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Neuroradiology Case of the Week

Case 422

July 2009

Gurshawn Singh MS II, Rajiv Mangla MD

Clinical Presentation: Patient is a 17-year-old female with a right paraspinal mass,

Imaging Findings: Initial CT showed a hyperdense lesion within the right paraspinous muscles at the T8 level. The adjacent vertebrae and intervertebral discs were normal. MRI performed shows hypointensity on T1 weighted images and hyperintensity on T2 weighted images in the same location which showed intense enhancement on post-contrast images. The imaging findings could not differentiate a malignant or benign condition at this point. The follow-up imaging showed progressive decrease in size of lesion with mature bone formation having central lucency. The adjacent cortex was preserved.

Diagnosis: Myositis ossificans circumscripta (pseudomalignant myositis ossificans)

Discussion: Myositis ossificans is a rare condition which can manifest in two forms. One form, Myositis ossificans circumscripta, is usually post-traumatic but in many cases history of trauma is not present. This form is also known as pseudomalignant myositis ossificans (PMO) due to its difficult differential diagnosis as it can be mistaken for other malignant conditions of soft tissues. The other form of myositis ossificans is congenital in nature and is also known as fibrodysplasia ossificans progressiva. This is a rare autosomal dominant disease with complete penetrance present via a mutation in bone morphogenic proteins and involves the ossification of skeletal muscles precipitated when trauma is present. Fibrodysplasia ossificans progressiva presents with pain and swelling of the soft tissues. Characteristic features are congenital malformations of great toes and most notably later on in life is progressive heterotopic ossification of tendons, ligaments, fasciae, and striated muscles. The pathophysiology of this disease is still unknown. Histologically, muscle fibers are usually destroyed and mononuclear infiltrates within the muscles and connective tissue are present. Fibroblasts will proliferate to replace damaged muscle fibers and are abundant in areas of newly formed bone tissue forming the heterotopic skeleton.
     Myositis ossificans circumscripta is a localized form and is often confused with malignant conditions when a clear history of trauma is not present. It may be difficult to differentiate even on imaging and histopathology. Imaging will show areas of bony masses penetrating into muscles in the areas of body where bone is normally not present e.g. in tendons, ligaments, or skeletal muscles. This condition can be differentiated from malignant sarcomas by virtue of intact cortex of underlying bone, radiolucent cleft, dense calcification in the periphery, and progressive decrease in volume on serial imaging. Normal radiographs are the preferred method of imaging in myositis ossificans. Radiographs can show remodeling of heterotopic bone. CT scanning can be more useful in detecting early swelling and ossification and might be necessary for the early diagnosis. The calcification starts after 7 days of presenting with symptoms and may not be seen even on CT until 6 weeks. MRI in early stages shows a nonspecific enhancing lesion within the muscles which might be difficult to differentiate from infection or malignant conditions like soft tissue sarcoma.
     The diagnosis of this condition can be a diagnostic challenge in early stages. Our case presented with painful swelling in the back which was hyperdense and partially calcified on CT. Biopsy was also performed and revealed florid fibroblastic proliferation of uniform spindle cells with extensive myxoid change. The most important diagnostic finding was ectopic ossification showing varying degrees of maturation which could be only visualized on repeat biopsy after some time. The follow-up CT in our case showed progressive mineralization, formation of mature bone, and decrease in volume.

References:

1. Mahboubi S, Glaser DL, Shore EM, Kaplan FS. Fibrodysplasia ossificans progressiva. Pediatr Radiol. 2001 May;31(5):307-14. PMID: 11379597 [PubMed]
2. Goldman AB. Myositis ossificans circumscripta: a benign lesion with a malignant differential diagnosis. AJR Am J Roentgenol. 1976 Jan;126(1):32-40. PMID: 175681 [PubMed]