Bony Lesions of the Orbit
Fibrous dysplasia of bone may involve one bone (monostotic) or more than one (polyostotic). When the orbit is affected, the condition is usually monostotic. However, the tumor may cross suture lines to involve multiple orbital bones. Patients often present during the first 3 decades of life. Narrowing of the optic canal and lacrimal drainage system can occur. Plain radiographic studies show a ground-glass appearance with lytic foci. Cysts that contain fluid also appear. As a result of arrest in the maturation of bone, trabeculae are composed not of lamellar bone but of woven bone with a fibrous stroma that is highly vascularized. Histologically, the bony trabeculae often have a C-shaped appearance (Fig 14-18).
Fibro-osseous dysplasia (juvenile ossifying fibroma), a variant of fibrous dysplasia, is characterized histologically by spicules of bone rimmed by osteoblasts (Fig 14-19). At low magnification, ossifying fibroma may be confused with a psammomatous meningioma.
Osseous and cartilaginous tumors of the orbit are rare; of these, osteoma is the most common. It is composed of mature bone and is slow growing and well circumscribed. Most commonly, an osteoma arises from the frontal sinus.
Other primary osseous and cartilaginous tumors that have been reported rarely in the orbit include both benign entities (eg, osteoblastoma) and malignant sarcomas (eg, Ewing sarcoma, osteogenic sarcoma).
Excerpted from BCSC 2020-2021 series: Section 4 - Ophthalmic Pathology and Intraocular Tumors. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.