Case report: natural development of osteosarcoma from precancerous lesion.

We encountered a very rare case that suggested the natural development of osteosarcoma from a precancerous lesion. The patient presented with a huge osteosarcoma in the distal femur on the initial consultation to our hospital. He had undergone X-ray examination twice previously, due to a knee injury. The findings of the lesion detected by the first X-ray examination were similar to a fibrous cortical defect (FCD), differing from those of an osteosarcoma lesion detected by second and last X-ray examinations. We retrospectively estimated the growth rate of the FCD-like and osteosarcoma lesions and found that FCD-like lesion was not osteosarcoma, but might have been a precancerous lesion. We also speculated that this osteosarcoma lesion might have appeared 18 months before the patient consulted our hospital.