What causes the beveled edge sign of a skull lesion on radiography and CT?
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Answer:
The beveled edge sign is a lytic calvarial lesion in which the inner and outer tables are destroyed to different extents, so their margins do not coincide and the defect is seen as two overlapping lucencies of unequal size. It is also called the hole within a hole appearance. It is characteristic of Langerhans cell histiocytosis, particularly the solitary eosinophilic granuloma of bone, where the inner table is usually the more extensively destroyed. A central residual island of unresorbed bone within the defect, the button sequestrum, may accompany it and is a further pointer to the diagnosis, though it is also described in osteomyelitis, radiation necrosis, epidermoid and metastasis. The differential for a lytic skull lesion includes metastasis, multiple myeloma, osteomyelitis, lymphoma, epidermoid, haemangioma and a surgical or traumatic defect; a non-sclerotic, geographic margin with a bevelled edge in a child or young adult strongly favours Langerhans cell histiocytosis.
Why is it called so?
The mismatched inner and outer table margins give the defect a bevelled, chamfered edge rather than a single sharp rim.
Pathophysiology
Langerhans cell histiocytosis produces a proliferating histiocytic infiltrate within the diploic space. Because the lesion begins in the diploë and expands outwards in both directions, it destroys the two cortical tables independently rather than as a single cylinder, and the rate of destruction differs between them. The result is two non-concentric defects of different diameter superimposed on one another in projection, producing the double-contour lucency and the sloping margin. Bone that remains unresorbed as the lesion enlarges around it survives as the central button sequestrum.
Alternative names: Hole within a hole appearance, double contour sign
Other associated named signs: Button sequestrum, geographic skull, floating teeth sign, and the vertebra plana of Langerhans cell histiocytosis
References
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