Fibrocartilaginous mesenchymoma, a unique osseous lesion: case report with review of the literature.

Fibrocartilaginous mesenchymoma is a rare osseous tumor that primarily arises in the long bones of children and adolescents. This lesion can grow quickly and reach a considerable size, despite its benign nature. It has proved challenging to diagnose and can be mistaken for a spectrum of benign and malignant bone tumors. The histological presentation of unique epiphyseal plate-like cartilage with destruction of the surrounding cortical bone and exhibition of dense fibrous stroma are important indicators for the diagnosis of fibrocartilaginous mesenchymoma. An 11-year-old boy presented with a left proximal humerus mass thought to be an aneurysmal bone cyst. The patient was lost to follow-up and came back 3 years later with massive growth of the lesion. Owing to the aggressive nature of the tumor, a left forequarter amputation was performed. Histological examination demonstrated numerous islands of cartilage with an exuberant spindle cell component characteristic of FCM. No distant metastases or local recurrences were identified at 2 years post-amputation. Because of the rapid growth of this lesion, it should be considered in the differential diagnosis of bone lesions in children and young adults.