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Findings: Plain radiographs illustrate a non-calcified, soft-tissue mass adjacent to the proximal humerus. MR demonstrates a mass located within the triceps muscle. T1 weighted axial MR images (a) demonstrates a mass with homogeneous signal intensity isointense with muscle, and T2 weighted axial images (b) of the mass demonstrate heterogeneous hyperintense signal. After gadolinium administration (c), the mass diffusely enhances. Edema is present in the surrounding muscle belly. Repeat plain films two weeks later demonstrate a circumscribed radiopaque lesion with a central lucent zone.

Diagnosis: Myositis Ossificans.

Discussion: Myositis Ossificans is a benign, ossifying soft-tissue lesion that primarily occurs within skeletal muscle. It is commonly associated with soft tissue trauma, although in almost one-third of the cases there is no history of injury. It is usually seen in the second and third decades, but has been documented in all age groups. Classification is based on imaging and histologic appearance, and is divided into early, intermediate, or late phase.

In the early phase, proliferating mesenchymal cells are seen on histological exam. This presents as a soft tissue mass radiographically. The mass develops a surrounding margin of lamellar bone with a non-ossifying central cellular area. Subtle calcifications develop 2-6 weeks after the onset of symptoms and matures into a well-defined ossified soft tissue mass by around 6 weeks. Therefore, plain radiographs and CT images during the first few weeks can lead to misdiagnosis. MR may suggest the diagnosis during this early and intermediate phase. MR of the early phase is characterized by an identifiable mass with poorly defined margins and an isointense signal intensity to muscle on T1 weighted images and hyperintense signal intensity on T2 weighted images. Most distinct in the early stages is the marked amount of adjacent edema, unusual in non-hemorrhaged primary neoplasms. During the intermediate stage, the lesion develops a mineralized core, with a surrounding rim of calcification. This is seen as an isointense core on T1 weighted images with a hypointense rim both on T1 and T2 weighted images. In the mature phase, the lesion develops as a well-defined calcified mass with internal fat marrow and no surrounding edema. On T1 and T2 weighted images, the mature lesion displays a rim of decreased signal intensity representing the peripheral mineralization/ossification, with a central area of high signal intensity approximating fat.

It is important to be able to recognize myositis ossificans radiographically. Its plain radiograph and MR appearance can be difficult to distinguish from a soft tissue sarcoma or an inflammatory process, such as an abscess. This is especially true in the early stages. The enhanced rim can mimic an abscess or necrotic tumor, and adjacent soft tissue edema can suggest infection.

References:
Shirkhoda A, Armin AR. MR Imaging of Myositis Ossificans:
Variable Patterns at Different Stages. Journal of Magnetic
Resonance Imaging. 1995;5(3):287-292.

Jelling J. MR Imaging of Soft-Tissue Masses. Mass-Like Lesion.
MRI Clinics of North America. 3(4):727-741, 1995.

Ogilvie-Harris MB. Pseudomalignant Myositis Ossificans:
Heterotopic New-Bone Formation Without a History of Trauma.
JBJS 26A. 1980;8:1274-1283.

Rooser B. Pseudomalignant Myositis Ossificans. Clinical, Radiologic,
and Cytologic Diagnosis in Five Cases. Acta-Orthop-Scand. 1989;60(4):457-460.

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