The adoption of GIST also suggests that these tumors may arise from a multipotential progenitor cell. Malignant GIST tumors showing leiomyomatous differentiation most commonly present in the muscular wall of the jejeunum and ileum, and much less commonly in the large intestine. The tumors are generally well circumscribed, with a white firm cut surface. Large tumors may show central necrosis and hemorrhage. Some tumors may be endophytic, bulging into the lumen of the intestine.
Smaller tumors have an intact overlying mucosal surface.
Many tumors are exophytic and bulge from the serosal surface into the peritoneal cavity or adjoining tissue. Adjoining peritoneal surfaces and the liver are the most likely sites of metastatic tumor. Malignant GIST tumors usually have a spindle cell histology. The most reliable parameter for a diagnosis of malignancy apart from the demonstration of metastases is mitotic rate. Tumors over 4cm in diameter should be considered to have malignant potential although this is a less reliable guide than mitotic activity.