernicious anemia. Most patients have no symptoms. The diagnosis is usually made incidentally on upper endoscopy (Figure 1).
The majority of these tumors are benign, but metastases have been reported in 3 - 5% of patients. Small tumors can be removed endoscopically, while larger tumors or those demonstrating invasion, require surgical excision. Ongoing endoscopic surveillance is required every six months since recurrence remains high. The role of concomitant antrectomy (removing part of the stomach) aimed at reducing the gastrin levels should be considered when surgical excision is required to remove the tumors, yet it must be individualized to each patient. Somatostatin analogues have been utilized to reduce the gastrin levels and have been shown to reduce recurrences, yet current guidelines to do not advocate their routine use. Five-year survival rates, approaching 98% illustrate the benign nature of this type of tumor.
Type 2 Gastric Carcinoids. Type 2 gastric carcinoids, like Type 1, are gastrin dependent and often multifocal. Type 2 gastric carcinoids develop secondarily in the context of high gastrin hormone levels due to gastrin secreating tumor (gastrinoma or Zollinger-Ellison syndrome (ZES)) associated with MEN I. In contrast to type 1 patients, these patients have elevated gastric acid and present with the clinical manifestations of ZES [direct them to ZE section]. Type 2 gastric tumors represent 5% of gastric carcinoids and are equally distributed between males and females. Histologically they appear similar to Type 1 tumors, however their malignant potential is greater. Regional lymph nodes (lymph nodes surruonding the stomach) have been reported in up to 30% of cases and liver metastases in 10%. Treatment of these tumors is focused on removal of the source of gastrin (typically a duodenal gastrinoma), together with resection of the gastric carcinoid if it is large and unable to be resected endoscopically. The 5-year survival is approximately 90% for type 2 gastric carcinoids.
Type 3 Gastric Carcinoid. Type 3 or sporadic tumors, account for 20% of gastric carcinoids. Unlike types 1 and 2, these tumors are not associated with elevated gastrin levels. They are usually solitary, large (>2cm) and occur most frequently in male over the age of 50. Regional lymph node involvement is found in up to 50% of cases and liver metastases develop in over two thirds of the patients. An 'atypical' carcinoid syndrome can develop in 5 to 10% in patients with type 3 carcinoids. This syndrome is a result of histamine release from the tumor and is characterized by a patchy bright red flush, cutaneous edema, salivary gland swelling and increase lacrimation. 24-hour urinary histamine levels and serum CgA serve as tumor markers and can be useful in following response to therapy. Sporadic Type 3 gastric carcinoid should be treated similar to the more common adenocarcinoma of the stomach, with an enbloc resection and an appropriate lymph node clearance. Unfortunately, unlike type 1 and 2 carcinoid, type 3 has a 5-year survival of only 50% overall and 10% in those patients with distant metastases.
Type 4 Gastric Carcinoids. These tumors consist of poorly differentiated endocrine carcinomas and mixed exocrine-endocrine carcinomas. Atrophic gastritis has been seen in up to 50% of these patients. The tumors are usually greater than 5 cm, often ulcerating and surgical unresectable. The prognosis is poor with the median survival of only 8 months reported in the literature. Chemotherapy has been attempted in a limited number of patients yet the rarity of these tumor does not allow for standard therapeutic protocols.
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