Fibrocartilaginous mesenchymoma of the tibia with predominant microcystic features: A case report and literature review

We report a case of fibrocartilaginous mesenchymoma (FCM) with multicystic changes in the fibrous component within the tumor, although the fibrous component usually shows cellular proliferation of spindle cells in the background of collagen fascicles. A 16-year-old boy complained of right knee pain after physical examination for several months. A bone tumor in the right proximal tibia was incidentally detected by plain radiography after a traffic accident. A surgical specimen obtained after enblock resection revealed irregular cartilaginous islands with enchondral ossification within the fibrous background and dominant cystic changes. Genetic testing for the GNAS mutation was negative, which excluded the possibility of cartilaginous fibrous dysplasia. In cases of bone tumors showing osteolytic changes with marginal sclerosis and cystic changes, the possibility of FCM should be considered.