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

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 describes a multiloculated, expansile lytic lesion with thin internal septa that create a bubbly, soap-bubble appearance on X-ray. This pattern is most strongly associated with a giant cell tumor because these tumors arise in the epiphysis of long bones after physeal closure and expand, thinning the cortex while preserving a well-defined border with internal septations. The knee region (distal femur or proximal tibia) is a classic site, and the lesion tends to be locally aggressive but without the aggressive matrix mineralization seen in other sarcomas.

Other tumors have different imaging hallmarks. Osteosarcoma typically shows aggressive periosteal reactions like a sunburst pattern or Codman triangle and a mix of lytic and sclerotic areas with soft-tissue mass. Ewing sarcoma often presents with an onion-skin periosteal reaction and diaphyseal involvement, sometimes with systemic symptoms. Chondroblastoma occurs in adolescents, is epiphyseal, and may have calcifications within the lesion but does not usually produce the classic expansive soap-bubble appearance of a GCT.