Case Report Surgical treatment of a mandibular ameloblastic carcinoma with metastases to the mandibular lymph nodes in a pony A. M. Derham* † , A. Kushnir † , S. Hoey † , H. Jahns † , A. R. Hollis ‡ and C. M. Kearney † † University College Dublin, University Veterinary Hospital, UCD, Belfield, Dublin 4, Ireland; and ‡ Centre for Equine Studies, Animal Health Trust, Kentford, Newmarket, Suffolk, UK *Corresponding author email: ann.derham@ucdconnect.ie Keywords: horse; cancer; jaw; mandibulectomy; tumour Summary A 4-year-old Connemara filly was presented with a rapidly growing oral mass on the right rostrolateral mandible and a right mandibular lymphadenopathy. Radiographs of the rostral mandible revealed a lytic, infiltrative mass consistent with soft tissue and mineralised material, and displacement of tooth 403. A biopsy showed characteristic histopathological features of an ameloblastic carcinoma. Subsequent FNA of the right abnormally firm and enlarged mandibular lymph node confirmed the metastatic spread. Computed tomography of the head was performed for surgical planning, including sentinel lymph node mapping to rule out other lymph node involvement. An aggressive lesion, consistent with soft tissue of the rostral right mandible was identified with intralesional contrast injection showing drainage of contrast within the right mandibular lymph nodes (Sentinel node). Based on the infiltrative and destructive nature reported in ameloblastic carcinoma in humans, a rostral mandibulectomy was performed, along with complete mandibular lymphadenectomy. The horse recovered uneventfully from surgery. In the early post-operative period marked lymphoedema of the ventral mandibular region was observed, which resolved 4–5 days post-operatively. Optimal cosmesis was maintained post-operatively, and no recurrence has been observed to date – 12 months post-surgery. Introduction Ameloblastoma is a rare, benign but locally aggressive odontogenic epithelial tumour that is typically non-painful and slow growing. It arises from epithelial remnants of the developing tooth root sheath and dental lamina (Munday et al. 2017). In humans, ameloblastomas represent approximately 1% of all tumours in the jaw (Avon et al. 2003) with more than 80% of cases arising from the mandible (Gorlin et al. 1963). In the dog and horse, these are also rare but are still the most common odontogenic tumours (Pirie and Dixon 1993; Gardner 2002). In the horse, as in humans, the most common site of ameloblastoma is also the mandible (Gardner 1994) with most tumours involving the cheek teeth of the caudal mandible, and ameloblastoma is less commonly associated with the mandibular symphysis and rostral mandible (Kutzler et al. 2007). Malignant variants of ameloblastoma are extremely rare in domestic animal species and have only been reported in three dogs (Jim enez et al. 2007; Hatai et al. 2013; Aydogan et al. 2014) and two horses (De Cock et al. 2003; Reardon et al. 2017) to date. None of these cases were associated with metastasis. Histopathological features that differentiate ameloblastic carcinomas from the benign counterparts are marked cellular atypia, high mitotic rate and multifocal areas of necrosis (Munday et al. 2017). This case report describes the diagnostic work-up and surgical treatment of an ameloblastic carcinoma of the rostral right mandible with local metastasis to the right mandibular lymph node in a 4-year-old Connemara filly. History, clinical findings, and diagnosis A 4-year-old, Connemara filly, was presented to University College Dublin Veterinary Hospital for evaluation of a mass involving the right mandible that had rapidly progressed in size over the previous 7 days. On presentation, the pony was in good body condition with vital clinical parameters within normal limits. The right mandibular lymph node was markedly enlarged and firm on palpation. Oral examination revealed a large (approx. 4.5 cm L 9 3 cm H 9 2 cm W) firm, raised, ulcerated mass of the right rostral mandible involving the bone and gingiva. The mass expanded caudally within the gingival stroma; however, rostrally, there was no clear demarcation between the mass and mandibular bone. Axial compression of the deciduous third incisor was present, and eruption of the second incisor was reduced compared to the contralateral side (Fig 1). Radiographs of the rostral mandible revealed an expansile infiltrative lesion composed of both soft tissue and mineral opacity, with associated moth-eaten lysis of the right rostral mandible, and displacement of tooth 403. A deep (2 cm) incisional biopsy was taken from the affected gingiva on the rostrolateral aspect of the mandible. Histopathological examination revealed large polygonal cells arranged in variable-sized islands and interconnecting trabecules supported by a dense fibrous stroma infiltrating the oral submucosa (Fig 2). The cells had distinct borders and only occasionally prominent intercellular bridging (odentogenic epithelial cells). The nuclei were centrally located with fine stippled chromatin and prominent small one to two nucleoli. There was moderate anisocytosis and anisocaryosis and 12 mitotic figures were observed per 10,4009 field. Multifocally, there was central necrosis of the larger islands and single cell necrosis. Some neoplastic islands contained bone. The diagnosis of carcinoma was based on the lack of peripheral basilar epithelial cells, high mitotic rate, moderate anisocaryosis and central necrosis of some islands. The presence of odontogenic epithelium in the tumour was confirmed by immunohistochemical (IHC) staining using pan- cytokeratin and vimentin antibodies. The neoplastic cells were positive for both markers (Supplementary Item 1). © 2019 EVJ Ltd 1 EQUINE VETERINARY EDUCATION Equine vet. Educ. (2019)  () - doi: 10.1111/eve.13130