Primary aneurysmal cyst of soft tissue. Report of a case with ultrastructural and MRI studies.

We report a case of primary aneurysmal cyst of soft tissues in a 57-year-old woman presenting with a painful mass in her left arm. Conventional radiography showed a radiolucent soft tissue mass surrounded by a ring of bone. MRI displayed an unusual, ill-defined soft tissue lesion that was not connected to the nearby humerus and appeared to be an aggressive tumour. Microscopically, the mass consisted of multiple anastomosing cavernous channels surrounded by a peripheral band of mature trabecular bone. These bloody channels were separated by fibrous septa containing fibroblasts, histiocytes and multinucleated giant cells, as well as fibromyxochondroid material. Some of these giant cells lined the septa and partially occupied the lumen of the channels. Ultrastructurally, the features observed in this tumour were similar to those described in aneurysmal bone cyst; the giant cells lining the septa were an additional observation. Whereas most bone tumours have a well-known extraosseous counterpart, this unique lesion is not well recognized by surgical pathologists and the few published cases have been reported under different names. Gross, microscopic, radiological and ultrastructural findings are presented to familiarize pathologists with this underdiagnosed condition.