Low-Grade Malignant Fibrous Histiocytoma Originating From the Medial Rectus Muscle

Fibrous histiocytomas (FHs) involving the orbit are uncommon. To our knowledge, here the authors report the first patient with an orbital low-grade malignant FH originating from the medial rectus muscle.

A clinical report relating clinical features as well as imaging and histopathologic findings is presented.

A 26-year-old female presented with a left eye proptosis that had been present for 4 months. A small left exotropia was observed and ocular motility evaluation revealed a -3 adduction and -1 abduction of the left eye. B-mode ultrasonic scans demonstrated a 2.5 × 2.0 cm mass with medium internal echo and a clear boundary located in the nasal region of the left orbit. Magnetic resonance imaging (MRI) showed a 2.8 × 2.0 cm oval extraconal orbital mass, with isointensity on T1-weighted MRI and hyperintensity on T2-weighted MRI. An anterior orbitotomy was performed and the tumor was completely removed. The mass originated from the medial rectus muscle. Histopathologic evaluation confirmed a low-grade malignant FH. After surgery, the left exotropia was no longer present. Ocular motility revealed a -1 adduction and -1 abduction of the left eye. No recurrence was evident at 1 year after surgery.

This rare patient demonstrates that FH may originate from the medial rectus muscle. With careful surgical dissection, a near normal function of the involved muscle can be restored.