An 11 year old boy presented with the complaints of pain in the right arm near the shoulder. X-ray examination revealed an expansile lytic lesion in the upper third of humerus. The most likely diagnosis is

Ans. is 'b' i.e., Unicameral bone cyst Of the given options only Giant cell tumor and Unicameral bone cyst cause lytic lesion. Among these two, GCT is easily ruled out as it is seen only after epiphyseal closure (i.e., about 20 yrs of age)Unicameral bone cyst (or Simple bone cyst)Though typically causes non-expansile lytic lesion but can cause expansile lesions.More about each optionUnicameral bone cystThese are the only true cysts of bonesThese are not neoplastic lesions.They appear in bone that is still in the active phase of growth i.e., early childhood or adolescenceX-ray appearance is typicalThey present as oval lytic lesion at the metaphysis (though may sometimes migrate to diaphysis as the bone grows)Long axis of the cyst lies parallel to the long axis of the bone.There is no periosteal reactionTypically non-expansile but may be expansible.Parosteal OsteosarcomaIt develops from the periosteum at the periphery of the bone and grows outwardsOn X-raythey present as dense mass of homogenous new bone that extends away from the bone cortex.OsteochondromaThese are not bone tumors but actually a hyperplastic or dysplastic bone disturbance which seems to originate from displaced or aberrant cartilage of the growth plate.The osteochondroma stops growing when the nearest epiphyseal plate fuses', growth after this suggests malignancyX-ray featuresThey develop in the metaphyseal region of long bonesOsteochondromas always originate near an epiphyseal line and grow at right angles to the long axis of the bone.Its distinguishing feature is the continuity of the bone cortex between the normal bone and the cortex of the osteochondroma.Its density varies from that of normal bone to densly calcified bone.Giant cell TumorAge group: 20 to 40 years*, appears after closure of epiphysis. *Main tumor cells of GCT are Mono nuclear stromal cells which fuse to from multinucleated giant cells.X-ray appearanceEccentric osteolytic expansile lesion at epiphysesIt provokes no new bone formation* or calcification. Any bone formation or periosteal reaction rules out GCT.Soap bubble appearance. *