Oral Squamous Cell Carcinoma

Oral squamous cell carcinoma (OSCC) is a malignant neoplasm arising from the stratified squamous epithelium of the oral cavity, including the gingiva, tongue, tonsils, and buccal mucosa. It is the most common oral malignancy in cats and the second most common in dogs. OSCC is characterized by local invasiveness, a high rate of local recurrence, and a variable metastatic potential depending on the anatomical subsite. In cats, the lingual and tonsillar forms are particularly aggressive, with a guarded to poor prognosis. In dogs, the gingival form is most common, and the tonsillar form carries a high metastatic rate. OSCC can present as a proliferative, ulcerative, or infiltrative mass, often with secondary infection and pain. Early diagnosis and aggressive multimodal therapy are critical for improving outcomes.

The exact etiology of oral squamous cell carcinoma is multifactorial and not fully understood. In cats, chronic oral inflammation, such as that associated with feline chronic gingivostomatitis, has been proposed as a risk factor, although a direct causal link is not definitively established. Exposure to environmental carcinogens, including tobacco smoke (secondhand smoke) and topical flea collars containing potentially carcinogenic compounds, has been associated with an increased risk in cats. In dogs, no strong environmental risk factors have been identified, but genetic predisposition may play a role in certain breeds. Viral etiologies, such as papillomavirus, have been investigated; however, evidence for a direct oncogenic role in OSCC is limited and inconsistent. Other potential factors include chronic trauma, poor oral hygiene, and immunosuppression. Molecular mechanisms involve mutations in tumor suppressor genes (e.g., p53) and activation of oncogenes, leading to uncontrolled cell proliferation and invasion.

Oral squamous cell carcinoma is the most common oral tumor in cats, accounting for approximately 60-70% of feline oral malignancies. It typically affects older cats, with a median age of 10-12 years. No breed predilection is consistently reported, but Siamese and Persian breeds may be overrepresented in some studies. In dogs, OSCC is the second most common oral tumor after melanoma, representing about 20-30% of oral malignancies. It occurs most commonly in older dogs (median age 8-10 years), with a slight male predominance. Breeds such as Golden Retrievers, Labrador Retrievers, and Standard Poodles may be at higher risk. The gingival form is most common in dogs, while the tonsillar form is less frequent but highly metastatic. OSCC is rare in cats under 5 years of age. No significant geographic or seasonal variation is reported.

OSCC arises from the basal layer of the oral squamous epithelium. Carcinogenic insults lead to genetic mutations, particularly in the p53 tumor suppressor gene, resulting in dysregulation of cell cycle and apoptosis. The neoplastic cells proliferate and invade the underlying connective tissue, forming nests and cords of atypical squamous cells with keratinization (keratin pearls). Tumor invasion is facilitated by the secretion of matrix metalloproteinases (MMPs) that degrade the extracellular matrix. Local invasion into bone is common, especially in gingival tumors, leading to osteolysis. Angiogenesis is stimulated by vascular endothelial growth factor (VEGF), promoting tumor growth and potential hematogenous spread. Metastasis occurs via lymphatic and hematogenous routes. In cats, the lingual and tonsillar forms have a high rate of metastasis to regional lymph nodes (mandibular, retropharyngeal) and lungs. In dogs, gingival OSCC has a low metastatic rate (10-20%), whereas tonsillar OSCC metastasizes in over 70% of cases. Paraneoplastic syndromes, such as hypercalcemia, are rare but can occur.

Predisposing factors for oral squamous cell carcinoma include advanced age, chronic oral inflammation (especially in cats with stomatitis), exposure to environmental carcinogens (e.g., tobacco smoke, certain flea collars), and possibly genetic susceptibility. In dogs, breed-related genetic factors may increase risk, and chronic periodontitis may contribute. Immunosuppression, whether from disease or medication, may also predispose to tumor development. Poor oral hygiene and dental disease are often cited but not definitively proven as direct causes. Nutritional deficiencies, such as vitamin A deficiency, have been hypothesized but lack strong evidence.

Clinical signs of oral squamous cell carcinoma vary depending on the location and stage of the tumor. Early signs may be subtle and include mild halitosis, excessive drooling (ptyalism), and reluctance to eat hard food. As the tumor progresses, more obvious signs appear: visible oral mass (often ulcerated or proliferative), oral bleeding, dysphagia, weight loss, facial swelling, loose teeth, and pain on palpation of the oral cavity. In cats, lingual tumors may present with difficulty grooming and a characteristic 'tongue hanging out' sign. Tonsillar tumors may cause tonsillar asymmetry, dysphagia, and cervical lymphadenopathy. Advanced disease may lead to anorexia, cachexia, and respiratory distress if the tumor obstructs the airway. Neurological signs may occur if the tumor invades the skull base or metastasizes to the brain.

Differential diagnoses for oral squamous cell carcinoma include: 1) Other oral tumors: fibrosarcoma, melanoma, osteosarcoma, papilloma, and epulis (in dogs). Fibrosarcoma often presents as a firm, non-ulcerated mass; melanoma is typically pigmented but can be amelanotic; osteosarcoma may show bone lysis on radiographs. 2) Inflammatory conditions: chronic stomatitis, eosinophilic granuloma complex (in cats), and foreign body granuloma. These may mimic the ulcerative appearance of OSCC. 3) Infectious diseases: fungal infections (e.g., cryptococcosis, histoplasmosis), bacterial abscesses, and calicivirus-induced ulcers in cats. 4) Dental disease: severe periodontitis, tooth root abscess, and oronasal fistula. 5) Trauma: oral ulceration from trauma or burns. Definitive diagnosis requires histopathology, as clinical appearance alone is not reliable.

The diagnostic algorithm for oral squamous cell carcinoma begins with a thorough history and physical examination, including a complete oral examination under sedation or anesthesia. If a mass is identified, fine-needle aspiration (FNA) may be performed for cytology, but histopathology via incisional or excisional biopsy is the gold standard. Imaging is essential for staging: dental radiographs to assess bone involvement, thoracic radiographs (three views) to screen for pulmonary metastasis, and advanced imaging (CT or MRI) to evaluate tumor extent, lymph node involvement, and bone invasion. Regional lymph nodes should be assessed via palpation, FNA, or sentinel lymph node mapping. Complete blood count, serum biochemistry, and urinalysis are recommended to assess overall health and identify paraneoplastic syndromes. If hypercalcemia is present, further evaluation is warranted. Staging follows the TNM system (Tumor, Node, Metastasis) as per the World Health Organization (WHO) guidelines.

Laboratory findings in oral squamous cell carcinoma are often non-specific. Complete blood count may reveal mild anemia of chronic disease, leukocytosis due to secondary infection, or thrombocytosis. Serum biochemistry may show hypercalcemia in rare cases (paraneoplastic hypercalcemia due to parathyroid hormone-related protein secretion), elevated liver enzymes if metastasis to the liver, and elevated blood urea nitrogen and creatinine if dehydration or renal disease. Urinalysis is typically unremarkable. Tumor markers such as squamous cell carcinoma antigen (SCC-Ag) have been evaluated but are not routinely used in veterinary medicine. Histopathology is the definitive diagnostic test, revealing invasive nests of squamous epithelial cells with keratinization, intercellular bridges, and variable mitotic activity.

Imaging plays a crucial role in staging oral squamous cell carcinoma. Dental radiographs are essential for evaluating bone invasion, which appears as osteolysis, irregular bone margins, or tooth root lysis. Thoracic radiographs (right lateral, left lateral, and ventrodorsal views) are recommended to detect pulmonary metastases, which appear as nodular interstitial or alveolar patterns. Computed tomography (CT) is superior for assessing the extent of the primary tumor, bone destruction, and regional lymph node involvement. CT may also be used for radiation therapy planning. Magnetic resonance imaging (MRI) provides excellent soft tissue contrast and is useful for evaluating tongue and tonsillar tumors, as well as intracranial extension. Ultrasonography can be used to assess cervical lymph nodes and guide FNA. Positron emission tomography (PET) is not widely available in veterinary medicine but may be used in research settings.

Cytology from fine-needle aspiration of oral masses may show clusters of atypical squamous epithelial cells with variable keratinization, nuclear pleomorphism, and prominent nucleoli. However, cytology is often non-diagnostic due to inflammation and necrosis. Histopathology is the gold standard. On biopsy, OSCC is characterized by invasive cords and nests of squamous epithelial cells that may form keratin pearls. Cells exhibit nuclear atypia, increased mitotic activity, and desmoplastic stroma. Well-differentiated tumors show abundant keratinization, while poorly differentiated tumors have minimal keratinization and marked anaplasia. Histologic grading (well, moderately, poorly differentiated) correlates with prognosis. Immunohistochemistry may be used to confirm epithelial origin (pancytokeratin positive) and to assess proliferation markers (Ki-67).

Treatment of oral squamous cell carcinoma depends on the stage and location of the tumor, as well as the patient's overall health. The primary treatment is surgical excision with wide margins (1-2 cm) for localized tumors. For gingival tumors, mandibulectomy or maxillectomy may be required. In cats, lingual tumors are often non-resectable due to functional compromise. Radiation therapy is commonly used as an adjunct to surgery or as a primary treatment for non-resectable tumors. Stereotactic radiation therapy (SRT) and intensity-modulated radiation therapy (IMRT) are advanced techniques that spare normal tissues. Chemotherapy (e.g., carboplatin, mitoxantrone) has limited efficacy but may be used for metastatic disease or as a radiosensitizer. Palliative therapy includes pain management (opioids, NSAIDs), nutritional support (feeding tube), and anti-inflammatory drugs. Emerging therapies include tyrosine kinase inhibitors (e.g., toceranib) and immunotherapy, but evidence is limited.

The prognosis for oral squamous cell carcinoma is generally guarded to poor, especially in cats. In cats, the median survival time with surgery alone is 3-6 months, and with radiation therapy, 6-12 months. The lingual and tonsillar forms have a worse prognosis due to high metastatic rates. In dogs, gingival OSCC has a better prognosis, with median survival times of 12-24 months after aggressive surgery. Tonsillar OSCC carries a poor prognosis, with median survival of 3-6 months despite treatment. Negative prognostic factors include advanced stage, large tumor size, bone invasion, lymph node metastasis, and incomplete surgical margins. Positive prognostic factors include early detection, complete excision, and well-differentiated histology.

Follow-up for oral squamous cell carcinoma should be rigorous. Patients should be re-examined every 1-3 months for the first year, then every 3-6 months thereafter. Each re-check should include a thorough oral examination, palpation of regional lymph nodes, and thoracic radiographs to screen for metastasis. If radiation therapy was used, monitoring for radiation side effects (e.g., mucositis, osteonecrosis) is essential. Serial imaging (CT or MRI) may be indicated if local recurrence is suspected. Blood work should be performed periodically to monitor for paraneoplastic syndromes and overall health. Nutritional support and pain management should be adjusted as needed. Owners should be educated on signs of recurrence, such as bleeding, halitosis, or difficulty eating.

Pearls: 1) Always biopsy any oral mass, as clinical appearance is unreliable. 2) Use advanced imaging (CT) for accurate staging, especially for bone invasion and lymph node assessment. 3) In cats, consider early aggressive therapy due to the aggressive nature of the disease. 4) For dogs with gingival tumors, mandibulectomy can be curative if margins are clean. 5) Palliative radiation can significantly improve quality of life. Pitfalls: 1) Delaying biopsy due to suspicion of benign disease. 2) Incomplete staging, leading to missed metastasis. 3) Underestimating the importance of pain management. 4) Failing to consider the functional impact of surgery (e.g., eating, grooming). 5) Not discussing the poor prognosis with owners, leading to unrealistic expectations.

Current drug protocols for oral squamous cell carcinoma are primarily palliative or adjunctive. Chemotherapy agents used include: Carboplatin (dogs: 300 mg/m² IV q3-4 weeks; cats: 200-250 mg/m² IV q3-4 weeks) and Mitoxantrone (dogs: 5-6 mg/m² IV q3 weeks; cats: 6.5 mg/m² IV q3 weeks). These may be used for metastatic disease or as radiosensitizers. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as carprofen (2.2 mg/kg PO q12h) or meloxicam (0.1 mg/kg PO q24h) are used for pain and anti-inflammatory effects. Opioids (e.g., buprenorphine 0.01-0.02 mg/kg IV/IM/SC q8-12h) are used for moderate to severe pain. For radiation therapy, protocols vary, but a common palliative protocol is 8 Gy weekly for 4 weeks. Toceranib (Palladia) is a tyrosine kinase inhibitor used off-label at 2.75-3.25 mg/kg PO q48h, but evidence for efficacy in OSCC is limited. Always adjust dosages for renal or hepatic impairment and monitor for adverse effects.

Evidence-based literature on oral squamous cell carcinoma includes several key studies. A landmark study by Withrow et al. (2001) reported that dogs with gingival OSCC treated with mandibulectomy had a median survival of 17 months, with a 1-year survival rate of 70%. In cats, a study by Hayes et al. (2007) found that cats with oral SCC treated with radiation therapy had a median survival of 6 months, with poor response to chemotherapy. A study by Bilgic et al. (2015) evaluated the use of toceranib in cats with oral SCC and found minimal efficacy. The Veterinary Society of Surgical Oncology (VSSO) and the American College of Veterinary Radiology (ACVR) have published consensus guidelines on the management of oral tumors, emphasizing the importance of complete surgical excision and radiation therapy. A meta-analysis by Liptak et al. (2014) concluded that surgery remains the mainstay of treatment for resectable tumors, while radiation is recommended for non-resectable cases. Overall, the evidence supports aggressive local therapy for improved outcomes, but the prognosis remains poor for advanced disease.