MR imaging and ultrasonography findings of early myositis ossificans: a case report.

Myositis ossificans (MO) is a benign soft tissue lesion with non-neoplastic heterotopic bone formation. MO in the intermediate and mature stages can be easily diagnosed if characteristic imaging findings such as a peripheral zonal pattern of ossification with variable thickness is observed. However, it is difficult to correctly diagnose early MO because it can mimic malignancy clinically, radiologically, and histopathologically. We report a case of early pseudosarcomatous phase of non-traumatic MO with atypical imaging findings. A 59-year-old woman presented with pain followed by a mass in the left thigh within a week. MR imaging and ultrasonography showed an intramuscular lesion with preserved muscle fascicles in the vastus lateralis muscle. Intralesional ossification or calcification was not seen on ultrasonography. A diagnosis of myositis ossificans was made by ultrasonographically guided biopsy.