To report a rare case of primary orbital synovial sarcoma.

A 55-year-old male visited our clinic with a 2-month history of decreased visual field in the left eye. The best corrected visual acuity of the left eye was 1.0, and 2.5 mm of proptosis was observed. There was slight limitations in motility and a mild visual field defect in the inferotemporal quadrant of the left eye. Funduscopic examination of the left eye revealed papilledema. Orbital magnetic resonance imaging (MRI) demonstrated the presence of an orbital mass that extended inferotemporally into the left orbit. On magnetic resonance imaging, a well-demarcated and 2.7 × 2.0 × 2.2 cm sized tumor was identified with intermediate signal intensities on T1/T2-weighted scans with slight homogenous enhancement. We approached the orbital mass using a lateral conjunctival incision with left canthotomy and cantholysis and performed partial surgical resection of the mass because the mass encircled the optic nerve. The mass was diagnosed as synovial sarcoma based on the findings of both histopathology and immunohistochemical staining. Postoperatively, the patient underwent radiation therapy to the rest of the tumor site.

Synovial sarcoma should be considered in the differential diagnosis of an orbital mass lesion encircling the optic nerve. Radiation therapy is useful to treat completely unresectable primary synovial sarcoma.