Imaging description

The diagnosis of fibrous dysplasia is usually made radiographically. It demonstrates expansile lytic lesions and cortical thinning with endosteal scalloping of the affected long bones. The expanded medullary spaces reveal a characteristic ground glass appearance. Some small lesions are surrounded by a thick sclerotic border or rind. These findings are strongly suggestive of polyostotic fibrous dysplasia and no further imaging is necessary (Figure 78.1).

The diagnosis of a myxoma is best made on MRI where the signal characteristics of the lesion are determined by its high water content. Myxomas have markedly low signal intensity on T1-weighted images compared with adjacent muscles and high signal intensity on T2-weighted images. The gadolinium-enhanced T1-weighted images reflect its solid consistency because it shows internal enhancement. The degree of enhancement is mild to moderate and it is almost always heterogeneous. In more than half the patients the myxoma is multifocal (Figure 78.2).

Ultrasound is useful in differentiating a cyst from a solid lesion such as a myxoma. It shows small areas of internal echoes in myxomas.

Importance

The association of intramuscular myxoma with fibrous dysplasia is well established and it is known as the Mazabraud syndrome. This association was first described by Henschen in 1926. In the English literature it was first described in 1971 by Wirth et al. In Mazabraud syndrome the myxoma(s) is most frequently associated with polyostotic fibrous dysplasia although several cases have been reported with the monostotic form of the disease. The reason for the association between these two entities is not clear but one explanation is the common origin and histiogenesis of fibrous dysplasia andmyxoma. The intramuscular myxoma in Mazabraud syndrome is often multifocal, but it can also be a solitary lesion.