Beveled edge sign | Radiology Signs

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. Lesions are usually in the frontal and parietal bones of a child or young adult, may be solitary or multiple, and are often painful with a palpable soft tissue swelling. The differential is refined by age and number: metastasis and myeloma in an adult, where multiple lesions have less sharply defined margins and myeloma classically gives uniform punched-out lucencies without sclerosis; epidermoid or dermoid, which has a sclerotic rim; venous lakes and arachnoid granulations, which are normal and occupy known anatomical sites; osteomyelitis with a ragged margin; and haemangioma with a radiating trabecular pattern. Skeletal survey or whole-body MRI defines the extent of disease, and coexisting vertebra plana, mandibular floating teeth or diabetes insipidus supports the diagnosis.

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. The infiltrate itself is a clonal proliferation of myeloid dendritic cells, most carrying BRAF V600E or another MAP kinase pathway mutation, which provokes an eosinophil-rich inflammatory response with local osteoclast activation.

Alternative names: Hole within a hole appearance; hole-within-a-hole sign; double contour sign; bevelled edge sign

Other associated named signs: Button sequestrum, geographic skull, floating teeth sign, and the vertebra plana of Langerhans cell histiocytosis

References

1. Zaveri J, La Q, Yarmish G, Neuman J. More than just Langerhans cell histiocytosis: a radiologic review of histiocytic disorders. Radiographics. 2014;34(7):2008-24. PMID: 25384298.