Pulmonary neoplasia: the surgeon's approach

Primary pulmonary neoplasia is less common than metastatic neoplasia in dogs and cats. The diaphragmatic lobes are most frequently involved, with the right lung lobes more often affected than the left. Specific anatomic localization of tumor origin is not always possible, and more than one tumor type may be present; therefore, classification of primary lung tumors usually is based on the predominant histologic pattern.

Adenocarcinoma is the most common histologic type found in dogs and cats; squamous cell carcinoma and anaplastic carcinomas are less common. Primary pulmonary tumors of connective tissue origin (e.g., osteosarcoma, fibrosarcoma, and hemangiosarcoma) are rare. Although most pulmonary tumors are malignant, benign tumors (i.e., papillary adenoma, bronchial adenoma, fibroma, myxochondroma, and plasmacytoma) occur. Pulmonary neoplasms are highly aggressive and tend to metastasize early. Most anaplastic carcinomas and squamous cell carcinomas have metastasized at the time of diagnosis, whereas approximately half of adenocarcinomas have done so. Metastasis is often to the lung itself or to regional lymph nodes or both.

Metastatic pulmonary neoplasia is an important differential diagnosis for nodular lung disease. Tumors with a high likelihood of resulting in pulmonary metastasis include mammary carcinoma, thyroid carcinoma, hemangiosarcoma, osteosarcoma, transitional cell carcinoma, squamous cell carcinoma, and oral and digital melanoma.

Physical examination findings

The most common clinical finding in dogs with primary pulmonary neoplasia is a nonproductive cough; other signs include hemoptysis, fever, lethargy, exercise intolerance, weight loss, dysphagia, and anorexia. Lameness may be associated with metastasis to bone or skeletal muscle or with development of hypertrophic osteopathy or in feline lung-digit syndrome. Hyporexia, weight loss, lethargy, dyspnea, and cough are common clinical signs in cats with primary lung tumors; respiratory signs may be present in as few as one third of affected cats.

Diagnostic imaging

Thoracic radiographs should be obtained in animals suspected of having pulmonary neoplasia. The most common finding with primary pulmonary neoplasia in dogs is a solitary nodular density in the periphery of a dorsocaudal lung lobe. Multiple miliary lesions are less common. The radiographic pattern may be classified as solitary nodular, multiple nodular, or disseminated-infiltrative. Multiple discrete lesions within a single lobe or multiple lobes usually represent metastatic neoplasia rather than multicentric primary neoplasia. Feline bronchoalveolar carcinoma may appear as a mixed bronchoalveolar pattern, an ill-defined alveolar mass, or a mass with cavitation. Bronchial disease is typically seen in affected cats (i.e., bronchointerstitial pattern, peribronchial cuffing, or bronchiectasis) and may represent airway metastasis. Because radiographic signs of pulmonary neoplasia in cats are not specific (many inflammatory diseases will cause similar changes), lung fine-needle aspirates may be the most helpful diagnostic tool.

Thoracic evaluation should include a three-view radiographic study (opposite lateral views and an orthogonal view). Lung lesions may go undetected in recumbent lateral radiographs when the affected lung is dependent because of the recumbent atelectasis that occurs. Thoracic radiographs are relatively insensitive indicators of pulmonary neoplasia because nodules must be at approximately 0.5 to 1 cm in diameter to be reliably recognized. Radiographs should also be evaluated for sternal or hilar lymphadenopathy and/or pleural effusion. It may be difficult to differentiate metastatic pulmonary neoplasia from pulmonary metastasis of a primary pulmonary tumor. Compared with primary lesions, metastatic tumors generally are smaller and more well circumscribed and usually are located in the peripheral or middle portions of the lung. Multiple nodules associated with primary lung tumors often consist of one large mass and smaller secondary nodules. When multiple nodules are metastases, there are usually several large masses and a variety of smaller lesions. Contrast-enhanced CT is the most sensitive means for detecting pulmonary lesions.

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