Myositis ossificans circumscripta (MOC) is a form of heterotopic ossification in extraskeletal soft tissues that is benign, but may appear clinically and radiologically as a malignant neoplasm1.
An 18-year-old woman had sudden pain in her left thigh with functional impairment, without trauma. Examination revealed an indurated mass on the upper third of the thigh. Laboratory analyses were normal except for a C-reactive protein level at 35 mg/l (normal < 3 mg/l). Magnetic resonance imaging (MRI), computed tomography (CT), and radiographs of the left thigh (Figure 1) were typical of MOC. Treatment with steroids and bisphosphonate infusion was effective for pain.
MOC is a rare benign disorder and imaging is characteristic2. It can be observed at any age, but especially in adolescents and young adults. MOC affects mainly extremities2 and trauma is often implicated (70%)2. Bone scintigraphy seems very sensitive, showing increased uptake over the lesion3. CT scanning illustrates the typical imaging features of MOC with a rim of calcification around the lesion, a perilesional edema without bone marrow or cortical abnormalities4. MRI findings depend on the stage of maturation and histological pattern of the lesion3. T2-weighted images show a heterogeneous localized mass with high signal intensity in the central area3. Without the characteristic imaging signs, biopsy is necessary to determine the difference between bone and soft-tissue malignancy1,5 such as paraosteal sarcoma, rhabdomyosarcoma, malignant fibrous histiocytoma, or osteoblastic metastasis.