MR imaging findings of an unusual case of myositis ossificans presenting as a progressive mass with features of fluid-fluid level.

We present a case of myositis ossificans involving the upper arm in which features of fluid-fluid level became apparent on magnetic resonance imaging (MRI). Serial MR images obtained over a 6-month period exhibited progressive appearances in the absence of dense mineralization in the early and mid phases, which simulate neoplastic conditions. Twenty-four weeks following the biopsy, MR images revealed that the mass had decreased in size with the disappearance of most of the features of fluid-fluid level. To the best of our knowledge, no example of myositis ossificans accompanied by features of fluid-fluid level, which was closely monitored by MRI, exists in the literature. These features of image examination should be evaluated cautiously to avoid unnecessary surgical intervention, especially in instances where lesions exhibit expansive appearance. This case provided beneficial information regarding the sequence of changes in terms of MR appearance of myositis ossificans.