A 10-year-old Hanoverian mare was evaluated for a right buccal swelling that recurred 3 months following surgical resection. Ultrasonographic examination showed a broadly pedunculated subcutaneous mass at the level of 106-109 and 406-409 cheek teeth associated with an erosive mucosal lesion on the inside of the cheek. Histological examination of a biopsy specimen revealed a well-demarcated, malignant, dermal schwannoma. Following subcutaneous placement of platinum coated Ir192 wires under general anaesthesia, low-dose radiation of 5 gray per day was delivered for 14 days. Short-term complications included loss of patency of the right nasolacrimal duct, erythema, dermatitis, leukotrichia and left-sided deviation of the muzzle. Ten months later, there has been no tumour recurrence. Findings suggest that the use of interstitial brachytherapy should be considered for a malignant, dermal schwannoma that has recurred or is not amenable to surgery.