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Neuroradiology Case of the Week

Case 397

Yasser Elshatory, Taik-Kun Kim, MD, and PL Westesson, MD, PhD, DDS

April 2009

Clinical Presentation: Patient is an 18-year-old woman who presented with a palpable mass on her back.

Imaging Findings: A well-circumscribed calcified intramuscular mass is noted in the paraspinous region on CT. Calcification is flocculent in shape. The mass has no connection with the spine. T1-weighted images with contrast reveal a well-enhancing mass with a tail like extension along intermuscular spaces. However, there is no infiltration into the surrounding tissue. A non-disrupted subcutaneous fat layer is seen overlying the lesion.

Diagnosis: Myositis ossificans of the erector spinae muscle

Discussion: The differential for extraskeletal or heterotopic ossification includes hereditary and non-hereditary clinical entities. However, hereditary heterotopic ossification, fibrodysplasia (myositis) ossificans progressiva, should be considered when heterotopic ossification is present in multiple sites, the patient is affected in childhood, and the course is severe and progressive, as it is a rare entity with an incidence of 1 in 10,000,000. The more common non-hereditary heterotopic ossification, myositis ossificans circumscripta, is often restricted to a single site, and is more common in adults. In fact, a more reasonable differential should include benign and malignant diseases of the paraspinal area, which includes diseases arising from muscle or surrounding connective tissue, such as rhabdomyosarcoma, malignant fibrohistiocytoma, and myositis ossificans, as well as diseases arising from adjacent bone, such as osteogenic sarcoma. Ruling out the latter is particularly important, since osteogenic sarcoma also represents a condition associated with heterotopic ossification, unlike rhabdomyosarcoma and malignant fibrohistiocytoma, in which calcification is not a prominent feature, or if present, displays more poorly defined areas of calcification. Primary osteogenic sarcoma of the spine is rare, however, comprising less than 2% of all osteogenic sarcomas. A good case for myositis ossificans circumscripta over osteogenic sarcoma can be made if imaging reveals a clear margin between the lesion and the adjacent bone. In the present case, a non-disrupted subcutaneous fat layer overlying the lesion, and adjacent muscle and bone with well-defined, non-disrupted boundaries argued against a malignant lesion such as osteogenic sarcoma. However, a tail-like extension along an intermuscular space seen on T1-weighted images with contrast was worrisome, and prompted CT biopsy and subsequent open biopsy. Pathological analysis confirmed a diagnosis of myositis ossificans circumscripta.
     The most common sites for myositis ossificans are in the thigh and arm (80%), with the intercostal spaces, erector spinae muscles, pectoralis muscles, glutei and chest wall representing collectively only 20% of observed sites. Thus, the present case of myositis ossificans of the erector spinae muscle represents a less common site for myositis ossificans. A history of trauma, either major, or minor but repetitive can sometimes be elicited. Excision of the soft tissue mass of the erector spinae while curative for myositis ossificans circumscripta would likely be associated with significant morbidity following surgical trauma.

References:

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