C. Soft tissue aneurysmal bone cyst
The gross and microscopic findings were characteristic of a soft tissue (extraosseous) aneurysmal bone cyst (ABC) with USP6 locus rearrangement, confirming the diagnosis (Figure 2). Aneurysmal bone cysts are benign but locally aggressive tumors most commonly found in association with the long bones and spines of children and young adults.1,2 Extraosseous or soft tissue ABCs pathologically and genetically resemble those associated with bone but lack a bony attachment.2 Soft tissue ABCs are a rare phenomenon with approximately 40 cases reported in the literature.3 Extraosseous ABCs were first described in 1972 by Salm and Sissons as soft tissue lesions with identical histopathological findings as ABCs.4 Aneurysmal bone cysts are classically described as bony-shelled, blood-filled cysts with fibrous septa, which contain giant cells, woven bone, fibroblasts, and histiocytes.5- 7 In 1999, Dal Cin et al described chromosomal rearrangement of 17p13 in all ABCs, including those associated only with soft tissue, confirming similar neoplastic pathogenesis.8 Furthermore, USP6 locus rearrangement has been determined to be sensitive for ABCs in a more recent study by Li et al.9 Magnetic resonance imaging is identical to osseous ABCs, demonstrating an expansile lesion, T2-weighted intense, with strong contrast enhancement and fluid levels.2,6,7,10 These lesions tend to occur within the first 2 decades of life, and of cases reported there is no sex predominance.2,10 Aneurysmal bone cysts, including the soft tissue variety, are difficult to diagnose given their rarity and similar presentation, clinically and radiographically, to other more common tumors such as giant cell tumors, extraskeletal osteosarcomas, ossifying fibromyxoid tumors, and myositis ossificans.2,3,6,7