- Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumor of the gastrointestinal tract, resistant to cytotoxic chemotherapy and radiation therapy. KIT and PDGFRA mutations have been identified as tumor initiating events in 85% of adult patients with GIST, but 85% of GISTs in pediatric patients lack KIT and PDGFRA mutations (wild-type) and imatinib is not as effective in eliciting objective responses.