MR imaging in longitudinal stress fractures: differential diagnostic features to Ewing sarcomas.

Purpose: To compare MR imaging features of stress fractures, simulating mal ignancies, and Ewing sarcomas. Materials and Methods: MR imaging studies of 4 patients with longitudinal stress fractures of the tibia (n = 2) and the femur (n = 2) simulating malignancy were retrospectively compared with the MRI scans of 10 patients with histologically proven Ewing sarcoma (femur n = 5; tibia n = 3, fibula n = 1; humerus n = 1). The diagnosis of stress fr actures was confirmed by follow-up examinations. An additional biopsy was p erformed in two patients. Results: Despite negative x-ray examinations, MRI showed the fracture line in all was irregular and there was no well define d margin towards normal fatty marrow. In contrast Ewing sarcomas were sharp ly demarcated in 9/10 cases. Extraosseous enhancing soft tissue was found i n Ewing sarcomas as well as in stress fractures. In stress fractures the en hancing mass was repair tissue. Areas of necrosis within the enhancing mass was seen in (8/10) Ewing sarcomas; only. In follow-up studies we observed a decrease of the marrow edema inpatients with stress fractures. Occurence of low signal areas in T-1- and T-2-weighted sequences within the initial e nhanced extraosseous tissue corresponded to bony callus on x-rays:films. Co nclusions: Repair tissue in stress fractures can imitate malignancy. The ir regularity of the marrow edema without well defined margins, the lack of ne crosis in the small enhancing tissue and the proof of the fracture line in the MRI are criteria to differentiate stress fractures from Ewing sarcomas. Short-term follow up studies are helpful to underline the diagnosis.