Soap-bubbly appearance | Radiology Signs

Answer:

A soap-bubbly lesion is an expansile lytic bone lesion whose interior is crossed by coarse, incomplete bony ridges, so that the lucency appears divided into multiple rounded loculi. On CT most of the apparent septa are not true septations but endosteal ridges on the inner surface of a thinned, remodelled cortex, which is a useful discriminator from genuinely multiloculated lesions. The pattern indicates slow expansion with concurrent endosteal resorption and periosteal apposition, so it belongs to lesions with a geographic margin rather than to aggressive permeative tumours. The classical causes are solitary plasmacytoma and multiple myeloma, in which a vertebral lesion may show thickened residual trabeculae producing the mini brain appearance on axial CT, and hypervascular metastases from renal cell and follicular thyroid carcinoma, which expand and blow out the cortex rather than simply destroying it. Non-malignant mimics that must be considered by age and site include giant cell tumour, aneurysmal bone cyst, brown tumour of hyperparathyroidism, fibrous dysplasia, chondromyxoid fibroma and, in the mandible, ameloblastoma and the odontogenic keratocyst. The practical consequence is procedural: an expansile lytic lesion of renal or thyroid origin can bleed heavily, so cross-sectional imaging should assess vascularity and preoperative embolisation should be considered before biopsy or fixation, while a solitary lesion in a patient over 40 requires myeloma screening before it is labelled a metastasis.

Why is it called so?

The rounded lucent loculi separated by thin curved ridges resemble a cluster of soap bubbles or foam.

Pathophysiology

Slowly growing intramedullary tumour resorbs the endosteal surface faster than the periosteum can lay down new bone, but not so fast that the periosteum is outrun. The cortex is therefore progressively thinned and displaced outward rather than destroyed, and residual buttresses of unresorbed trabecular and endosteal bone persist as ridges within the expanded cavity. In plasmacytoma the plasma cell infiltrate activates osteoclasts through RANKL and inhibits osteoblasts through DKK1, producing purely lytic expansion with no reactive sclerosis. In renal and thyroid metastases the abundant tumour vascularity supports rapid but still expansile growth, which is why these lesions may be pulsatile and haemorrhagic.

Alternative names: Soap bubble appearance; multiloculated lytic lesion

Other associated named signs: Mini brain sign of vertebral plasmacytoma, and the fallen fragment sign and rising bubble sign of simple bone cyst

References

1. Spinnato P, Filonzi G, Conficoni A, Facchini G, Ponti F, Sambri A, et al. Skeletal Survey in Multiple Myeloma: Role of Imaging. Curr Med Imaging. 2021;17(8):956-965. PMID: 33573573.