Question
Osteoclastoma
Answer
Osteoclastoma (Giant Cell Tumour of Bone) is a locally aggressive, usually benign, bone tumour typically arising in the epiphysis of long bones (most commonly around the knee — distal femur/proximal tibia) after epiphyseal fusion, in young adults (20–40 years).
Gross: Eccentric, expansile, lytic lesion that may extend to the subchondral bone; red-brown, haemorrhagic, soft “chocolate-coloured” cut surface, sometimes with cystic degeneration.
Radiology: Eccentric, purely lytic, “soap-bubble” appearance with well-defined but non-sclerotic margins, extending to the subchondral bone surface, typically without periosteal reaction.
Microscopy: Characteristic biphasic population of cells: numerous osteoclast-like multinucleated giant cells (containing 50–100+ nuclei) uniformly scattered throughout a background of mononuclear stromal cells (the actual neoplastic component), with the nuclei of the giant cells resembling those of the surrounding mononuclear cells (a key diagnostic clue).
Behaviour: Locally aggressive with a tendency to recur after curettage; a small percentage (~1–5%) undergo malignant transformation or show pulmonary metastasis (usually indolent/“benign metastasizing” behaviour) despite bland histology. Treated by curettage (with adjuvant therapy) or wide excision for aggressive/recurrent lesions.