Femoral mesenchymal chondrosarcoma with secondary aneurysmal bone cysts mimicking a small-cell osteosarcoma

Mesenchymal chondrosarcoma is a rare but aggressive, high-grade malignancy of primitive cartilage-forming mesenchyme that arises most commonly from skeletal sites. Although there are radiological findings suggestive of the diagnosis, imaging features often overlap with those of other skeletal sarcomas. The definitive diagnosis relies on the histological finding of a typical bimorphic appearance, consisting of nests of small, round, poorly differentiated cells and more mature cartilaginous tissue. To highlight this, we present the case of a 21-year-old man who was referred to our institution with a history of right knee pain. Initial imaging and histological evaluation of a core biopsy of the lesion suggested osteosarcoma of the distal right femur; after review, however, the correct diagnosis of mesenchymal chondrosarcoma was made. Adequate tissue sampling and thorough histological evaluation of biopsy specimens is vital for the accurate diagnosis of primary bone malignancies, especially those of chondroid origin.