Primary aneurysmal cyst of soft tissues (extraosseous aneurysmal cyst).

In this report we describe a soft-tissue aneurysmal cyst located in the left shoulder of a 20-year-old woman. Although the tumor initially was clinically considered a myositis ossificans, the microscopic features were identical to those found in classic aneurysmal bone cyst (ABC). Light microscopy showed a lesion composed of a peripheral band of mature trabecular bone surrounding a multicystic central area made up of anastomosing cavernous blood channels separated by fibrous septa that contained osteoid, mineralized myxochondroid material and osteoclastic giant cells. Whereas most bone tumors have an extraosseous counterpart, ABC has been found outside the bone only once, in an artery, but never in soft tissues. Moreover, major reviews state that aneurysmal bone cyst occurs exclusively within bones. This unique case is a very rare occurrence of one of these lesions in the soft tissues.