Low grade central osteosarcoma dedifferentiating into a high grade leiomyosarcoma in a young adult

An 18 –year-old male patient presented with left knee joint pain of seven month duration. The initial MRI scan was suggestive of an infective process, while the subsequent scan revealed radiological features suggestive of an osteosarcoma in the left distal femur. The biopsy was reported as an osteosarcoma in a different institution. Following neoadjuvant chemotherapy wide local excision of the tumour was performed.  On histologic evaluation, a low grade central osteosarcoma was diagnosed with progression to high grade leiomyosarcoma. Low grade central osteosarcoma is an uncommon bone forming tumour which occasionally dedifferentiate into a highgrade sarcoma, most commonly to conventional osteosarcoma. The young age of the patient and dedifferntiation into a leiomyosarcoma are unusual features encountered in this case. Furthermore, we discuss the challenges encountered in diagnosing a low grade central osteosarcoma which has deceptively benign features radiologically and histologically.