A "soap bubble" appearance in the femur or tibia seen on X-ray is indicative of which type of bone tumor?

The "soap bubble" appearance observed on X-ray is characteristic of a giant cell tumor of bone. This type of tumor typically arises around the knee, particularly at the distal femur and proximal tibia. The appearance results from the tumor’s lytic nature, which causes localized bone destruction and a prominent fibrous and osteoclastic component within the lesion.

Giant cell tumors are generally benign but can exhibit aggressive behavior and have a potential for local recurrence. The "soap bubble" description comes from the well-defined, eccentric lytic lesions that expand the bone and present with significant cortical thinning, reinforcing the diagnosis when visualized through imaging techniques.

Other bone tumors mentioned, such as osteosarcoma, chondrosarcoma, and fibrous dysplasia, have different radiographic features. Osteosarcoma typically presents as a mixed lytic and blastic lesion, often with periosteal reaction or "sunburst" appearance. Chondrosarcoma may show calcified cartilaginous elements on imaging. Fibrous dysplasia presents as a "ground glass" appearance rather than a soap bubble.

Thus, the specific imaging characteristics, especially the distinctive "soap bubble" appearance, are strongly indicative of a giant cell tumor of bone.