Giant cell tumours are so named because they contain multinucleate osteoclast-type giant cells. These tumours are benign but locally aggressive. They can metastasize, and can rarely undergo sarcomatous transformation. They typically occur eccentrically in the metaphysis of long bones, usually around the knee. If fusion of the physeal plate has occurred, GCTs extend to the subarticular bone; they are otherwise limited in growth by the open physeal plate.

On FDG-PET/CT, low uptake in a bone lesion is likely benign. High uptake however, as is present in this case, is less specific, with high FDG accumulation occurring in benign bone lesions, including: giant cell tumour, chondroblastoma, Langerhans' cell histiocytosis, amongst others. Malignant bone lesions with high uptake may be primary or metastatic.