Soft tissue aneurysmal bone cyst of left hemithorax: An extremely rare case.

Aneurysmal bone cyst (ABC) is a benign expansile cystic lesion that can affect any bone of the skeleton, especially the femur, tibia, and humerus. Lesions with histologic features of an ABC can be originated within soft tissue in exceedingly rare cases. Extra-skeletal ABC may mimic a variety of benign and malignant lesions and can be confused with other common or rare giant cell-rich tumors of soft tissue. Clinical, radiological and histologic correlation are crucial in reaching the correct diagnosis. Here we report a case of an extra-skeletal ABC arising in left hemithorax in a 13-year-old girl and discuss the common differential diagnosis of this rare entity.