Findings: (Images 1 - 4)
An oval-shaped, solid 4 cm mass is identified within the posterolateral aspect of the right proximal thigh, superficial to the muscular fascia along the deep aspect of the subcutaneous fat. (Images 3a & 4a) It demonstrates complex signal characteristics and a relatively well-defined border, correlating with the calcified rim seen circumscribing the soft tissue mass on plain films (Images 1a & 2a).

Inflammation is noted both around the mass itself, along the periphery of the vastus lateralis and other quadriceps muscles. Edema is also present within the hamstring musculature. The underlying femur is normal.

Discussion:
The peripherally calcified rim, muscular edema and the clinical history (that the calcifications were not present on a plain film taken approximately two weeks ago) strongly support the diagnosis of posttraumatic lesion such as myositis ossificans or idiopathic inflammatory pseudotumor.

The current examination is performed without contrast. A contrast study would determine whether there is internal enhancement within the mass. A myositis would not enhance internally, whereas a soft tissue tumor would. Additional unlikely diagnostic considerations include aggressive fibromatosis, synovial sarcoma, and actinomycosis (due to penetrating wound).

Myositis ossificans is a post-traumatic soft tissue mass which develops characteristic peripheral calcification over a 6 - 8 week period following acute injury. Other less common etiologies include burns, tetanus, or an intramuscular hematoma. Imaging findings lack specificity in the early days after an injury and are difficult to distinguish from more ominous lesions such as a sarcoma. MR imaging findings evolve in parallel with radiographic findings. Initially, heterogeneous intramuscular signal alteration will progress to a mass-like region of high signal intensity on T2-weighted images during the first days to weeks after injury. Rim- like calcification develop over a 6 - 8 week period and can be documented on plain film and CT. MR images reveal (after 6 - 8 weeks) a corresponding hypointense thin rind of signal alteration with central fat signal intensity. Clinical history, especially early on, is essential to aid the radiologist in suggesting this very important diagnosis. Failure to suspect this entity may lead to an unnecessary biopsy, which could result in a false diagnosis of osteosarcoma due to the presence of abundant osteoid and mitotic spindle cells.

Sources:
1. David A. May, David G. Disler, Elizabeth A. Jones, Avinash A. Balkissoon, and B. J. Manaster Abnormal Signal Intensity in Skeletal Muscle at MR Imaging: Patterns, Pearls, and Pitfalls.RadioGraphics 2000; 20: 295-315.