Purpose or Case Report: Myositis ossificans is a pseudotumor that originates from skeletal muscle that may be clinically and histologically mistaken for a malignant soft tissue tumor. The correct diagnosis of myositis ossificans is important, as it can prevent unnecessary biopsies or surgery. This case report illustrates the imaging features of myositis ossificans on plain radiography, ultrasound (US), and magnetic resonance imaging (MRI).
The subject of the case report is a 16-year-old female who presented to emergency department with 11 days of posterior right thigh pain with no known history of trauma. Physical examination revealed significant point tenderness in the right posterior thigh. Initial radiographs were normal. A targeted US demonstrated a mixed echogenicity lesion in the deep soft tissues of the posterior lateral right thigh. A follow up US, 2 weeks after the initial one, showed a lesion of mixed echogenicity with a central hypoechoic area encircled by a peripheral hyperechoic area. That peripheral hyperechoic area has some posterior acoustic shadowing suggesting calcifications. An MRI obtained approximately 1.5 months after the initial hospital visit demonstrated a well-defined lesion with heterogeneous T2 hyperintense and T1 intermediate signals along the short head of the right biceps femoris muscle. Mild heterogenous enhancement was seen of the mass and peripheral enhancement within the adjacent soft tissue. Radiographs obtained on the same day as the MRI showed a faintly ossified round mass in the posterior lateral soft tissues of the right thigh. A follow up radiograph obtained 4 months later demonstrated an interval decrease in size of the calcified mass, and the patient’s pain had improved by this time.
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