Oral Masses: Cystic, Benign & Malignant
ACVIM 2008
Jennifer E. Rawlinson, DVM, DAVDC; John R. Lewis, VMD, FAVD, DAVDC
Ithaca, NY, USA; Philadelphia, PA, USA

Cystic Masses

Swellings either within the oral cavity or associated with surrounding maxillofacial structures warrant close examination as these swellings could be inflammatory, neoplastic, cystic or developmental in origin. With sedated oral examination, intraoral radiography, and biopsy a diagnosis if not at least a category of disease can be reached. As treatment planning and prognosis for oral masses varies dramatically, narrowing of the diagnosis to at least a general characterization will be helpful.

Oral masses that are characterized as cystic in nature include abscesses, true cysts, and odontomas. Odontomas will be discussed under benign masses as they are really odontogenic tumors. They mistakenly fall into the cystic category on initial diagnoses periodically as significant amounts of fluid can accumulate around unerupted dental tissue. Abscesses are a localized collection of infected purulent fluid formed by the disintegration of tissue. Most abscesses are formed by invasion of tissues by bacteria, but some are caused by fungi, protozoa or even helminthes, and some are sterile. Common oral causes for formation of abscesses include fractured teeth with pulp exposure, non-vital intact teeth, feline tooth resorption, foreign body, retained tooth roots (either traumatic or post-extraction), bony sequestration, osteomyelitis and severe periodontal disease. Treatment will vary depending on the etiology, abscess location and size, and radiographic findings.

A true cyst is a pathologic cavity filled with fluid, lined by epithelium, and surrounded by a definitive connective tissue wall. The cystic fluid is either secreted by the cells lining the cavity or derives from the surrounding tissue fluid.1 Cysts usually form within the jaw of the animal because most cysts originate from the numerous rests of odontogenic epithelium that remains after tooth formation. Clinical presentation includes a firm or fluctuant nonpainful swelling that appears radiolucent on intraoral images. These masses though benign in nature can be very destructive due to their expansile nature, and large regions of normal anatomy can be destroyed prior to their discovery. Unerupted teeth and/or association with dental structures is common though not necessary. The two most common forms of dental cysts seen in domestic small animals are radicular and dentigerous cysts.

Radicular cysts are very rare in the veterinary field. Radicular cysts most likely result when inflammatory products from non-vital teeth stimulate epithelial cell rests within the periodontal ligament to proliferate. This creates a fluid-filled swelling associated with the root of one or more fully erupted teeth. A dentigerous cyst is a cyst that forms around the crown of an unerupted tooth. Fluid accumulates in the layers of reduced enamel epithelium and/or between the epithelium and the unerupted tooth crown.1 Common unerupted teeth in the dog are the mandibular first premolars and the mandibular third molars. It is not unusual to find dentigerous cysts associated with these unerupted teeth on routine survey intraoral radiographs. Therefore, intraoral radiography is recommended for all "missing" teeth. In veterinary medicine, the most common treatment for both radicular and dentigerous cysts is extraction of the associated tooth/teeth with complete debridement of the cystic region. A radiographic recheck is usually recommended at 6 months postoperatively to ensure complete resolution of the cyst.2

Benign Masses

Some oral neoplasms of dogs and cats may be locally invasive but do not metastasize to distant sites, or may not even be locally invasive. Benign oral tumors rarely present with oral bleeding or halitosis unless they are large enough to be traumatized by opposing teeth upon closure of the mouth. Bleeding is a common complaint in dogs with a large acanthomatous ameloblastoma. Benign oral masses often present as a focal swelling of gingiva and/or alveolar mucosa, and are often circumscribed and rarely ulcerated. Benign tumors that are locally invasive (e.g., canine acanthomatous ameloblastoma) may cause disfigurement from invasion of the maxilla or mandible and displaced but often firmly-seated teeth. Mandibular lymph nodes often palpate within normal limits. Appetite and activity level are usually unaffected. Non-neoplastic differential diagnoses include: 1) normal anatomy (e.g., incisive papilla palatal to maxillary incisors; lingual molar salivary gland in cats); 2) craniomandibular osteopathy (CMO, most commonly seen in West Highland White, Scottish, and Cairn Terriers); mandibular swellings associated with CMO are often bilateral; 3) osteomyelitis/bone sequestrum, usually seen in the incisive bone or bilaterally in the caudal mandible or maxilla; often appears as bony swelling with gingival recession, erosion, exposed bone and fetid odor. Cocker Spaniels and Dachshunds may be overrepresented; 4) dentigerous cyst (arising in area of a tooth that has not yet erupted)--see above; 5) eosinophilic granuloma (occurring in dogs and cats on tongue, lips, and soft palate: breed predilection in Cavalier King Charles Spaniels and Arctic breeds).3

Epulis (plural=epulides) is a general term referring to a gingival mass of any type. This term has been adapted in veterinary nomenclature to refer to tumors arising from epithelial remnants within the periodontal ligament. Benign oral tumors can occur at any age, but middle-aged and geriatric pets are overrepresented. Odontogenic tumors (those arising from tooth-forming tissue) can occur at any age, but tumors in young pets are more likely to be of odontogenic origin. Epulides are most common in brachycephalic dogs such as Boxers and Pugs. These breeds are also prone to development of gingival hyperplasia. Gingival hyperplasia can present as a focal gingival mass that mimics epulides. The correct clinical description for this condition is gingival enlargement, since hyperplasia should be reserved for histologically confirmed cases. Odontogenic tumors of periodontal ligament origin are by far the most common benign oral neoplasms in dogs. Historically, epulides have been classified as fibromatous, ossifying, and acanthomatous. Fibromatous and ossifying epulides are minimally invasive. Ossifying epulides may contain dentin-like or cementum-like tissue in a fibrous tissue swelling. Acanthomatous epulis is the most locally invasive. The nomenclature regarding epulides in dogs has changed. Fibromatous and ossifying epulides have been placed under the same heading of peripheral odontogenic fibromas. Based on histologic appearance, acanthomatous epulis is now referred to as canine acanthomatous ameloblastoma.4

A list benign oral tumors that are less common than odontogenic tumors of periodontal origin include: other odontogenic tumors (ameloblastoma, odontoma), papilloma, osteoma, plasmacytoma, hemangioma. Ameloblastoma can be characterized as central (intraosseous) or peripheral (extraosseous ameloblastoma). Both can be locally invasive, but central ameloblastoma often exhibits cystic bony changes. Odontoma is not a true tumor. It is considered to be a hamartoma, an accumulation of normal epithelial and mesenchymal odontogenic cells that are arranged in an abnormal manner but allowing for induction of dental hard tissues. A compound odontoma has hard tissues that are produced in a relatively organized manner, producing tooth-like structures (denticles). A complex odontoma produces dental hard tissue that bears no resemblance to a tooth. Oral papillomas usually occur in dogs <2 years old. Detection of canine oral papillomavirus-DNA in canine oral squamous cell carcinomas suggests that a progression of viral papillomas into carcinomas may occur.5 Osteomas often present as slow growing masses on the hard palate.

Oral tumors in cats are rarely benign. One benign tumor is the feline inductive odontogenic tumor (FIOT), also called inductive fibroameloblastoma, occurs most commonly in the rostral maxilla of cats <2 years of age and may be locally invasive but has not been reported to metastasize. Amyloid-producing odontogenic tumor (APOT), previously referred to as calcifying epithelial odontogenic tumor (CEOT), may be locally invasive in both dogs and cats but has not been reported to metastasize.

Malignant Masses

Malignant masses are neoplasms that have the potential to be locally invasive and can metastasize to distant sites. Although malignant oral tumors can occur at any age, middle aged and geriatric pets are over-represented.

Canine papillary squamous cell carcinoma is an exception in that it occurs most commonly in dogs less than one year of age. The most common canine oral tumor is malignant melanoma (melanosarcoma), seen most commonly in dogs greater than 10 years of age. Squamous cell carcinoma, fibrosarcoma, osteosarcoma, osteochondrosarcoma, mast cell tumor and hemangiosarcoma are some other more common malignant oral tumors in dogs. The most common feline oral tumor is squamous cell carcinoma, representing approximately 70% of oral tumors in cats.6 Average age of onset is ten years, but cats as young as five months have been affected.7 Other malignant feline oral tumors include fibrosarcoma, osteosarcoma and melanoma, in order of frequency.

Risk factors for most oral tumors have not been determined. Breeds with pigmented oral mucosa may be predisposed to malignant melanoma. One study looked at risk factors in cats with squamous cell carcinoma: exposure to flea collars was associated with a 5x increased risk. High intake of canned cat food was associated with a 3x increased risk. Eating canned tuna was associated with a 5x increased risk.8 Sexual activity is a risk factor for canine transmissible venereal tumor which can manifest as a primary or metastatic tumor on the lips, buccal mucosa and tonsils.

History and presenting complaint of malignant oral tumors may include disfiguring swelling of the mandible or maxilla, oral bleeding or halitosis. Dysphagia may be noted if the mass is large enough to affect function. Physical examination findings depend on tumor type. Melanomas may be pigmented, pink, or red and are often lobulated, friable and partially necrotic. Fibrosarcomas are often smooth, firm and may cause generalized disfigurement but rarely bleed spontaneously. Squamous cell carcinoma is firm, pink, locally invasive into underlying bone, and may be proliferative, ulcerative, or a combination. Osteosarcoma can manifest as a hard diffuse swelling of the maxilla or mandible, but also includes a fleshy intraoral proliferation. Osteochondrosarcoma, also referred to as multilobular tumor of bone, is locally invasive but slow to metastasize, and sometimes has a characteristic radiographic "popcorn ball" appearance. Appetite and activity level are often unaffected until tumors reach a large size.

Differential diagnoses that may mimic malignant tumors include gingival hyperplasia, granulation tissue, swelling due to foreign body, tooth root abscess, normal anatomy (incisive papilla, lingual molar gland in cats), benign tumors, osteomyelitis/bone sequestrum, CMO, dentigerous cyst, cheek chewing lesions and sublingual chewing lesions. Eosinophilic granulomas may also mimic aggressive oral tumors, therefore an incisional biopsy is an important. Removal of a circumscribed mass to the normal level of the surrounding gingiva will often serve as an adequate biopsy, but will not prevent local recurrence. Incisional biopsies of large masses are warranted to provide information prior to performing radical maxillectomy or mandibulectomy. Aspiration of mandibular lymph nodes is warranted for staging of any patient with an oral tumor. Based on results of incisional biopsy, surgery or radiation therapy may be treatment options. Chemotherapy may be used as an adjunctive therapy but is rarely effective by itself against most oral tumors.

Piroxicam at 0.3 mg/kg once daily in dogs may slow the progression of carcinomas. Possible complications of surgery include recurrence of primary tumor, intraoperative or postoperative bleeding, dehiscence, and iatrogenic ranula or cervical mucocele.

In previous studies, median survival time after surgical excision has been approximately 12 months depending on tumor type.9 However, when clean surgical margins are obtained in the absence of micrometastasis, patients can be cured. Prognosis with squamous cell carcinoma is good if it is found while still surgically resectable.

Pearls

  • Melanomas larger than 2 centimeters in diameter have a high incidence of micrometastasis at the time of diagnosis...

  • Tonsillar squamous cell carcinoma carries a grave prognosis: 98% of these tumors metastasize early to regional lymph nodes...

  • When submitting excisional biopsies, ask the pathologist to evaluate margins for presence of neoplastic cells....

  • Clean margins do not rule out the possibility of recurrence, but animals with clean histological margins have a better long-term prognosis...

  • Dental radiography is invaluable in providing information about oral tumors...

  • Benign tumors will often displace teeth, whereas more aggressive tumors will cause resorption of roots, which may manifest as missing or very mobile tooth crowns...

  • Benign tumors may have a smooth layer of reactive bone surrounding the tumor whereas a malignant tumor often exhibits destruction of cortical bone with a classic "sunburst" appearance caused by periosteal reaction...

  • Right or left total mandibulectomy (previously referred to as hemimandibulectomy) and maxillectomy surgeries carry with them the potential for profuse bleeding...

  • Blood type and cross match may be warranted preoperatively...

  • Pigmentation is not a reliable indicator of tumor type...

  • Approximately 40% of oral melanomas may be amelanotic: other tumors other than melanoma may be pigmented if the oral mucosa is normally pigmented...

  • Removal of regional lymph nodes (mandibular, parotid, and retropharyngeal) may be beneficial at the time of oral surgery...

  • A single surgical approach to all three lymph nodes has been described...

  • If the biological nature of a tumor does not match its histology results, ask for re-evaluation of the sample by the pathologist...

  • Fibrosarcoma can sometimes appear as a histologically benign tumor but may be clinically aggressive...

  • Nonhealing tooth extraction sites may be a clinical manifestation of squamous cell carcinoma: biopsy of suspicious tissue is warranted at the time of extraction...

  • Dehiscence and inappropriate healing often occur when oral surgery is performed on irradiated sites...

  • Surgical removal of oral tumors provides the best long term prognosis...tumors that are detected early are likely to be operable...

  • New pet owners should be advised to socialize puppies and kittens to allow them to feel comfortable with periodic examination of the oral cavity...

  • Dogs adapt remarkably well after major mandibulectomy or maxillectomy...

  • Cats can also adapt but may require a feeding tube to provide nutritional support postoperatively...

  • Lymph node aspirates and chest radiographs are important for preoperative patient staging, but micrometastasis cannot be totally ruled out by these tests.

References

1.  White SC, Pharoah MJ. Oral Radiography Principles and Interpretation, 2004.

2.  Verstraete FJM. Self-Assessment Colour Review of Veterinary Dentistry. 1999.

3.  Lewis, JR. Veterinary Clinical Advisor, pp. 769-773.

4.  Verstraete FJM, et al. J Comp Pathol. 1992;106(2):169-82.

5.  Teifke JP, et al. Vet Microbiol. 1998;60(2-4):119-30.

6.  Harvey, CE, Emily PP. Small Animal Dentistry, 1993.

7.  Ogilvie GK, Moore AS. Managing the Veterinary Cancer Patient : a Practice Manual, 1995.

8.  Bertone ER, et al. J Vet Intern Med 2003;17(4):557-62.

9.  Kosovsky JK, et al. Vet Surg. 1991;20(6):397-401.

Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

John Lewis, VMD, FAVD, DAVDC
University of Pennsylvania
Philadelphia, PA

Jennifer Rawlinson, DVM, DAVDC
Cornell University
Ithaca, NY


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