·       A lesion of uncertain origin.
·       Usually benign but may recur after local removal or curettage.
·       In about 10% cases if behaves as frankly malignant tumour. Metastasising through the blood stream to the lung.
·       Common sites- Epiphyseal region
o   The lower end of the femur
o   The upper end of the tibia
o   The lower end of the radius
o   The upper end of the humerus
Those ends of the long bones at which most growth occurs (growing ends)

Pathology
Characteristically it occurs in the end of the bone, occupying the epiphysical regions and often extending almost to the joint surface. It destroy the bone surface, but new bone forms beneath the periosteum, so tumour, giving a faintly loculated appearance (soap bubble appearance). Pathological fracture is common.
The tumour has a reddish fleshy appearance; it comes away in pieces quite easily when curetted, but is difficult to remove completely from surrounding bone. Aggressive lesions have a poorlydefined edge and extend well into surrounding bone.
Histologically, the striking feature is an abundance of multinucleated giant cells scattered on a background of stromal cells (spindle calls) which little or no visible inter cellular tissue.
Clinical Features:
The patient is usually a young adult.
i.             Gradually increasing local swelling in the end of a long bone.
ii.          Pain the end of a long bone
iii.       There may be a history of trauma
iv.       Pathological fracture occurs in 10-15% of cases
v.          May be tender on firm palpation.

Differential Diagnosis
i)                  Subacute osteomyelitis
ii)               Syphilitic osteo-periostitis
iii)            Other bone tumours
iv)            Metastatic suprarenal neuroblastoma (Histologically)

Investigations
X-rays
i)     A radiolucent area situated eccentrically at the end of along bone and bounded by subchondral bone plate.
ii)   The centre has a soap-bubble appearance due to ridding of the surrounding bone.
iii)  The cortex is thin and sometimes ballooned; aggressive lesions extend into the soft tissue.

CT & MRI                        -   Will reveal the extent of the tumour, both within the bone and beyond.
Arthoscopy                       -   To establish whether the articular surface has been broached or not.
FNAC or Open biopsy    -   To confirm the diagnosis.
  
Treatment
·       Well-confined, slow-growing lesions benign histology-Thorough curettage and stripping of the cavity with burrs and gouges, followed by swabbing with hydrogen peroxide or by the application of liquid nitrogen; the cavity is then packed with bone chips.
·       More aggressive tumours- Excision followed, if necessary, by bone grafting or prosthetic replacement.
·       If the affected bone is one that can resonably be dispensed with- such as the clavicle or fibula- Excision of part of the bone.
·       Tumours in awkward sites (e.g. spine) may be difficult to eradicate; supplementary radiotherapy is sometimes recommended but it carries a significant risk of causing malignant transformation.