Introduction to Oral Neoplasia in the Dog & Cat

Brook A. Niemiec, DVM, FAVD, Diplomate AVDC

The oral cavity is a very common place to encounter neoplastic growths, which may range from benign to malignant.^1-3 The key to successful therapy of oral masses is early detection and definitive diagnosis.

The best therapy for oral masses is complete surgical resection.^1,4-5 If surgery is performed early, the procedure is much less invasive. In addition, early detection and treatment decreases the odds for tumor metastasis.

The information presented in this article not only helps improve patient care but also improves practice revenue through proper client education (see Educating Clients About Oral Neoplasia).

TYPES OF MASSES

Benign Masses⁶
The most common oral growths were originally termed epulids (fibromatous and ossifying)¹; now they are identified as peripheral odontogenic fibromas.⁶ Another benign oral mass is the acnthomatous ameloblastoma, which was originally termed an epulis.

Peripheral odontogenic fibromas are overgrowths of the periodontal ligament (Figures 1A and 1B),¹ classifying them as hamartomas (benign masses resulting from an overgrowth of mature cells normally present in the affected tissue). These masses can grow very large, but are not aggressive. They do not cause bony reaction (Figure 1C), but the ossifying type produces bony proliferation.

Acanthomatous ameloblastomas are locally aggressive, but do not metastasize. The most common location of these fleshy tumors is in the incisor/canine region of large-breed dogs.¹ They originate within the bone but do not have radiating microtendrils (Figure 2).

Malignant Masses³
Squamous cell carcinomas (Figure 3) are the most common malignant oral tumors in cats, with fibrosarcomas (Figure 4) a distant second. Both are typically seen in older cats and locally aggressive but late to metastasize. These soft tissue tumors secondarily invade bone, sending out branches of cells that cannot be seen clinically.

Melanosarcomas (Figure 5) are the most common malignant oral tumor is dogs (typically seen in older dogs with dark pigmented gingiva).⁷ Melanomas are locally aggressive and also metastasize very early in the disease course. At time of diagnosis, 90% of melanomas have already spread. Fibrosarcomas (Figure 6) and squamous cell carcinomas (Figure 7) are also seen in dogs.

Cysts
Cysts are a subset of oral masses:

• Dentigerous cysts (Figure 8) are the most common cysts, which arise from the enamel-forming organ of an impacted tooth.1 They are most commonly found in mandibular first premolars of brachycephalic breeds. Their incidence in veterinary medicine has not been documented, but anecdotally they are found in approximately 50% of embedded teeth.
• Radicular cysts (Figure 9) are another type of cyst that develops from embryonic tissue remains, usually due to an inflammatory stimulus from the tooth.

As cysts grow, they cause bone loss by pressure.⁸ They can grow quite large in a short period of time, quickly resulting in weakened bone.

DIAGNOSIS

During routine physical examinations, an oral examination should be performed. Any oral masses are cause for concern (especially in felines) and should prompt a dental procedure and biopsy or, at minimum, very careful monitoring.

Clinical cues help determine the type of oral growth: In my experience, malignant growths tend to invade the tissues; benign growths tend to be more proliferative. However, because clinical examination of masses is unreliable, dental radiographs and biopsies should be performed for all oral masses.

Imaging
Most benign neoplastic growths (Figure 10) have no boney involvement on dental radiographs.^6,8

• If bone involvement does occur, it is expansive—the bone “pulls away” from the advancing tumor, leaving a decalcified, soft—tissue-filled space (Figures 11A and 11B).^6,8
• Bony margins are usually distinct.
• These types of expansive growths typically cause tooth movement (Figures 11B and 11C).⁸

Malignant oral neoplasms typically invade bone early in the course of disease, resulting in irregular, ragged bone destruction (Figures 12A and 12B).⁸ Initially, the bone will display a mottled “moth eaten” appearance, but radiographs late in the disease course reveal a complete loss of bone (the teeth appear to “float” in space) (Figures 12C and 12D). If the cortex is involved, an irregular periosteal reaction is often seen (Figure 12E).8

Cystic structures appear as radiolucent areas with smooth bony edges (Figure 13A).⁸ Similar to other benign growths, they grow by expansion and displace other structures (eg, teeth). Dentigerous cysts are typically seen as a radiolucent structure centered on the crown of an unerupted tooth (Figure 13B).⁸

Aspiration & Biopsy

Fine-needle aspiration is useful to differentiate inflammation from neoplasia, and can potentially determine tumor type.

Histopathologic testing is always necessary for accurate diagnosis of oral masses and prior to definitive therapy since, as mentioned earlier, benign and malignant tumors can appear clinically and radiographically similar (compare Figure 11B to Figure 12C).³

Additional Diagnostics

Additional diagnostic tests for questionable cases include:

• Complete blood panel
• Urinalysis
• Bacterial and/or fungal culture
• Fungal serology
• Computed tomography/magnetic resonance imaging to determine surgical margins.

TREATMENT

Benign Masses
Peripheral Odontogenic Fibromas
A surgical cure can be achieved by:

• Removal of the mass with narrow margins
• Extraction of the tooth (or teeth) from which the mass has arisen
• Debridement of the periodontal ligament.¹

However, since benign masses are typically slow growing and noninvasive, I tend to take a more conservative approach, especially when large teeth are involved. Conservative treatment involves:

• Excision of the mass to physiologic margins
• Monitoring the area.

If the mass regrows quickly, treat as discussed earlier. If regrowth takes a year or two, then reexcise at annual (or semi-annual) dental cleanings.

Acanthomatous Ameloblastomas
After quality dental imaging, these tumors can be removed by local excision with small margins. Depending on the text, recommendations vary from 0.5 to 1 cm.6 These tumors have a 90% control rate with radiation therapy and some reports cite that local chemotherapy may also be effective.^6,10

Malignant Masses
Squamous Cell Carcinoma & Fibrosarcoma
In cats, aggressive surgery (en bloc, with 1- to 2-cm surgical margins) is required for definitive treatment.^4,5 Radiation therapy may help with pain, but has not been shown to increase survival times significantly.

In dogs, recommended therapy is similar to therapy for cats; however, in the dog these tumors respond better to radiation therapy (up to one year survival rates for squamous cell carcinoma).¹¹

Melanoma
In dogs, optimum treatment incorporates a combination of aggressive surgery with 2-cm margins, radiation therapy, and chemotherapy. Despite combining all of these treatment methods, survival is generally less than a year. However, a vaccine has been recently released that shows promise as an adjunct therapy for this tumor.^12-14

Cysts
Cyst treatment requires surgery due to the potential for pathologic fracture. In addition, cysts can become infected, creating significant swelling, pain, and/or malignancy.¹⁵ If cyst formation has occurred, en bloc removal or extraction of the tooth and meticulous curettage of the lining can prove curative.

WHEN TO REFER

Simple excision of benign masses can be performed by the general practitioner. However, aggressive masses should be referred to a veterinary dentist, surgeon, or oncologist for definitive therapy. Three-view chest radiographs and a complete blood panel should be obtained prior to referral.

A MASS IS NOT ALWAYS WHAT IT SEEMS 

In my experience, approximately 1% of “very benign” oral growths in the dog turn out to be malignant (Figures A and B). Furthermore, it is not uncommon to see very destructive benign growths, such as eosinophilic granulomas (Figure C). Finally, osteomyelitis (Figure D) may demonstrate the same radiographic findings as malignant tumors, and aggressive tumors may show no bone involvement early in the course of disease.

It is crucial to interpret histopathology results in light of radiographic findings. The prudent practitioner will note the type and extent of bony involvement (if any) on the histopathology request form (and may include copies of the radiographs and pictures) to aid the pathologist.

A diagnosis of a malignancy without bony involvement should be questioned prior to initiating definitive therapy, such as aggressive surgery, radiation therapy, or chemotherapy. Conversely, a benign tumor diagnosis with significant bony reaction should be further investigated prior to assuming the tumor is not malignant.

Figure A. Intraoral picture of the left buccal mucosa of a dog with a small, soft mass. Histopathology revealed high-grade melanosarcoma, which was successfully treated (5-year survival) with only local resection. This case highlights the importance of early detection.

Figure B. Intraoral picture of the left buccal mucosa of a dog with a small, soft mass. Histopathology revealed low-grade fibrosarcoma, which was successfully treated (5-year survival) with a maxillectomy.

Figure C. Intraoral picture of the palate of a cat with significant soft tissue and bony destruction/infection. Histopathologic evaluation revealed eosinophillic granuloma, which was successfully treated with a combination of medical and surgical interventions.

Figure D. Intraoral picture of the left maxilla of a dog with significant soft tissue and bony destruction/infection. Histopathologic evaluation revealed osteomyelitis, which was successfully treated with a maxillectomy.

Educating Clients About Oral Neoplasia

Client education about oral neoplasia encourages them to embrace the importance of proper dental care. Carefully explain that:

• Prevention: Every dental cleaning (prophylaxis) includes a complete oral examination, which also screens for oral neoplasia
• Detection: Early detection and treatment decreases the odds for tumor metastasis
• Therapy: Prompt surgery to address an oral mass decreases the risks of the procedure since it is much less invasive as well as less painful for the pet.
• Monitoring: Encourage clients to perform dental homecare, including an oral examination for growths or masses. If they suspect anything is out of the ordinary, instruct them to contact the clinic immediately.
• Feline Fundamentals: Benign tumors are extremely rare in cats; therefore, counsel cat owners that any oral masses should be immediately evaluated and biopsied at the clinic.
• Education: Use photos and radiographs (see Furthering Your Knowledge: Oral Neoplasia) to illustrate the different types of oral masses, which helps clients identify masses early and portrays how these masses can proliferate and metastasize.

Furthering Your Knowledge: Oral Neoplasia

For additional information on oral neoplasia, specifically staging, treatment, and prognosis, refer to the following resources:

• Suggested Reading
  • Niemiec BA. Small Animal Dental, Oral and Maxillofacial Disease: A Color Handbook.London: Manson, 2010.
  • Verstraete FJ, Lomner MJ. Oral and Maxillofacial Surgery in Dogs and Cats, 1st ed. Philadelphia: Saunders, 2012.
  • Kudnig ST, Seguin B. Veterinary Surgical Oncology. Ames, IA: Wiley Blackwell, 2012.
• Client Education Resources:
  • avdc.org/oraltumors.html
  • dogbeachdentistry.com (videos, posters, books

References

1. Taney KG, Smith MM. Problems with the bones, muscles and joints. In Niemiec BA (ed): Small Animal Dental, Oral and Maxillofacial Disease: A Color Handbook. London: Manson, 2010, pp 189-223.
2. Wiggs RB, Lobprise HB. Veterinary Dentistry: Principles and Practice. Philadelphia: Lippincott Raven, 2007.
3. Dhaliwal RS. Malignant oral neoplasia. In Niemiec BA (ed): Small Animal Dental, Oral and Maxillofacial Disease: A Color Handbook. London: Manson, 2010, pp 225-235.
4. Lascelles BD, Thomson MJ, Dernell WS, et al. Combined dorsolateral and intraoral approach for the resection of tumors of the maxilla in the dog. JAAHA 2003; 39:294-305.
5. Felizzola CR, Stopiglia AJ, de Araujo VC, de Araujo NS. Evaluation of a modified hemimandibulectomy for treatment of oral neoplasms in dogs. J Vet Dent 2002; 19:127-135.
6. Debowes LJ. Problems with the gingiva. In Niemiec BA (ed): Small Animal Dental, Oral and Maxillofacial Disease: A Color Handbook. London: Manson, 2010, pp 159-181.
7. Ramos-Vara JA, Beissenherz ME, Miller MA, et al. Retrospective study of 338 canine oral melanomas with clinical, histologic, and immunohistochemical review of 129 cases. Vet Pathol 37:597-608, 2000.
8. Niemiec BA. Veterinary dental radiology. In Niemiec BA (ed): Small Animal Dental, Oral and Maxillofacial Disease: A Color Handbook. London: Manson, 2010, pp 63-77.
9. Verstraete Frank JM. Oral pathology. In Slatter D (ed): Textbook of Small Animal Surgery. Philadelphia: Saunders, 2003, pp 2648-2649.
10. Kelly JM, Belding BA, Schaefer AK. Acanthomatous ameloblastoma in dogs treated with intralesional bleomycin. Vet Comp Oncol 2010; 8(2):81-86.
11. Kinzel S, Hein S. Stopinski T, et al. Hypofractionated radiation therapy for the treatment of malignant melanoma and squamous cell carcinoma in dogs and cats. Berliner und Münchener Tierärztliche Wochenschrift 2003; 116:134-138.
12. Bergman PJ. Anticancer vaccines. Vet Clin N Am Sm Anim Pract 2007; 37:1111-1119.
13. Bergman PJ, McKnight J, Novosad A, et al. Long-term survival of dogs with advanced malignant melanoma after DNA vaccination with xenogenic human tyrosinase: A phase I trial. Clin Cancer Res 2003; 9:1284-1290.
14. Grosenbaugh DA, Leard AT, Bergman PJ, et al. Safety and efficacy of a xenogeneic DNA vaccine encoding for human tyrosinase as adjunctive treatment for oral malignant melanoma in dogs following surgical excision of the primary tumor. Am J Vet Res 2011; 72(12):1631-1638.
15. Neville BW, Damm DD, Allen CM, Bouquot JE. Oral and Maxillofacial Pathology, 2nd ed. Philadelphia: Saunders, 2002, p 609.

Brook A. Niemiec, DVM, FAVD, Diplomate AVDC, is chief of staff of Southern California Veterinary Dental Specialties, with offices in San Diego and Murrieta, California, and Las Vegas, Nevada. He has authored many articles and chapters, including the recently published Small Animal Dental, Oral and Maxillofacial Disease: A Colour Handbook (Manson Publishing). Dr. Niemiec founded the veterinary dental telemedicine website vetdentalrad.com, which also offers instructional videos and educational posters. He lectures extensively at national and international conferences and is the coordinator and instructor of the San Diego Veterinary Dental Training Center (vetdentaltraining.com). Dr. Niemiec received his DVM from University of California-Davis.