Aneurysmal cyst of soft tissue: report of a case with serial magnetic resonance imaging and biopsy.

We report a case of an extraosseous aneurysmal cyst arising in the left retroclavicular soft tissue of a 29-year-old woman. A magnetic resonance imaging (MRI) scan showed a solid lesion within soft tissue, abutting the clavicle without bone involvement. An incisional biopsy was interpreted as showing osteoclast rich nodular fasciitis with prominent vascularity. A second MRI 5 months later showed intralesional cystic change with areas of increased signal on T2-weighted images, still without any bony defect. The lesion was excised. Histological examination revealed large vascular spaces lined focally by giant cells. The remainder of the lesion was composed of an admixture of spindle cells and osteoclast-like giant cells. The histological and ultrastructural appearance was that of an aneurysmal bone cyst; however, in view of the lack of any bony involvement, a diagnosis of aneurysmal cyst of soft tissue was made. Primary aneurysmal cysts of soft tissue are rare; this is the third well-documented case in the literature, and the first to describe both the MRI appearance and the histological evolution from a solid to multiloculated lesion.