The term 'soap bubble' on imaging is a characteristic descriptor for which tumor?

• A. Giant cell tumor
• B. Osteosarcoma
• C. Ewing sarcoma
• D. Chondroblastoma

Answer: (A)

Soap bubble appearance reflects a multiloculated, expansile lytic process with internal septations creating bubble-like cavities. This pattern is most classically associated with giant cell tumor, particularly in skeletally mature patients around the knee where the lesion is eccentric and epiphyseal-to-metaphyseal in location. The cortex is thinned and expanded, but the periosteal reaction is not as aggressive as in osteosarcoma, and there isn’t the diaphyseal, onion-skin, or highly sclerotic features seen with the other tumors.

Osteosarcoma tends to show aggressive periosteal reactions (like sunburst) and mixed lytic-sclerotic changes with a rapid course. Ewing sarcoma usually presents in the diaphysis with onion-skin periosteal reaction. Chondroblastoma is an epiphyseal lesion in younger patients and often shows chondroid matrix and calcifications; it can be bubbly, but the classic, dominant descriptor is more characteristic of giant cell tumor.