Extragastrointestinal Stromal Tumors (EGISTs)
Until 2002, stromal tumor has been re-recognized and correctly named, which was confused with leiomyomas for such a long time. A breakthrough has been made on stromal tumor over this short span of a decade. Especially the use of imatinib, a tyrosine kinase inhibitor (TKI), allows stromal tumor to become a model for targeted therapy. Gastrointestinal stromal tumor (GIST) is the most common gastrointestinal mesenchymal tumor, accounting for 2% of all gastrointestinal tumors. GIST often occurs in the stomach (40–70%), small intestine (20–40%), colon (5–15%), and rectum (5%). On the contrary, primary extragastrointestinal stromal tumor (EGIST) accounts only a very small percentage of all stromal tumors. EGIST occurs frequently in the mesentery, omentum, and peritoneum but rarely in the retroperitoneum. Up to now, only 59 cases have been reported. EGIST is commonly seen in young and middle-aged people, with an incidence comparable between men and women. EGIST often grows very large before it is detected incidentally, resulting in a low rate of complete resection and a high rate of recurrence (even after complete surgical resection). Although located in different primary sites, EGIST is substantially similar to GIST in terms of pathology and immune phenotype. Under microscope, tumor cells are short spindle, nest, sheet, or vesicle shaped, with translucent cytoplasm. Immunohistochemistry reveals CD117 positive, as well as actin, calponin, and S100 positive or weakly positive. C-kit gene mutation is detectable. Under electron microscope, tumor cells appear as irregular fusiform with intertwined pseudopodia of varying lengths extending toward the surrounding area and rich in original intercellular connection. CT and MRI identify “abdominal mass” in the majority of patients with EGIST, which is diagnosed based on postoperative pathology. Currently, surgery remains the most common treatment for EGISTs. Although the use of imatinib has revolutionized treatment and improved outcomes for patients with GIST, little is known with regard to its efficacy in EGIST.
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