This Chest x-ray demonstrates classic findings of fibrous dysplasia involving the right 4th rib posteriorly. Fibrous dysplasia exists in two forms: monostotic (involving one bone) and polyostotic forms (involving multiple bones).
The monostotic form is by far the most common, the ribs being the most common site of fibrous dysplasia. Thus, fibrous dysplasia is the commonest benign expansile lesion of a rib. Fibrous dysplasia is a non-neoplastic process, causing replacement of normal bone with large fibrous stroma and islands of immature woven bone.