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Clinical Commentary Odontogenic tumours in the horse E. S. HACKETT* AND G. M. BAXTER Colorado State University, Department of Clinical Sciences, 300 West Drake Road, Fort Collins, Colorado 80523, USA.

The case outlined in ‘Surgical treatment of a rostral mandibular complex odontoma in a 3-year-old horse’ (Snyder et al. 2008) has subject details typical for complex odontomas reported in this species. The horse presented with a firm, nonpainful swelling located in the mandible. Complex odontoma has more frequently been associated with the maxilla in horses, underscoring the importance for histological confirmation of tumour type. In this report, the horse’s owner declined biopsy before surgical intervention. Biopsy with tumour typing and staging prior to operative planning is currently recommended. Tumour excision without the benefit of prior histological diagnosis is an acceptable treatment, as long as the excision provides the appropriate operative margin necessary to prevent local metastasis and regrowth. Surgical biopsy paired with excision negates the need for multiple procedures and avoids delay of treatment. En bloc surgical excision resulted in failure of regrowth 6.5 months post operatively. This report adds to the collective knowledge regarding tumours of odontogenic origin in horses and reminds practitioners of this rare yet important differential that many times results in a favourable prognosis. Odontogenic tumours are primary neoplasms arising from Fig 1: Causes of firm nonpainful swelling overlying maxillary dental tissue (Pirie and Dixon 1993). These tumours are rare in cheek teeth include tumours and cysts of odontogenic origin, periapical abscessation or other oral tumours. the horse and typically manifest as slow growing firm, nonpainful, focal swellings overlying the maxilla and mandible. There is no known breed or sex predilection. Differential classified due to high degree of malignancy and greater diagnoses of firm maxillary or mandibular swellings include likelihood of metastasis (De Cock et al. 2003; Benlyazid et al. other dental diseases such as odontogenic cystic formation and 2007). Odontomas have both epithelial and mesenchymal cell periapical abscessation of the cheek teeth (Dixon et al. 2000a). elements (Brounts et al. 2004). Ameloblastic odontomas Firm swellings can also be associated with oral neoplasia not of contain dentin and enamel and are poorly differentiated (Crabill dental origin including osteoma, osteosarcoma, fibrosarcoma, and Schumacher 1998). Complex odontomas are composed of lymphosarcoma, and squamous cell sarcoma (Head and Dixon tissues found in normal teeth arranged at random whereas 1999) (Fig 1). Histological examination of surgical biopsy is compound odontomas, composed of similar tissues, have a critical to ensuring correct diagnosis. more organised structure (Debowes and Gaughan 1998). Odontogenic tumours are classified based on tissue of Tumours can localise to both maxilla and mandible, but those origin and degree of differentiation evaluated histologically. found in the mandibular symphysis are considered likely to be Major classes reported in horses include ameloblastoma, of odontogenic origin (Pirie and Dixon 1993). Odontogenic ameloblastic odontoma, ameloblastic carcinoma, odontoma, tumours typically do not metastasise to distant tissues but local and complex or compound odontoma (Table 1). invasion is common (Peter et al. 1968). If untreated, these Ameloblastomas are epithelial cell tumours and do not contain tumours can enlarge and impact adjacent structures resulting in enamel (Crabill and Schumacher 1998). Ameloblastic altered tooth position, airway diameter, sinus drainage and carcinomas are similar to ameloblastomas and are further vascular integrity. Systemic side effects have been reported including weight loss and humoural hypercalcaemia (Rosol et al. *Author to whom correspondence should be addressed. 1994; Knottenbelt and Kelly 2005). EQUINE VETERINARY EDUCATION / AE / DECember 2008 653

TABLE 1: Odontogenic tumours in the horse: Tumour type, subject details, location, treatment and outcome

No. of Mean age Tumour type cases (years) Breed Sex Location Initial treatment Outcome Report

Ameloblastoma 11 10 Pt,Pn, F,M, Mandible (11) Euthanasia (6) Medical: death Vaughan and Bartels 1968; Epithelial origin Qh,Sd,St, MC Maxilla (1) Medical (1) Surgical: Hanselka et al. 1974; Tb,Tn Surgical (4) No regrowth (3) Summers et al. 1979; w/cryo (1) Euthanasia due French et al. 1984; to regrowth (1) Jones and Brumbaugh 1991; Pirie and Dixon 1993; Gardner 1994; Rosol et al. 1994; Dixon and Head 1999; Dixon et al. 2000b Ameloblastic carcinoma 1 30 Qh F Maxilla (1) Medical (1) Medical: Euthanasia De Cock et al. 2003 Epithelial origin due to dysphagia (1) Ameloblastic odontoma 4 5 Ab,Pn, M,MC Maxilla (4) Euthanasia (3) Surgical: Peter et al. 1968; Epithelial and Qh Surgical (1) Regrowth (1) Lingard and Crawford mesenchymal 1970; Roberts et al. 1978; Bellamy et al. 1991 Complex odontoma 4 7 Mg,Qh, F,M Maxilla (3) Euthanasia (1) Surgical: Schlumberger 1953; Epithelial and Wm Surgical (2) No regrowth (1) Dillehay and Schoeb 1986; mesenchymal Not reported (1) Dubielzig et al. 1986; Heufelder et al. 1994 Compound odontoma 2 0.8 Qh F,M Mandible (1) Surgical (2) Surgical: Brounts et al. 2004 Epithelial and Maxilla (1) w/oesophagostomy No regrowth (1) mesenchymal w/transfusion Regrowth and surgery repeated (1)

Breed abbreviations: Ab = Arabian, Mg = Morgan, Pt = Paint, Pn = Pony, Qh = Quarter Horse, Sd = Saddlebred, St = Standardbred, Tb = Thoroughbred, Tn = Tennessee Walking Horse, Wm = Warmblood.

Treatment of odontogenic tumours is complete surgical noticed that the tooth root looked abnormal and submitted excision. Prognosis following complete excision is good, the tooth for histopathological examination. Histopathology although this may be difficult in the presence of severe local confirmed ameloblastoma. Ameloblastomas occur more invasion. Tumours are best treated when small in size, although commonly in the mandible and while the tumour is odontogenic tumours rarely present in the subclinical phase. In radiolucent, expansion of the bony cortex and periosteal the mandible, partial hemimandibulectomy may be required to reaction may result in a surrounding thin shell of bone (Fig 2). allow wide excisional margins. In the maxilla, sinus entry is The mass involved the tooth roots of both 407 and 406. This common and staged surgeries may be necessary to remove the horse was treated with a combination of wide surgical excision tumour completely without undue stress to the patient. and cryosurgery similar to previous reports (French et al. 1984). Radiographic and computed tomography imaging may be Many oral tumours involve either a loose tooth or a mass that useful in operative planning to ensure complete excision is has developed after tooth removal. For this reason, we both possible and likely. Surgeons should plan for operative blood loss and consider oesophagostomy to allow adequate nutrition in the post operative period. If radical excision is elected, this procedure may result in poor cosmetic outcome. Surgical excision can be combined with other therapies to increase success and minimise post operative regrowth. Combination therapies may include surgical incision with cryosurgery, local or systemic chemotherapy, or radiotherapy. Radiotherapy is recommended in man if wide en bloc resection is not possible (Mendenhall et al. 2007). Invasive odontogenic tumours respond poorly to medical management and palliative surgical treatment. A horse recently presented to Colorado State University for surgical treatment of ameloblastoma with a history of dental abnormality. The horse was an 18-year-old Thoroughbred gelding with a 2 month history of firm nonpainful swelling overlying the right mandible. The 3rd premolar (407) was extracted by the referring veterinarian 10 days prior to Fig 2: Radiographic appearance of a mandibular ameloblastoma presentation. At the time of extraction, the veterinarian in an 18-year-old Thoroughbred gelding. 654 EQUINE VETERINARY EDUCATION / AE / DECember 2008

recommend that all teeth should be examined for Gardner, D.G. (1994) Ameloblastomas in the horse: a critical review and abnormalities at the time of removal. Differentials for loose report of an additional example. J. Oral Pathol. Med. 23, 41-44. teeth, especially in a young horse, should include odontogenic Hanselka, D.V., Roberts, R.E. and Thompson, R.B. (1974) tumour. In addition, a mass that develops at a site of tooth Adamantinoma of the equine mandible, a case report. Vet. Med. extraction is highly suggestive of a tooth origin tumour. small anim. Clin. 69, 157-160. Head, K.W. and Dixon, P.M. (1999) Equine nasal and paranasal sinus References tumours. Part 1: review of the literature and tumour classification. Vet. J. 157, 261-278.

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