Neoplasia of the Respiratory System in Small Animals
Tumors of the nose and paranasal sinuses account for 1%–2% of all canine or feline tumors. The incidence in dogs is twice that in cats; incidence is also higher in males of both species than in females. The mean age at time of diagnosis is 9.5–10 yr for dogs and 12 yr for cats. In dogs, nasal tumors are nearly all malignant, and slightly >60% are carcinomas, of which adenocarcinoma is the most common. In dogs, the ethmoturbinates tend to be the site of predilection. Dolichocephalic and mesocephalic breeds appear to be at higher risk than brachycephalic breeds. In cats, ≥90% of nasal tumors are malignant, the most common being lymphoma and the second most common being carcinomas. Tumors of the nose and paranasal sinuses typically are very invasive locally and metastasize infrequently; metastasis is more likely in carcinomas and usually occurs late in the disease. Common sites of metastasis are regional lymph nodes, lungs, and brain. Invasion of the paranasal sinuses tends to be greater in dogs than in cats. In general, survival of untreated animals is 3–5 mo after diagnosis.
Chronic nasal discharge is the most common clinical finding; it may be mucoid, mucopurulent, or serosanguineous. Initially, discharge is unilateral but often becomes bilateral. Periodic sneezing, epistaxis, and respiratory stertor may occur. Facial and oral deformities result from destruction of bony or soft-tissue sinonasal structures. Retrobulbar extension of these tumors results in exophthalmos and exposure keratitis. Secondary epiphora may occur if the nasolacrimal duct is blocked. Late in the disease, CNS signs (eg, disorientation, blindness, seizures, stupor, and coma) may develop if the tumor extends into the cranial vault.
Diagnosis is based on history and clinical findings and elimination of other causes of nasal discharge, sneezing, or facial deformation. Nasal radiographs or CT typically show increased density of the nasal cavity and frontal sinuses as well as evidence of bone destruction. CT is vastly superior to plain radiography in diagnosis of chronic nasal diseases. Definitive diagnosis is based on biopsy of tumor tissue, either with blind biopsy based on CT lesion localization or on rhinoscopic visualization with direct biopsy. Nasal hydropulsion using a high-pressure saline infusion into the nose often yields diagnostic samples and, if large volumes of tumor tissue break free, nasal obstruction will be relieved immediately.
Treatment largely depends on tumor type and extent of disease. The treatment of choice for canine nasal adenocarcinoma is radiation therapy. Aggressive surgical excision, chemotherapy, radiation therapy, or combinations for other tumor types afford a more favorable prognosis when diagnosis is made early.
Tumors of the larynx and trachea are rare in dogs and cats. Tumors of the larynx most frequently reported in dogs are oncocytoma, squamous cell carcinoma, mast cell tumor, melanoma, and osteosarcoma; in cats, they are squamous cell carcinoma, lymphosarcoma, and adenocarcinoma. Benign inflammatory polyps of the larynx also are seen in dogs and cats. Tumors of the trachea are particularly rare. Osteochondral dysplasia of the trachea (osteochondroma) is a benign tumor of the trachea primarily seen in dogs <1 yr old. Other benign mesenchymal tumors, carcinomas, and sarcomas are occasionally seen.
The most common signs of tumors of the larynx include inspiratory dyspnea, stridor, voice change (hoarse bark or loss of voice), coughing, and exertional dyspnea. Findings typically associated with tumors of the trachea are coughing, dyspnea, stridor, and rarely hemoptysis. Laryngeal and tracheal tumors may be associated with signs of fixed upper airway obstruction (inspiratory and expiratory dyspnea). The degree of dyspnea often relates to the degree of luminal obstruction.
Diagnosis is made from the history and clinical findings and by eliminating other causes of upper airway obstruction or coughing. The tumor mass may be seen radiographically or on laryngoscopy or tracheoscopy. Definitive diagnosis is made on biopsy.
Surgical excision and resection is the treatment of choice. Radiation therapy may be palliative for radiosensitive tumors such as squamous cell carcinoma, mast cell tumor, and lymphoma. Surgical resection of tracheal osteochondral dysplasia in dogs is curative.
Primary lung tumors are rare in dogs and cats; however, the reported incidence of lung carcinomas has increased at least 100% during the last 20 yr. This is attributed to an increased average life span, better detection and awareness, or possibly increased exposure to environmental carcinogens. Most primary lung tumors are diagnosed at a mean age of 10–12 yr in dogs and 12 yr in cats. There is no consistent breed or sex predilection in either species. Primary lung tumors usually originate from the terminal bronchioles and alveoli; they occasionally develop as a second coincidental tumor, which may make differentiation between primary and metastatic disease difficult.
Of the primary lung tumors in dogs and cats, ≥80% are malignant. Adenocarcinoma and alveolar carcinoma are the most common types. Primary lung sarcomas and adenomas are rare in both species. Metastatic spread of primary lung tumors is generally to other areas of the lungs, tracheobronchial lymph nodes, bone, and brain. Intrapulmonary spread via the airways occurs in ~50% of dogs with adenocarcinoma. Metastatic spread to the pleurae, pericardium, heart, and diaphragm may occur; miscellaneous extrathoracic sites include liver, spleen, and kidney. Dogs with papillary (bronchoalveolar) adenocarcinoma have a better prognosis than those with other lung tumors; however, histologic grade and detection of clinical signs are the most important determinants of prognosis and survival. Both recurrence and metastasis tend to occur earlier and with greater frequency in dogs with moderately or poorly differentiated tumors.
Primary lung tumors have variable manifestations, which depend on the location of tumor, rapidity of tumor growth, presence of previous or concurrent pulmonary disease, and awareness of the owner. Common signs include cough, inappetence, weight loss, reduced exercise tolerance, lethargy, tachypnea, dyspnea, wheezing, vomiting or regurgitation, pyrexia, and lameness. The most common clinical findings in dogs are cough, dyspnea, lethargy, and weight loss, although 25% of dogs with primary lung tumors have no clinical signs related to the tumor. Coughing is uncommon in cats; nonspecific signs, such as inappetence, weight loss, and tachypnea and dyspnea, are more common. In either species, tachypnea or dyspnea indicates massive tumor burden or pleural effusion. Pleural effusion is particularly common in cats with primary lung tumors. Lameness may be due to hypertrophic osteopathy (unusual in cats) or to metastasis to bone or skeletal muscle. Thoracic auscultation may be normal, reflect increased breath sounds compatible with pulmonary airway disease, or be muffled because of pulmonary consolidation or pleural effusion.
One-third or more of primary lung tumors are recognized incidentally during radiography for other problems, or at necropsy. Thoracic radiographs are essential for a tentative diagnosis in those animals exhibiting compatible clinical signs. Primary lung tumors in dogs may occur as single or multiple circumscribed mass lesions, as a diffuse lung pattern, or as a lobar consolidation. In cats, single circumscribed mass lesions are less common, whereas a diffuse lung pattern or lobar consolidation is more frequent. Pleural fluid accumulation is common in cats and less frequent in dogs. In either species, chest wall involvement and hilar lymphadenopathy may be seen. Tentative diagnosis can be made by excluding other causes of pulmonary disease with similar radiographic lung patterns. Definitive diagnosis requires biopsy.
Surgical resection of tumor via lobectomy of diseased lung lobes is the treatment of choice. Inoperable lesions or metastatic disease may be controlled with chemotherapy. Overall median survival time for dogs having surgical treatment for primary lung tumor is 120 days. Mean survival time for operable primary lung tumors without node involvement in dogs is 12 mo; if the lymph nodes are involved or multiple tumors are found at the time of diagnosis, survival time is only 2 mo. Recurrence or metastasis of tumor is a common cause of death.
A localized tumor may extend to the lungs by dissemination through hematogenous or lymphatic routes or by direct extension of tumor cells. Certain primary tumors, such as mammary adenocarcinoma, osteosarcoma, hemangiosarcoma, and oral melanoma, most commonly metastasize to the lungs. The lungs may be the only site of metastasis, or there may be concurrent metastasis in other organs; in the former, the diagnostic approach is to identify an occult primary tumor or to carefully review the medical history for disclosure of previous tumor removal. Because pulmonary metastasis occurs late in the clinical course of a malignant tumor, prognosis is poor.
The signs of metastatic pulmonary disease are similar to those of primary lung tumors except that coughing is less common. Severity of signs depends on the anatomic location of the tumor and whether the lesions are solitary or multiple.
Establishing a diagnosis is similar to that for primary lung tumors. Because of the limitations of routine radiography, small lesions (≤3 mm in diameter), which are present in ≥40% of cases with pulmonary metastasis, may not be seen. Thoracic CT can identify lesions not seen radiographically.
Radiography of the chest should precede removal of tumors with a known high incidence of metastatic spread to the lungs. The major goal of cancer therapy is prevention of metastasis rather than cancer eradication. Slow-growing or solitary metastatic lesions are best treated by surgical excision. Chemotherapy or radiation therapy may be useful with certain tumor types not amenable to surgical resection. Overall, the prognosis for animals with pulmonary metastasis is poor.