Soft-tissue aneurysmal bone cyst: report of a case with t(5;17)(q33;p13).

We describe a primary aneurysmal bone cyst (ABC) of the soft tissue of the distal thigh in a 10-year-old girl. Radiographs showed an oblong density in the soft tissue that was consistent with hemorrhage or calcification; the underlying bone was unremarkable. Pathologic findings were consistent with myositis ossificans on gross examination, but numerous hemorrhagic cysts were present in the lesion. Histopathologic findings showed features of an ABC. Cytogenetic analysis of the tumor revealed 46,XX,t(5;17)(q33;p13); 17p13 breakpoints have been reported in intraosseous ABC. Thirteen months after diagnosis, the patient had excellent function and no radiologic evidence of recurrent disease. Soft-tissue ABC is a rare, benign lesion that can have a similar radiologic appearance to myositis ossificans but has a histologic appearance identical to that of its intraosseus counterpart.