Case 4

I am presented with a non-contrast CT series, demonstrating a large mass in the medial compartment of the right thigh – it is heterogeneously hypo-attenuating with a distinct peripheral rim of calcification. There is no involvement of the adjacent femur.

I am now presented with selected MRI series showing multiple fluid-fluid levels within the mass – indicating the layering of blood products within a haematoma. There is a thin rim of very low signal intensity around the mass, indicating calcification.

The fluid sensitive series (such as STIR and PD) show high signal intensity in the adjacent medial compartment muscles – indicating reactionary oedema / inflammation.

These findings are most characteristic of myositis ossificans as a result of local trauma and haematoma formation.

The differential diagnosis includes soft tissue aneurysmal bone cyst and extra-osseous telangiectatic osteosarcoma.

I would ask about a history of trauma, and recommend follow up imaging to document the expected zonal ossification pattern of myositis ossificans, whereby the mass continues to ossify from the periphery. This distinguishes it from malignant lesions which do not show mature calcification peripherally.