Abstract
PURPOSE: We present a rare case of intramuscular alveolar soft part sarcoma (ASPS) of the lateral rectus (LR) muscle and the surgical technique used to maintain orthotropia after complete resection of the mass. OBSERVATIONS: A 5-year-old boy presented with progressive proptosis of the left eye due to an orbital tumor. The patient was previously diagnosed with ASPS of the left LR muscle from an incisional biopsy, and the tumor size increased despite 5 cycles of chemotherapy prior to presenting to our center. Magnetic resonance imaging showed a 28x19x15mm-sized contrast-enhancing intramuscular mass of the left LR muscle, and there was no evidence of nodal or distant metastasis. The mass was excised en bloc, along with the insertion and the posterior normal part of LR muscle. To maintain proper eye alignment after resecting LR muscle, a 4-0 Prolene® hang-back suture was placed between the scleral insertion and the periorbita of the posterior orbit and the left medial rectus muscle was injected with botulinum toxin. During the follow-up of 51 months after surgery, the patient had no evidence of recurrence or metastasis and remained orthotropic in primary gaze, with a good cosmetic result. CONCLUSIONS AND IMPORTANCE: ASPS of extraocular muscles is a rare tumor occurring mainly in children and young adults, and treatment may cause significant sequelae such as orbital exenteration, radiation-induced complications, and large-angle strabismus. Complete resection of tumor including the extraocular muscle is essential for treatment, and a subsequent reconstruction using a hang-back suture technique is useful to achieve proper eye alignment as well as a good cosmetic outcome.