Rare gastrointestinal stromal tumors (GIST): omentum and retroperitoneum.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms that arise in the gastrointestinal tract and rarely elsewhere in the abdomen. GISTs that develop outside the digestive tract are called extra-GISTs (EGISTs). The incidence of EGISTs is reported to be approximately 10% of all GISTs, and the median age is younger than that of conventional GISTs. EGISTs have similar histology and immunohistochemical features as conventional GISTs, with the majority of them in the omentum and mesentery. Most GISTs harbor a kinase-activating mutation in either KIT or PDGFRA. For EGISTs, the incidence of this type of mutation is 40-50%, which is somewhat lower than for conventional GISTs. EGISTs may have a worse prognosis compared with conventional GISTs with high mitotic indices, large size, and distant metastasis including lymph node involvement. In large abdominal tumors, the visceral origin is almost impossible to discern.