Giant-cell bone tumour or osteoclastoma, light micrograph. The large cells with multiple nuclei that give this form of tumour its name are visible throughout this stained section. This is a grade II tumour which is not malignant but has poorly defined boundaries. Giant-cell tumour is most common in young adults. It arises in the long bones of the leg or arm, most often around the knee. It grows rapidly, causing painful swelling, and may invade joints. Giant-cell tumours rarely undergo spread (metastasis) to other tissues. Treatment is surgical removal, followed by the use of liquid nitrogen to kill remaining cells. Magnification unknown.