OBJECTIVETo explore the clinicopathological and immunohistochemical features of extra-gastrointestinal stromal tumors (EGIST) arising from the omentum and mesentery and to investigate the cellular origin of these tumors, prognostic factors, and the relationships with gastrointestinal stromal tumors.

METHODSNineteen cases of mesenchymal neoplasms arising from the omentum and mesentery (previously diagnosed as smooth-muscle tumors or schwannomas) were studied morphological with a panel of immunohistochemistry including CD117 and CD34.

RESULTSAmong the 19 cases, 14 tumors were confirmed to be EGIST, of which 6 tumors arose from the omentum and 8 cases located at the mesentery. The size of tumors ranged from 3.5cm to 29.0 cm (mean 12.4cm) in diameter. Histologically, there were 9 cases of mainly spindle cell type, 2 cases of mainly epithelioid cell type and 3 cases of mixed cell type. all EGIST expressed CD117 (14/14) and a percentage of them expressed also CD34 (8/14) and/or SMA (6/14), anyhow, all EGIST were negative for desmin and S-100 protein. Six patients with tumors arising from the omentum were all alive without evidence of disease (tumor-free). Among 7 cases with tumors of the mesentery, three patients died of the disease, 1 alive with the disease and 3 patients alive without evidence of the disease.

CONCLUSIONSEGIST were identical by their histological and immunohistochemical features with gastrointestinal stromal tumors (GIST). This tumor may arise from the multipotential mesenchymal stem cells. EGIST have various clinical behavior, and the parameters used for predicting the prognosis of GIST may not be completely suitable for EGIST evaluation.