Understanding Ameloblastoma in Dogs
Canine acanthomatous ameloblastoma (CAA) is a significant oral tumor in dogs. While it's classified as benign due to its lack of metastatic potential, CAA's local invasiveness makes it a serious concern for both veterinarians and pet owners.
Origins and Classification
CAA arises from remnants of odontogenic epithelium—the tissue involved in tooth development—found within the gingiva. It's most commonly located in the tooth-bearing regions of the jaws, especially the rostral mandible (front part of the lower jaw). Previously called acanthomatous epulis, advances in veterinary pathology have clarified its distinct nature, separating it from other similar oral tumors.
Breed and Age Predisposition
- Middle-aged dogs are most often affected.
- Certain breeds show higher prevalence: Golden Retrievers, Cocker Spaniels, Akitas, and Shetland Sheepdogs.
- No breed is entirely immune; any dog can develop CAA.
Clinical Presentation
The tumor typically presents as a proliferative mass on the gums. You might notice:
- An exophytic lesion (growing outward) with an irregular or smooth surface
- The mass feels firm and is attached to underlying bone
- Ulceration or necrosis may occur
As CAA grows, it can cause:
- Displacement or loosening of teeth
- Destruction of both cancellous and cortical bone
Other symptoms include facial swelling, oral bleeding, drooling, bad breath (halitosis), pain while chewing, difficulty eating or swallowing, or sometimes no symptoms at all—discovered only during a dental exam.
Diagnosis and Imaging
- Dentists use radiographs or CT scans to assess bone involvement. CT scans offer detailed images showing bone lysis (destruction), tooth displacement, and jaw deformation.
- The tumor may be peripheral (in the gum) or intraosseous (within bone breaking into surrounding tissues).
A thorough diagnosis involves clinical assessment, biopsy with histopathology, imaging studies, and sometimes cytologic or immunohistochemical analysis. Large tissue samples examined by experienced pathologists are crucial for accuracy.
Tissue Characteristics
Under the microscope:
- Tumor consists of islands/sheets of squamous epithelial cells bordered by palisading cells with reverse nuclear polarization.
- The supporting stroma varies: dense gingival tissue, fibroblastic periodontal ligament tissue, or loose bone marrow tissue.
Cytology shows clusters of epithelial cells—sometimes with spindle cells—exhibiting mild anisocytosis (size variation), high nuclear-to-cytoplasmic ratios, finely stippled chromatin, and rare mitotic figures.
Aggressiveness and Recurrence
Although CAA does not metastasize, it's locally aggressive. Incomplete removal leads to high recurrence rates—up to 91% after marginal excision. The tumor infiltrates underlying bone distinctly and can return if not fully excised.
Treatment Options
- Surgical excision with wide margins (1–2 cm of normal tissue) is the gold standard for cure.
- Rim excision may be considered for small tumors (<2 cm) with minimal bone involvement.
If surgery isn't possible or declined:
- Radiation therapy offers about an 80% three-year progression-free survival rate but carries risks such as osteoradionecrosis and secondary malignancies in irradiated tissues.
Chemotherapy (e.g., intralesional bleomycin) has been tried but causes localized side effects like wound formation and tissue reactions; it's less favored today.
Molecular Insights
Molecular studies reveal that over 60% of CAAs harbor activating mutations in the HRAS gene, particularly p.Q61R variant; some have BRAF mutations. These genetic features align closely with human ameloblastomas, making CAA a valuable model for studying RAS-driven tumors and exploring targeted therapies. Immunohistochemical detection of HRAS p.Q61R helps distinguish CAA from other oral cancers like squamous cell carcinoma.
Prognosis and Quality of Life
- If completely excised: 1-year survival rates reach 97–100%.
Mild recurrences occur mainly after incomplete surgery. Dogs generally adapt well even after partial jaw removal—most regain good function and quality of life post-surgery. Regular monitoring ensures early detection if recurrence occurs later on. Late metastasis is not a concern for this tumor type.
