Solitary fibrous tumor (SFT) is a rare tumor of the pleura, mediastinum, pericardium and other organs affecting predominantly middle-aged patients. SFT arising in the orbit is extremely rare, and its malignant form is even rarer. The authors herein describe a case of malignant solitary fibrous tumor of the orbit.

A 67-year-old male presented with a 4-month history of right proptosis. On ophthalmologic examination, 9-mm proptosis was observed in the right eye, and extraocular movements were limited in all directions of gaze. CT scan and MR imaging showed a lobulated, well-enhancing mass adjacent to the lateral rectus muscle in the superolateral retrobulbar space. Excisional biopsy through a lateral orbitotomy was performed. Histopathological examination showed proliferation of spindle cells with a fascicular pattern interspersed with bands of collagen, increased cellularity, cellular pleomorphism, hemorrhage, necrosis and high mitotic activity. Immunohistochemical staining revealed diffuse positivity for CD34, CD99, Ki67 and p53, and malignant SFT was diagnosed.

Malignant SFT should be considered in the differential diagnosis of acutely progressing unilateral proptosis.