Odontogenic tumors (OTs) arise from those embryonic tissues destined to develop into teeth and associated structures. They originate from remnants of ondontogenic epithelium (rests of Malassez and rests of Serres located within the periodontal ligament stroma and gingiva) in the tooth-bearing areas of the mandible and maxilla. OTs are considered neoplasms that range from benign to invasive in biologic behavior.

Historically, OTs have been categorized as being epulides or an epulis. These terms have no specific histopathologic connotation and are clinical terms used only to describe any localized swelling on the gingiva. Many veterinarians and pathologists commonly use both terms incorrectly today. The term epulis should not be used in defining a diagnosis. Confusion exists as several different pathologic entities are categorized as epulides, and they are often mistakenly considered benign and inconsequential.

Many epulides are actually odontogenic tumors (or cysts) and definitive diagnosis requires a biopsy and histopathologic assessment by a pathologist with formal training in the pathology of odontogenic tumors and cysts. Treatment planning should occur only after a definitive diagnosis has been established.   

Ameloblastomas

Ameloblastomas are non-inductive, soft tissue tumors that, in spite of their benign appearance, should be considered locally invasive neoplasms that do not metastasize. In dogs, two subtypes may be encountered: 1) central (intraosseous) ameloblastoma and 2) peripheral (acanthomatous) ameloblastoma. The central (intraosseous) ameloblastoma often presents as a facial swelling with bony distortion, however, the main complaint may be an obvious tooth displacement or malocclusion. Because they are usually painless, patients are usually asymptomatic. On X-rays, it is common to see bone lysis with unilocular or multilocular cystic structures around tooth roots. Peripheral (acanthomatous) ameloblastomas (PAs) are often noted as gingival enlargements discovered as incidental findings during routine dental screening exams. They are considered to be benign tumors with invasive properties into surrounding bone. The infiltration of bone distinguishes the PA from other ameloblastomas. On X-rays, patterns are characterized by bony infiltration, alveolar bone resorption and tooth displacement. The preferred treatment modality is surgical excision with appropriate margins. Recurrence is common and may be disastrous. Incomplete surgical removal, before a diagnosis is established, is the most common reason for recurrence. 

Ameloblastomas are considered rare in cats and those that do exist are sometimes confused with other more common types of ondontogenic tumors in this species.

Peripheral Odontogenic Fibroma

Many of the gingival enlargements previously described as fibromatous and ossifying epulides have been reclassified as peripheral odontogenic fibromas (POFs). POFs are slowly growing, benign neoplasms that are common in dogs and may be seen in cats. Histopathology is necessary to definitively diagnose POFs. These tumors contain proliferative fibroblastic connective tissue as well as proliferative ondontogenic epithelium associated with foci of a variety of hard tissues, suggesting induction. Complete surgical excision, with appropriate margins, is usually curative.

Amyloid Producing Ondontogenic Tumor

This OT type is rare in dogs and cats and presents as a gingival swelling on either jaw in patients between 8 to 13 years of age. These tumors are locally invasive, however, not metastatic. On radiographs, the lesions are often cystic and may resemble an ameloblastoma. The presence of amyloid (Congo red stain), which tends to calcify, is a characteristic histopathological finding of this tumor. Surgical excision, with appropriate margins, is necessary to completely remove this invasive neoplasm.

Feline Inductive Odontogenic Tumor

This tumor is unique to young cats (usually 8 to 18 months). Feline inductive odontogenic tumors (FIOTs) are raised, submucosal, soft tissue masses most commonly located on the rostral maxilla. This tumor is considered locally invasive, however, it does not metastasize. Complete surgical excision is required. 

Odontomas

Odontomas are benign inductive tumors diagnosed in young dogs and cats of 6 to 18 months. Some consider odontomas as actually being malformations and not neoplasms. These are well-differentiated, non-invasive tumors characterized by the simultaneous occurrence of soft and hard dental tissues (enamel, dentine, cementum, dental papilla). An odontoma in which rudimentary tooth-like structures (denticles) are present is referred to as a compound odontoma. An odontoma in which the arrangement of dental tissues is haphazard, is called a complex odontoma.