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Gastric Stromal Tumors
Stromal tumors of the stomach are rare tumors that arise from the mesenchyma, the connective tissue and blood vessels that support an organ. The parenchyma, on the other hand, represents the functional tissue of the organ. Within the stomach, the parenchyma includes the epithelial glandular tissue within the mucosa and the mesenchyma consists of the supporting tissues, or stroma. The components of the stroma include smooth muscle cells, nerve cells, lipocytes, vascular structures, and epithelioid cells. Gastric stromal tumors arise from these cell types.
Prevalence and Epidemiology
The prevalence of gastric stromal tumors varies by type. The most common is the gastrointestinal stromal tumor (GIST), with 10 to 20 cases per million persons, representing 5000 to 6000 cases in the United States annually. GISTs make up 2% to 3% of all gastric tumors. Lipomas represent 2% to 3% of benign gastric tumors, neurogenic tumors account for 4%, and vascular tumors comprise 2%. The remainder of the gastric stromal tumors are exceedingly rare.
There is an increased risk for GIST with neurofibromatosis, Carney's syndrome, and germline mutations of KIT.
Clinical Presentation
Gastrointestinal bleeding (33%) and abdominal pain (19%) are the most common presenting symptoms associated with GIST. Anemia, hematochezia, hematemesis, bloating, abdominal pain, palpable mass, and abdominal distention are additional features.
Pathology
Historically, GISTs were referred to as leiomyoma, leiomyosarcoma, epithelioid leiomyosarcoma, and leiomyoblastomas, based on the thought that the tumors arise from smooth muscle cells. However, it is now felt that GISTs arise from the interstitial cell of Cajal, which is a primitive gut stem cell in the muscularis propria that expresses KIT, a tyrosine kinase receptor. Distinguishing GIST from other stromal tumors is by the expression of KIT, and 95% of GISTs express KIT. CD117 immunohistochemistry stains are positive for KIT and are diagnostic of GIST. The expression of KIT in GIST is the premise behind medical therapy.
GISTs can be benign or malignant. The determination of a benign or malignant GIST is based on the number of mitoses observed per high-power field. Malignant GISTs can recur and metastasize to the liver and peritoneal surface, with distant metastasis being rare. Metastatic GISTs do not manifest with lymphadenopathy. Therefore, if lymphadenopathy is present, other malignant tumors, such as adenocarcinoma and lymphoma, should be considered.
Other gastric stromal tumors are diagnosed histologically. Schwannomas stain positive for S-100 protein. True leiomyomas and leiomyosarcomas arise from smooth muscle cells and are rare in the stomach.
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