Lung Cancer

In dogs and cats, tumors affecting a lung lobe (“pulmonary” tumors) are most commonly caused by cancer cells that have traveled through the bloodstream and implanted in lung tissue. These are termed metastatic tumors. Primary lung lobe tumors—those that originate within the lung itself—are far less common. Most primary tumors arise from glandular tissue and are often classified as adenocarcinoma or bronchogenic carcinoma. Less commonly, they may develop from supporting tissues of the lung, such as:

• fibrosarcoma (from fibrous tissue)
• chondrosarcoma (from cartilage)
• hemangiosarcoma (from blood vessels)
• or other tissue sources

Dogs and cats living with smokers, or in environments with certain pollutants (for example, asbestos), have a higher risk of developing primary lung lobe tumors. Brachycephalic breeds (bulldogs, pugs, etc.) are also at increased risk compared with long-nosed breeds.

Pets with primary lung tumors are typically middle-aged or older. Roughly 25% of dogs and cats with a primary lung tumor show no clinical signs; tumors may be found incidentally when chest radiographs are taken for another reason. When signs occur, they may include:

• cough
• weight loss
• lethargy

Large tumors, or those compressing important structures, can cause breathing difficulty or regurgitation, where food comes back up abruptly after eating. Some animals develop lameness because the lung tumor has spread elsewhere or because it triggers swelling and bone changes in the toes and lower limb bones, a condition called hypertrophic osteopathy.

Figure 1. Lung lobe tumor in a caudal (rear) lung lobe of a dog measuring 47.8 mm (nearly 2 inches) in diameter.

Lung tumors are identified using chest x-rays or a chest CT (computed tomography) scan (Figure 1). On radiographs, lung tumors are difficult to detect until they reach at least about 0.5 inch in diameter. Tumors that are large or near the chest wall can sometimes be sampled with a needle and syringe, and the sample is evaluated microscopically to determine tumor type. Because affected animals are commonly older, blood and urine tests are assessed to look for other concurrent disease. Screening tests—such as abdominal ultrasound—may also be recommended to search for a primary tumor elsewhere in the body.

A single lung lobe tumor is treated with surgical removal, either through a larger incision on the side of the chest or through minimally invasive methods using several smaller incisions and a thoracoscope. When multiple lung lobe tumors are present, metastatic disease (spread from another site) is most likely, and alternatives such as chemotherapy may be considered.

Dogs and cats have a right and left lung, and each lung is divided into lobes that can be removed individually. Most lung lobe tumors are removed through a lateral chest incision called a thoracotomy, typically positioned just behind the front leg. The incision passes between ribs, which are gently separated and then re-approximated after the lobe is removed. The blood vessels and the airway tube (bronchus) supplying the lobe are sealed either with sutures or a stapling device; staplers are commonly used because they are faster and simpler than hand suturing.

A chest tube is placed to remove air or fluid after surgery and can also be used to administer local anesthetic to reduce pain. Chest tubes are commonly left in place for 12–24 hours postoperatively. Because these procedures are painful, multiple pain-prevention and pain-control strategies may be used, including:

• epidural anesthesia (pain medication injected around the spinal cord)
• intravenous medications
• fentanyl patches (transdermal drug delivery)
• tablets or liquid medications once the pet is awake

Following surgery, the pet remains hospitalized until the chest tube is removed, breathing is stable, and pain can be controlled with oral medications. Activity is restricted for 10–14 days to allow healing. Some dogs may wear a bandage for 1–2 weeks after surgery, or an Elizabethan collar to prevent licking or chewing at the incision. Depending on the microscopic diagnosis of the removed tissue, chemotherapy may be recommended. Your veterinarian may also suggest follow-up chest radiographs at future visits to monitor for spread or recurrence.

Anesthesia carries risk in older animals, and risk increases in those with airway disease. Many pets require ventilation during surgery to maintain adequate oxygenation. Bleeding or air leakage may occur if vessels or the bronchus fail to seal appropriately. Pain control is essential for several days after surgery because painful animals may not inflate their lungs well and can develop low oxygen levels. Despite these potential issues, most dogs and cats do survive the procedure.

Because the incision is located just behind the front leg, some dogs may have difficulty walking—especially on stairs—for up to two weeks after surgery, and swelling along the incision line is common for several days.

In many cases, average survival after lung lobectomy for a primary lung lobe tumor is one year or longer. Survival tends to be longer when the tumor is small or located at the lung lobe periphery (15–17.5 months), when lymph nodes are small (20 months), and when the tumor can be completely removed. Tumor type also matters: survival is longer in dogs with completely excised adenocarcinomas (19 months) than in dogs with squamous cell carcinomas (8 months). The most favorable prognosis occurs in pets with small-diameter, well-differentiated papillary carcinomas identified before clinical signs develop and with no evidence of lymph node spread. Cats are more likely than dogs to have spread of primary lung tumors, so prognosis in cats may be less favorable.

Because secondhand smoke increases lung tumor risk, the best available prevention is for owners to stop smoking. Even when owners smoke outdoors, smoke residue can be carried indoors on hair and clothing.