A Case of Secondary Chondrosarcoma Arising in Osteochondroma Diagnosed With 18 F-FDG PET/CT.

A man in his seventies with an incidentally detected scapular osteochondroma showed progressively increasing FDG uptake on serial 18 F-FDG PET/CT, while the CT component initially revealed no definite signs of malignant transformation. On the final PET/CT, however, CT images showed a periosteal reaction suggestive of extracortical extension, raising strong concern for malignancy. Biopsy was followed by tumor resection, and postoperative pathology confirmed secondary peripheral chondrosarcoma (grade 3). The patient later developed widespread metastases. This case underscores that increased or rising FDG uptake in an osteochondroma may indicate early malignant transformation, emphasizing careful assessment even when CT changes are minimal.