24 Lower Respiratory Tract Tumors
Most lung tumors originate from the epithelium of the airways and alveolus. When classified by histologic pattern and cellular morphology, most lung tumors are adenocarcinomas (differentiated). Squamous cell carcinomas, anaplastic carcinomas, and sarcomas are less frequent, but more biologically aggressive. Benign tumors are rare. Occasionally, lymphoma, malignant histiocytosis, and lymphomatoid granulomatosis are found.
Typically, dogs with lung tumors are between 9 and 10 years old. Breed and gender preferences have not been consistently noted.
Up to 56% of dogs with lung tumors have no clinical signs. A chronic, nonproductive cough that is unresponsive to antimicrobials and only temporarily responsive to corticosteroid therapy is most frequent. Systemic signs of inappetence and weight loss are frequent findings. Other less common signs are dyspnea, tachypnea, hemoptysis, and cyanosis. Lethargy, lameness, dysphagia, fever, cranial vena cava obstruction, diarrhea, ascites, and spontaneous pneumothorax are found infrequently.
Environmental chemical exposure is not well-documented in dogs, but airborne carcinogens are suspected causes of respiratory tract neoplasms. Second-hand smoke exposure increases the risk of lung cancer in brachycephalic breeds when compared with dolichocephalic breeds.
Lung neoplasms can be detected radiographically in almost any pattern. They can be solitary or disseminated, which includes multifocal or diffuse parenchymal patterns. The most typical are the solitary mass or diffuse interstitial patterns. Pleural effusion may be observed, and lobar consolidation is sometimes noted.
Differential diagnoses for lung tumors can be generated from the radiographic lung pattern. Pneumonia, abscesses, fungal granulomas, lymphomatoid or eosinophilic pulmonary granulomatosis, hematomas, pulmonary thromboembolism, cysts, bullae, and primary and metastatic lung tumors can be seen radiographically as solitary or multifocal lung masses. Pneumonia, primary and metastatic lung neoplasms, hemorrhage, edema, fibrosis, and granulomatous diseases are observed in disseminated pulmonary patterns. Multicentric neoplasms involve the lung parenchyma simultaneously with other abnormalities including lymphoma, malignant histiocytosis/metastatic histiocytic sarcoma, and, rarely, mast cell tumor.
Detection of lung disease with plain survey radiography is limited to pulmonary masses larger than 3 to 5 mm in diameter. Computed tomography (CT) and magnetic resonance imaging provide additional information about the extent of the primary tumor’s invasion into other thoracic structures such as the mediastinum and esophagus. It has been demonstrated that CT is more sensitive than plain thoracic radiography for detecting tracheobronchial lymphadenopathy. In one study, lymphadenopathy (with confirmed metastatic disease) was detectable with CT despite the finding of normal-size lymph nodes with plain survey films.
Metastasis can be vascular (by means of lymphatics or blood vessels) or alveolar (by local cell migration in the airways) or through the pleura. Local metastasis to structures in the thorax is more common than systemic spread. Regional lymph nodes (tracheobronchial and mediastinal) are more commonly affected than the pleura, pericardium, and heart. Dogs with locoregional metastasis may be presented for clinical signs referable to secondary invasion.
Solitary or multifocal masses seen with thoracic radiographs in an older dog raises the suspicion for primary or secondary lung neoplasms. Signs referable to the respiratory tract in an older dog should be investigated with thoracic radiography. Three view thoracic radiographs including left lateral, right lateral, and dorsal ventral or ventral dorsal views have been shown to improve detection of primary or metastatic lung tumors when compared with only two views. Aerated lung around the lesion helps demarcate the location, size, and shape of the mass. Although it is tempting and less costly to do only one lateral view, the heart, liver, pleural fluid, and atelectasis may obscure lung parenchymal masses.
Most solitary lung tumors are in the periphery of the lungs and are believed to originate from the small airways. Lymphomatoid granulomatosis can be considered an uncommon differential for most of the radiographic presentations common to dogs with primary lung tumors. It appears as a combination of mediastinal, sternal, or hilar lymphadenopathy, often with either diffuse alveolar-interstitial pulmonary infiltrate, interstitial-nodular infiltrate with large multifocal masses, or lobar consolidation. Pleural effusion is sometimes present. The radiographic appearance of dogs with malignant histiocytosis/metastatic histiocytic sarcoma is similar.
Plain radiographs and fine-needle cellular aspiration may be all that is needed before therapeutic thoracotomy is performed. A complete blood cell count, serum biochemical panel, and urinalysis are indicated to rule out other diseases and suitability for general anesthesia. Screening for coagulation defects with determination of a platelet count with an activated clotting time or prothrombin/partial thromboplastin times is appropriate before needle aspiration. Abdominal radiography/ultrasound examination is useful to help assure that the pulmonary mass is not metastatic from an abdominal tumor.
Thoracic ultrasound imaging is useful to localize lung lesions that are in contact with the thoracic wall and enables the sampling of pleural fluid. Ultrasound images of lung neoplasms are characterized by variable echogenicity and irregular borders. In the absence of vessels or a fluid-filled bronchus commonly observed with alveolar lung disease, pulmonary neoplasm is considered a likely diagnosis. Pleural effusion is readily seen and, because fluid improves resolution, soft-tissue lesions are enhanced, whereas pleural gas prevents visualization of solid structures in the lung.
Better diagnostic accuracy is achieved when needle placement for lesion aspiration is guided by ultrasound. Ultrasound guided fine-needle aspiration or tissue biopsy is most useful for lesions adjacent to the thoracic wall when little aerated lung is present between the mass and the thoracic wall to obscure visualization.
There are no distinctive hematologic or biochemical abnormalities that support a diagnosis of primary pulmonary tumors. Normocytic, normochromic anemia may be present.
Fine-needle cellular aspiration (blind or ultrasound-guided) is recommended even for dogs with diffuse pulmonary neoplasia. In most studies, it has been shown to be 80% to 90% successful in distinguishing a lung tumor from a nonmalignant cause and can be useful in identifying infectious agents as well. A specific cytologic diagnosis may be hindered by inflammation (nonseptic pyo- or pyogranulomatous inflammation) associated with lung neoplasms. Blind or guided fine-needle aspiration can be used to sample mass lesions and malignant pleural effusion. Pleural fluid obtained by percutaneous aspiration is often described as a modified transudate or exudate without evidence of neoplastic cells. Cytopathologic examination of tissue samples obtained by fine needle aspiration of the lung with a small-gauge (25 or 27 gauge, 1.5-inch needle on a 6-ml syringe is suggested) has been highly correlated with histopathologic examination of tissue obtained through thoracotomy. Cytopathologic diagnostic accuracy is improved with ultrasound guidance. Transthoracic fine needle aspiration is a minimally invasive procedure with rare to no adverse events reported after sampling. Many animals require no sedation or general anesthesia before a transthoracic fine needle aspiration.
When cutting core biopsy instruments are used to obtain a tissue sample (20 gauge Westcott needle, 18-20 gauge Menghini aspirate-biopsy needle [Tru-Cut]), complications are more frequent and potentially more severe. In one study, the overall diagnostic accuracy was 83%, but more than half of the animals had complications from the procedure. Asymptomatic pneumothorax was most common, and intrapulmonary hemorrhage, hemoptysis, and severe pneumothorax requiring removal of air were reported with rare deaths. Heavy sedation or general anesthesia is required. Transbronchial biopsy during bronchoscopic examination is seldom rewarding. Cytologic confirmation of neoplastic cells in fluid obtained from transtracheal wash or bronchoalveolar lavage is infrequently diagnostic, except in dogs with lymphoma.
Thoracotomy with lobectomy offers the opportunity for accurate histologic confirmation and is the also the treatment of choice for excisable tumors. Lymph node aspiration/biopsy can be obtained for clinical staging.
Although the equipment necessary for thoracoscopy is expensive, it is minimally invasive with a rapid postoperative recovery. It is useful for lung and regional lymph node biopsies and lobectomy. Most intrathoracic structures of the left and right hemithorax are endoscopically visible and a sound knowledge of the anatomy of intrathoracic structures is a prerequisite for diagnostic and therapeutic thoracoscopy. Because cardiac output was decreased even at low intrathoracic pressures, insufflation-aided thoracoscopy should be used with caution and at the lowest possible insufflation pressure.
Determining the extent of disease is important for prognosis. Thoracic radiographs along with a complete blood cell count and serum biochemical panel are the minimum tests for clinical staging, because the primary tumors, lymph node status, and presence of metastatic disease with or without concurrent illness can be assessed. The findings of palpably normal-size lymph nodes during thoracotomy or histologic metastasis-free lymph nodes have been demonstrated as predictors of longer survival when compared with dogs with enlarged or tumor-bearing lymph nodes. Dogs with a small (<2 cm diameter), solitary, or peripherally located mass have a better long-term outcome than dogs with large, multiple, or centrally located tumors.
Surgical excision whenever possible is the standard treatment for solitary lung tumors. Tracheobronchial lymph nodes should be palpated and, whenever possible, histologically evaluated. Three-view thoracic radiographs give an evaluation of lung parenchyma, suggesting that radiographically visible tumor, whether solitary or multifocal, is potentially completely resectable with complete or partial lobectomy via thoracotomy. CT may more accurately determine the extent of disease. If all visible tumor is removed and regional lymph nodes are normal size, most dogs with adenocarcinomas have a median survival time of more than a year after surgery. Histologic evidence of tumor in regional lymph nodes is associated with shorter disease-free interval and survival.