Mesenchymal chondrosarcoma diagnosed on FISH for HEY1‐NCOA 2 fusion gene

Mesenchymal chondrosarcoma ( MC ) is an extremely rare subtype of chondrosarcoma that has a small round‐cell sarcoma with focal cartilaginous differentiation, often with a pericytomatous vascular pattern. The non‐cartilaginous components are usually dominant, and such lesions might be confused with other small round‐cell tumors. Recently, a tumor‐specific HEY1‐NCOA 2 fusion gene was identified in MC . Here we report the case of a 9‐year‐old boy who was diagnosed with MC by detection of HEY1‐NCOA 2 fusion signals in almost 50% of tumor cells in tissue sections on fluorescence in situ hybridization ( FISH ). In this way, the tumor was definitively diagnosed as MC . This case suggests that the detection of the HEY1‐NCOA 2 fusion gene on FISH is of diagnostic value for MC .