Discussion Author: Tom Hash
Gastrointestinal stromal tumors (GIST) are the most common mesenchymal neoplasm of the gastrointestinal (GI) tract. They account for 1-3% of all GI tract tumors. GISTs occur anywhere in the GI tract, from the esophagus to the anus; the most frequent site of origin is the stomach, followed by small intestine. GISTs also can occur in the mesentery, retroperitoneum, and omentum. They usually are found in patients over the age of 50. There is no well-defined sex or race predilection.
GISTs most commonly involve the muscularis propria and typically have exophytic growth. The most frequent presentation on imaging is a mass arising from the intestinal wall which protrudes into the abdomen. They range in size from millimeters to more than 30 cm. GISTs are often well-encapsulated, heterogeneously enhance, and have central areas of low density corresponding with necrosis or hemorrhage. On barium studies the appearance is characteristic -- but nonspecific -- of a submucosal mass; however, occasionally there is an irregular surface due to focal mucosal ulceration. GISTs do not have associated adenopathy, a helpful diagnostic clue on CT or MRI.
Presenting symptoms depend upon location: GISTs in the stomach, small and large intestine, and anus most frequently present with GI bleeding, anemia, abdominal pain, or an abdominal mass; those arising in the esophagus most commonly present with dysphagia.
GISTs are distinguished from other mesenchymal tumors (e.g. leiomyomas) histologically. Uniquely, GISTs express KIT, a tyrosine kinase growth factor receptor.
30% of GISTs are malignant. The larger the tumor and the higher the mitotic rate the more likely GISTs are to be malignant. In the stomach, benign GISTs outnumber malignant ones three to one. Small intestine GISTs similar in size to gastric GISTs have a relatively more aggressive course. GISTs in the colon, anus, and esophagus are most often malignant. Malignant GISTs have a propensity to recur and to metastasize to the liver and peritoneum. Less commonly, they metastasize to the lungs, pleura, bone, retroperitoneum, and subcutaneous tissues.
Surgical resection is the primary modality for treatment of all GISTs. The tumors respond poorly to chemotherapy and radiation. However, imatinib mesylate (Gleevec), a tyrosine kinase inhibitor, is often used with success in treatment. A retrospective review of 54 patients has shown 18F-FDG PET to be superior to CT in predicting early response to imatinib mesylate therapy.
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