An unusual intraosseous lesion with fibroblastic, osteoclastic, osteoblastic, aneurysmal and fibromyxoid elements. "Solid" variant of aneurysmal bone cyst.

Four examples are reported of an unusual noncystic intraosseous lesion which does not conform to any hitherto recognized entity and which can be mistaken, not only by the general histopathologist but by the osteoarticular pathologist, for a variety of other conditions, including sarcoma and giant-cell tumor. They were in patients aged 5 to 13 years; three in the spine, one in the ethmoid. Local excision, supplemented by low-dose radiotherapy in cases with cord compression, produced a satisfactory outcome in all cases. At presentation the radiologic findings were nonspecific but, following treatment, an eggshell rim of bone developed in those lesions which had been incompletely excised. Histologically, they are characterised by florid fibroblastic or fibrohistiocytic proliferation, osteoblastic differentiation with osteoid production, areas rich in osteoclast-type giant cells, aneurysmal sinusoids, and occasional foci of degenerate calcifying fibromyxoid tissue. Because this combination of histologic features can be found in the solid parts of aneurysmal bone cyst and in no other condition, at this centre we have regarded this lesion as a variant of aneurysmal bone cyst devoid of any cystic component.