[MRI diagnosis in longitudinal stress fractures: differential diagnosis of Ewing sarcoma]

To compare MR imaging features of stress fractures, simulating malignancies, and Ewing sarcomas.

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 fractures was confirmed by follow-up examinations. An additional biopsy was performed in two patients.

Despite negative x-ray examinations, MRI showed the fracture line in all patients with stress fractures. In these cases marrow edema was irregular and there was no well defined margin towards normal fatty marrow. In contrast Ewing sarcomas were sharply demarcated in 9/10 cases. Extraosseous enhancing soft tissue was found in Ewing sarcomas as well as in stress fractures. In stress fractures the enhancing 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 in patients with stress fractures. Occurrence of low signal areas in T1- and T2-weighted sequences within the initial enhanced extraosseous tissue corresponded to bony callus on x-rays films.

Repair tissue in stress fractures can imitate malignancy. The irregularity of the marrow edema without well defined margins, the lack of necrosis 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.