Surgical management of an odontogenic collision tumour and oronasal fistula in a 14‐year‐old Warmblood mare

Summary Tumours of the equine nasal passages and paranasal sinuses are uncommon, and collision tumours, two independent neoplasms arising adjacently, are exceptionally rare. Cementomas and osteolipomas are both benign, slow‐growing tumour types, but are only sparsely described in horses. The aim of this case report was to describe the diagnostic work‐up, surgical management and outcome of a horse with a rare collision tumour consisting of a cementoma and an osteolipoma associated with maxillary cheek tooth 106. A 14‐year‐old Warmblood mare was evaluated for right‐sided nasal discharge, stridor and maxillary swelling. Diagnostic investigations included clinical examination, oral examination, standing CT and histopathology. The mass was removed under standing sedation via a combined rhinotomy and oral approach, including extraction of tooth 106. Postoperative complications, including an oronasal fistula, were treated using a combination of a modified levator labii superioris muscle transposition technique. Clinical until 12 months and CT follow‐up at 6 months were performed. CT revealed a large expansile mass involving tooth 106 with mixed mineralised and soft‐tissue components. Histopathology identified two distinct tumour types: a dorsally located osteolipoma and a ventral cementoma connected to the tooth root, consistent with a collision tumour. Surgical excision was successful, with no evidence of tumour recurrence at 6‐ and 12‐month follow‐up. An oronasal fistula developed postoperatively but resolved after staged surgical management. Limitations were that this is a single case report and long‐term (>1 year) recurrence risk remains unknown. Collision osteolipoma–cementoma tumours, although extremely rare, should be considered in horses with mixed‐attenuation sinonasal masses. CT and histopathology are essential for accurate diagnosis. Complete surgical excision is feasible, but postoperative fistula formation is a significant risk for odontogenic tumours requiring tailored management.