Theresa W. Fossum, DVM, MS, PhD, DACVS
Veterinary Medicine & Biomedical Sciences, Texas A&M University, College Station, TX, USA
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.
The average age of dogs and cats with primary lung tumors is over 10 years. Anaplastic carcinomas tend to occur at a slightly younger age (8 to 9 years) than adenocarcinomas. There does not seem to be a gender or breed predilection, although boxers may be overrepresented.
Nearly 25% of dogs with pulmonary neoplasia are asymptomatic at the time of diagnosis (i.e., pulmonary neoplasia is an incidental finding when thoracic radiographs are evaluated for an unrelated problem). If clinical signs are present, the owner may report that they have been apparent for weeks to months.
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. Weight loss, lethargy, and dyspnea are common clinical signs in cats with primary lung tumors; respiratory signs may be present in as few as one third of affected cats. In one study, 47% of cats hospitalized with a primary problem of respiratory distress that had pulmonary parenchymal disease on thoracic radiographs had neoplasia.
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.
Ultrasound- and CT-guided aspiration of thoracic masses may be performed. Diagnoses obtained by fine-needle aspiration (ultrasound-guided and blind) cytopathology accurately reflect the diagnosis obtained on histopathologic examination. CT-guided intrathoracic fine-needle aspiration and core biopsies are also diagnostically accurate. Complications include pneumothorax and pulmonary hemorrhage that rarely require treatment. Use of fine-gauge (25- or 27-gauge) needles may reduce complications.
Wide surgical resection is the treatment of choice for solitary nodules or multiple masses involving a single lobe if there is no evidence of distant metastasis or extrapleural involvement. Surgical resection occasionally is indicated for lung metastasis of a distant primary tumor (e.g., limb osteosarcoma). An intercostal thoracotomy is preferred over median sternotomy because it provides adequate exposure for lobectomy and lymph node biopsy. Partial lobectomy should be performed only when the tumor is located at the periphery of the lung lobe; otherwise, total lobectomy should be performed. Thoracoscopy can help determine whether pulmonary metastasis is present before a thoracotomy, particularly if the presence of metastasis is an important factor in determining whether resection of the pulmonary mass should be performed. Thoracoscopic removal of primary lung tumors in dogs is possible.
With large neoplastic lesions, positioning the animal in sternal recumbency or in lateral recumbency with the affected side down, and providing oxygen (i.e., nasal insufflation or oxygen cage) often is beneficial. Blood gas analysis or evaluation with pulse oximetry is warranted preoperatively in patients undergoing thoracic surgery to detect and define the severity of respiratory impairment. Unexplained abnormalities should be investigated because ventilatory impairment caused by nonsurgically correctable disease (i.e., diffuse micrometastasis) occasionally is identified. If possible, significant anemia should be corrected before surgery.
Thoracoscopic Biopsy or Partial Lobectomy
For larger masses requiring partial lobectomies, use thoracoscopy to determine the optimal position for making a minithoracotomy (keyhole) incision (thoracoscopic-assisted partial lobectomy). The size of the incision will depend upon the size of the lesion and the lung lobe to be exteriorized. Grasp the affected lung lobe with Babcock forceps, and pull it out of the chest until a stapling device (e.g., thoracoabdominal stapler, TA-30 or TA-55) can be placed between the mass and the hilus. Staple and resect the affected portion. Check the incision site for leakage, and then reposition the lung in the chest.
Partial lobectomy may be performed to remove a focal lesion involving the peripheral one half to two thirds of the lung lobe or for biopsy. Partial lobectomy may be performed through a lateral fourth or fifth space intercostal thoracotomy or median sternotomy. Identify the lung tissue to be removed, and place a pair of crushing forceps across the lobe proximal to the lesion. Place a continuous, overlapping pattern of absorbable suture (2-0 to 4-0) 4 to 6 mm proximal to the forceps. If necessary, place a second row of sutures in a similar manner. Excise the lung between the suture lines and clamps, leaving a 2- to 3-mm margin of tissue distal to the sutures. Oversew the lung in a simple continuous pattern with absorbable suture (3-0 to 5-0). Replace the lung in the thoracic cavity and fill the chest cavity with warmed sterile saline solution. Inflate the lungs and check the bronchus for air leaks. Remove the fluid before closing the thorax.
Partial lobectomy may also be performed with stapling devices (e.g., TA stapler). The stapling equipment comes in various sizes, which produce staple lines 30 mm, 55 mm, or 90 mm long. Select the staple size based on the width of the lung, so that the staple line extends across the entire width of the lung to be removed, but not beyond the edges. If air leaks or hemorrhage are noted, place a simple continuous pattern of absorbable suture along the lung margin. The stapling devices compress tissue to a thickness of 1.0 mm (2.5-mm staples; 30 mm length only), 1.5 mm (3.5-mm staples), or 2 mm (4.8-mm staples). Avoid stapling excessively thick or fibrotic lung because this may result in large air leaks or hemorrhage. Check the lung for leaks and close as described above.
Complete lobectomy is best performed through a lateral thoracotomy. Dogs can survive acute loss of up to 50 percent of their lung volume; however, transient respiratory acidosis and exercise intolerance may occur. Pneumonectomy causes compensatory changes in the contralateral lung and myocardium, even in normal dogs. Although residual lung volume, vital capacity, and maximal breathing capacity are substantially decreased initially, residual lung volume increases significantly after 3 months. Identify the affected lobe or lobes and isolate them from the remaining lobes with moistened sponges (laparotomy or 4 x 4s, depending on the animal's size). Identify the vasculature and bronchus to the lobe. Using blunt dissection, isolate the pulmonary artery supplying the affected lobe and pass a ligature of nonabsorbable or absorbable suture (2-0 or 3-0) around the proximal end of the vessel. Do not compromise the lumen of the parent vessel from which this vessel arises. Place a second ligature in a similar fashion distal to the site where the vessel is to be transected. A transfixing suture may be placed between these sutures proximal to the transection site to prevent the first suture from being inadvertently dislodged. Transect the artery between the distal two ligatures. Ligate the pulmonary vein in a similar fashion. Identify the main bronchus supplying the lobe and clamp it with a pair of Satinsky or crushing forceps proximal and distal to the selected transection site. Sever the bronchus between the clamps and remove the lung. Suture the bronchus proximal to the remaining clamp in a continuous horizontal mattress pattern or, in cats and small dogs, place a transfixing ligature around the bronchus. Before removing the clamp, secure a suture in the bronchus distal to the clamp. After removing the clamp, oversew the end of the bronchus in a simple continuous suture pattern. Fill the chest cavity with warmed sterile saline solution. Inflate the lungs and check the bronchus for air leaks. Before closure, check lungs that have been "packed off" to make sure they reinflate and are not twisted. Remove the fluid and close the chest as described above.
A Miller's knot or transfixation suture can be used successfully to ligate the vessels and bronchus in many animals. Stapling devices (e.g., 30 mm TA Stapler, white [2.5 mm staples] cartridge) may also be used for complete lobectomy, but make sure the bronchus and vessels are adequately ligated by the staples.