Pre-operative imatinib treatment of gastrointestinal stromal tumour (GIST) abolishes immunostaining for c-kit – A pitfall for the unwary pathologist

Gastrointestinal stromal tumour (GIST) is a neoplasm of the gastrointestinal tract, mesentery, or omentum that expresses the protein-tyrosine kinase KIT (CD117) and is the most common mesenchymal tumour arising at these sites.With the availability of effective but expensive treatment in the form of imatinib, accurate diagnosis of gastrointestinal stromal tumour (GIST) is extremely important.We describe a case of a 64 year old male with biopsy proven GIST of the gastric cardia which was unequivocally positive for CD117 and CD34. We subsequently received his proximal gastrectomy specimen containing an ulcerated tumour in the gastric cardia which was positive for actin and desmin, but did not express CD117, CD34, S100 or bcl-2. The case was therefore misdiagnosed as a typical leiomyoma rather than a GIST. It was recognized at a subsequent clinicopathological meeting that the patient had been treated with imatinib post-biopsy but pre-operatively unbeknown to the pathologist reporting the gastrectomy specimen who therefore misdiagnosed as a typical leiomyoma.