Gastrointestinal Stromal Tumor (GIST) with Chondroid-Myxoid-Chordoid Features Mimicking Extraskeletal Myxoid Chondrosarcoma

Epithelioid gastrointesinal tumors (GISTs) are less likely to have c-kit gene mutations (and express CD117) than spindle cell GISTs. CD117 negative/c-kit negative GISTs can have platelet-derived growth factor alpha (PDGFRα) gene mutations, overexpress PDGFRα protein and respond to imatinib mesylate. Many cases of CD117-negative/CD117weakly positive, c-kit mutation negative and PDGFRα mutation positive myxoid epithelioid GISTs and one case of CD117-positive GIST with chondro-myxoid features mimicking chondrosarcoma have been reported. We report a case of myxoid epithelioid GIST with predominance of chondroid and chordoid areas resembling an extraskeletal myxoid chondrosarcoma that was strongly positive for CD117, PDGFRα and DOG1 (Discovered on GIST 1) by immunohistochemistry, but lacked c-kit and PDGFRα gene mutations. It is possible that CD117 is strongly positive if a myxoid epithelioid GIST has chondroid/chordoid appearance, but a larger study is needed to confirm this association. CD117 expression in GISTs is important, because GISTs showing CD117 positivity respond to imatinib. No comment can be made about the prognostic significance of chondroid/chordoid appearance in the GISTs.