Gastrointestinal stromal tumor and gastric adenocarcinoma collision tumors.

Gastrointestinal stromal tumors sometimes occur together with gastric carcinoma, but true collision tumors featuring these 2 components are very rare. The authors describe here 2 collision tumors containing a gastrointestinal stromal tumor with intermingling elements of gastric adenocarcinoma. The gastrointestinal stromal tumors were 5.5 to 6 cm spindle cell tumors, and one patient had an additional prepyloric stromal tumor (2.5 cm). All gastrointestinal stromal tumors had low mitotic counts less than 5/50 high-power fields and were positive for KIT, DOG1, and CD34. Different KIT exon 11 mutations (single nucleotide substitution, complex insertion-duplication) in all tumors indicated that the 2 gastrointestinal stromal tumors in one patient were independent primary tumors. The adenocarcinoma components displayed gland-forming intestinal type to signet ring cell morphology with focally accompanying dysplastic epithelium, immunohistochemical positivity for CDX2, and varying keratin 7/20 expression. We hypothesize that development of gastric adenocarcinoma within a gastrointestinal stromal tumor may be based on displaced gastric epithelium in a long-standing stromal tumor with events of intermittent ulceration and epithelial regeneration.