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Fibrous dysplasia is another type of fibro-osseous lesion that can be found in the paranasal sinuses. Two forms of fibrous dysplasia exist: monostotic and polyostotic. Monostotic (70-85%) lesions involve only one bone, while the polyostotic (15-30%) form can affect multiple bones. The monostotic form involves the facial skeleton approximately 25% of the time, with the mandible and maxilla being the most common locations in the head and neck. Interestingly, the polyostotic form may present in patients with McCune-Albright syndrome; this syndrome also presents with precocious puberty, abnormal skin pigmentations, and early skeletal bone maturation.

While risk of malignancy is somewhat low in patients with the polyostotic form of fibrous dysplasia (0.5%), patients with McCune-Albright syndrome have an approximately 4% risk of malignant degeneration. Biopsy may not always be warranted as fibrous dysplasia has a characteristic ‘ground glass’ appearance on CT scan.  Fibrous dysplasia usually presents in younger patients, with growth actually decreasing  around the age of puberty; because of this, treatment is usually conservative, with surgical intervention reserved for symptomatic patients, or those with cosmetic deformity.