A histopathological review of gastrointestinal related mesenchymal tumors: the hidden GIST.

BACKGROUND The diagnosis of gastrointestinal stromal tumors is based on documentation of c-KIT and platelet-derived growth factor-alpha receptors or specific c-KIT mutations. Before the diagnosis of GIST was possible, all cases had been classified as sarcomas or benign tumors. OBJECTIVES To identify cases of GIST formerly diagnosed as abdominal or retroperitoneal mesenchymal tumors. METHODS We reviewed the archive material on all surgical cases diagnosed as gastrointestinal related malignant mesenchymal tumors or GIST in our medical center during the last decade (1995-2004). RESULTS Sixty-eight cases of retroperitoneal soft tissue sarcoma were identified. Thirty-eight were reconfirmed to be GIST, 19 were newly diagnosed as GIST (the hidden cases), 8 cases were re-diagnosed as mesenchymal tumors, and 3 cases of sarcoma remained sarcomas. Of all the GIST tumors, c-KIT-positive and PDGFRalpha-positive tumors were more characteristic of primary gastric tumors, while c-KIT-positive and PDGFRalpha-negative tumors were found in the colorectal area. The c-KIT-negative and PDGFRalpha-positive cases were of gastric origin. CONCLUSIONS Any c-KIT-negative malignant mesenchymal mass located near the proximal gastrointestinal tract should also be stained for PDGFRalpha to differentiate between GIST and other soft tissue sarcomas. Practically, formerly diagnosed abdominal or retroperitoneal soft tissue sarcomas should be reviewed to identify patients with misdiagnosed GIST and thereby avoid future unnecessary and ineffective chemotherapy.