Odontogenic Tumours

Introduction

Odontogenic tumors (OT) arise from remnants of the embryonic tissues destined to develop into teeth and associated structures. They originate from remnants of odontogenic epithelium (rests of Malassez and rests of Serres located within the periodontal ligament stroma and gingiva, respectively), odontogenic mesenchyme, or a combination of the cellular elements that comprise the tooth-forming apparatus. Clinical behavior ranges from hamartoma-like proliferations to benign and invasive neoplasms.

Histologically, OT may mimic some stage of the developing tooth bud. There may be soft tissues of the dental organ or dental pulp, or they may contain hard tissue elements of enamel, dentin, and/or cementum. With the exception of the canine peripheral odontogenic fibroma (formerly fibromatous epulis) and canine acanthomatous ameloblastoma (formerly acanthomatous epulis), OT are considered uncommon in animals. One classification utilises the tissue of origin as the basis for distinguishing OT. The three resulting groups are:

• Those consisting of odontogenic epithelium without odontogenic mesenchyme;
• Tumors arising from odontogenic epithelium and mesenchymal cells, with or without dental hard tissue formation;
• Those consisting primarily of mesenchyme, with or without odontogenic epithelium.

Epithelial Tumours

Ameloblastoma

Ameloblastoma is an epithelial tumor that originates from remnants of the dental organ (rests of Malessez), dental lamina, cells of epithelial origin lining odontogenic cysts or possibly from basal epithelial cells of the oral mucosa. It is a non-inductive (no effect on surrounding mesenchyme) tumor described in many domestic species, and occurs in the tooth-bearing areas of the mandible, maxilla and incisive bone. Because there is no production of enamel or dentin, it remains a soft tissue tumor and is considered the least differentiated of the epithelial OT. In spite of a benign appearance histologically in animals, ameloblastoma typically is a locally invasive, slow growing neoplasm that does not metastasise.

• Central (or intraosseous) ameloblastoma - is a relatively uncommon odontogenic tumor in dogs and is rarely seen in cats. It usually manifests as a gross swelling with distortion of bone often resulting in tooth displacement or malocclusion. It is usually painless, with slow expansion and patients may be asymptomatic. Central ameloblastoma most often appears radiographically as an osteolytic, unilocular or multilocular cystic lesion around tooth roots, with well-defined, sclerotic margins. Jaw expansion usually occurs. Wide surgical excision (mandibulectomy or maxillectomy) is the treatment of choice and considered curative.

• Canine acanthomatous ameloblastoma - the second common histologic variant of ameloblastoma found in dogs is the canine acanthomatous ameloblastoma (CAA). This used to be called acanthomatous epulis and was distinguished biologically and histologically from other types of epulides by their tendency to infiltrate cancellous bone. The acanthomatous epulis was later recognized as a type of ameloblastoma and termed a peripheral ameloblastoma. The term peripheral ameloblastoma was later replaced by canine acanthomatous ameloblastoma, to differentiate it from the peripheral ameloblastoma in humans, which is a non-invasive tumor type.

• Acanthomatous ameloblastoma - also arises from remnants of odontogenic epithelium located in the gingiva (rests of Serres) in the tooth-bearing areas of the jaws. Although CAA is generally accepted as arising from gingival epithelium, it may also arise intraosseously and then break out of bone. The infiltration of bone distinguishes the CAA from central ameloblastoma. The radiographic pattern is dominated by bony infiltration, alveolar bone resorption and tooth displacement. Because acanthomatous ameloblastoma is locally aggressive and invades bone, bloc excision of the tumor with at least 1 cm of normal-appearing tissue is required to be curative. If the tumor is completely excised, the prognosis is excellent.

Amyloid-Producing Odontogenic Tumor

This OT type is rare in dogs and cats and presents as an ‘epulis’ on either jaw in patients between 8 to 13 years old. It has been previously referred to in veterinary literature as a calcifying epithelial odontogenic tumor. Amyloid-producing odontogenic tumor (APOT) appears as a gingival enlargement which grows by expansion. It is locally invasive but not does not metastasize. It often has a cystic appearance on radiographs and complete excision is considered curative.

Feline Inductive Odontogenic Tumor

This tumor type is unique to young cats (usually 8 to 18 months). It was originally described as inductive fibroameloblastoma. Feline inductive odontogenic tumor (FIOT) is a raised submucosal soft tissue mass typically located on the rostral maxilla. The tumor may be locally invasive, but metastasis has not been recorded. Wide surgical excision is the treatment of choice, and considered curative, with local recurrence expected after incomplete excision.

Mesenchymal Tumours

Peripheral Odontogenic Fibroma

Many of the tumors previously described as fibromatous and ossifying epulides have been reclassified as peripheral odontogenic fibroma (POF). Peripheral odontogenic fibroma is a slow growing, benign neoplasm, common in the dog and uncommon in the cat. The surface epithelium appears normal, and radiographic features vary according to the presence and amount of mineralized products. POF is characterized by a low-grade neoplastic proliferation of fibroblastic connective tissue of variable cellularity in which a variety of bone, osteoid, dentinoid (dentin-like) or even cementum-like material is present. Definitive treatment typically requires en bloc resection of the mass and underlying bone, and is considered curative if excised completely.

Mixed (Mesenchymal and Epithelial) Tumours

Odontoma

Odontomas are benign inductive tumors diagnosed in young dogs and cats, generally appearing at 6 to 18 months of age. Because odontoma contains well-differentiated cells, it is characterized by the simultaneous occurrence of a composite of soft and hard dental tissues (enamel, dentin, cementum, dental papilla) of both epithelial and mesenchymal origin. The dental tissues in odontomas may or may not exhibit a normal relation to one another. An odontoma in which rudimentary tooth-like structures are present indicates advanced cellular differentiation and is referred to as a compound odontoma. An odontoma in which the conglomerate of dental tissues is disorderly, bearing no resemblance to a tooth, is called a complex odontoma. The radiological appearance is typical: either a sharply defined mass of calcified material surrounded by a narrow radiolucent band (complex odontoma), or a variable number of tooth-like structures (compound odontoma) are present. Because odontomas are not true neoplasms, marginal excision and/or curettage to remove the abnormal material may be curative.

Reactive Lesions

Reactive lesions are non-neoplastic gingival enlargements that are commonly grouped under the previously noted nondescript classification of epulides. The term ‘epulis’ (singular) has no specific histopathologic connotation and is a clinical designation for any localized, exophytic swelling on the gingiva. Reactive lesions include focal fibrous hyperplasia, pyogenic granuloma, peripheral giant cell granuloma and reactive exostosis. Marginal excision of these lesions, without inclusion of adjacent normal tissue, is generally sufficient, as local recurrence is uncommon.