Minute Mesenchymal Chondrosarcoma within Osteochondroma: An Unexpected Diagnosis Confirmed by HEY1-NCOA2 Fusion

Mesenchymal chondrosarcoma is rare and can be challenging to diagnose. Herein, we report a minute mesenchymal chondrosarcoma within an osteochondroma. A 12-year-old girl presented with an asymptomatic exophytic lesion of the rib. The tumor was clinically diagnosed as osteochondroma, and was excised after observation for 3years. The resected specimen revealed an unexpected minute (0.9cm) "monophasic" mesenchymal chondrosarcoma in the apex of the lesion. The sarcoma consisted of monomorphic spindle cells without hyaline cartilage.

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