The lung is the essential respiration organ whose principal function is to transport oxygen from the atmosphere into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere. There are two type of lung cancer diagnosed in dogs. The first is primary lung cancer which is defined as lung tumors that originate in the lung tissue. The second type is metastatic lung cancer whichis cancer that originates elsewhere in the body such as a leg bone, the mouth, or the thyroid gland, but has spread to the lung via the bloodstream.
Primary lung cancer, or tumors originating in the lung, are relatively uncommon in dogs (less than 1% of all cancers in dogs), although the number has been increasing. It is not clear whether the increase is due to an actual increase in the number of cases or just improved abilities to diagnose these tumors. Primary lung tumors are almost always malignant and are usually carcinomas (often adenocarnimas), although can be hemangiosarcomas. They usually present as a large solitary mass visible in the lung on a chest x-ray.
Carcinomas develop from the epithelial tissues in the lungs. They may be primarily derived from the lung tissue itself, or the airways or bronchioles. Canine lung cancer is aggressive and frequently metastasizes to the regional lymph nodes and tissues of the thorax. Adenocarcinoma has a tendency to metastasize (spread) to the central nervous system. Adenocarcinomas are further classified based on their location (eg bronchial, bronchoalveolar, or alveolar carcinoma).
A classification system based on the prevailing histologic pattern may be used to categorize most primary lung neoplasms as adenocarcinomas, squamous cell carcinomas, anaplastic carcinomas, or bronchioloalveolar carcinomas. Only the neoplastic cell type and the presence or absence of metastatic disease have been found to be of value as prognostic indicators.
Metastatic lung tumors are those that spread from a primary tumor elsewhere in the body. The lungs are a common site to which other types of cancer spread, such as with dogs diagnosed with bone cancer. Metastatic lung tumors are usually found in multiples, not as a single mass. Be aware that dogs with multiple lung masses may be the result of a fungal infection and not cancerous. That is why it's imperative to obtain an accurate diagnosis if lung masses are found in your dog.
The reason why a dog develops cancer, is generally not straightforward. Cancer is non-lethal genetic damage of cells (mutations in the DNA). Mutations have been found in canine lung cancer with changes in the regulation of cell death and replacement deviate from the norm. Little is known about the exact causes of these mutations, however,some chemicals may be involved in initiating or promoting the cancers. There may be a genetic factor at play as well.
The environment in which your dog lives is an important factor to the potential threat of developing lung tumors. Just as with humans, secondhand smoke in the dog's home due to a smoking owner correlates with an increased incidence of lung cancer in dogs.
According to the Centers for Disease Control and Prevention, more than 126 million Americans who don't smoke are exposed to secondhand smoke in their homes, vehicles, workplaces, and public places. This exposure causes thousands of lung cancer and heart disease deaths among nonsmokers every year, according to the California Environmental Protection Agency.
Making the leap from the effects of secondhand smoke on humans to their effects on pets isn't a big one, says veterinarian Carolynn MacAllister of Oklahoma State University. "There have been a number of scientific papers recently that have reported the significant health threat secondhand smoke poses to pets," MacAllister said. "Secondhand smoke has been associated with oral cancer and lymphoma in cats, lung and nasal cancer in dogs, as well as lung cancer in birds." Dogs living in home with smokers have a 60% risk for developing this form of cancer.
Studies have also shown that dogs with short and medium-noses are more susceptible to lung cancer, "because their shorter nasal passage aren't as effective at accumulating the inhaled secondhand smoke carcinogens," MacAllister said. "This results in more carcinogens reaching the lungs."
Asbestos can also be a cause of a specific form of lung cancer, called mesothelioma, just as it is in humans.
Male and female dogs get lung cancer at approximately the same rate. There may be a slight increase in risk associated with living in an urban area. There are no known predispositions with respect to breed and the development of lung tumors. However, most lung cancer is seen in medium to large sized dogs and is most frequently diagnosed in older dogs. The average age of diagnosis is 11 years.
Short-nosed breeds exposed to cigarette smoke in the home have twice the risk of getting lung cancer as medium- or long-nosed breeds exposed to a similar amount of cigarette smoke.
Symptoms of lung cancer are coughing (which may also produce phlegm or blood), exercise intolerance (lethargy), weight loss or loss of appetite, and other respiratory signs such as trouble breathing or shortness of breath.
The most common abnormal clinical sign reported in dogs with lung cancer is chronic cough. Usually this is a non-productive cough, which means the dog is not coughing up fluid or mucus. Occasionally pets will cough up small amounts of blood or phlegm. If the tumor is large and is causing compression of the trachea or a major airway, the animal may experience difficulty breathing or dyspnea. Other causes of dyspnea associated with lung cancer include fluid accumulation around the lungs, known as pleural effusion, and widespread cancerous involvement of the lungs, leaving little normal lung tissue.
The clinical signs can also be vague and not specific to the respiratory tract. Approximately 25 percent of pets with lung cancer may not show any clinical signs of illness at all. Occasionally dogs with lung cancer will limp or show signs of lameness. This can occur because of spread of tumor to the bones or due to a secondary effect that the tumor has on bone growth. The latter condition results in excess bone growth and swelling of the limbs, and is referred to as hypertrophic osteopathy. Several other conditions may cause similar clinical signs to those seen in animals with lung cancer which include heart failure, pneumonia, metastatic cancer, heartworm infection, fungalinfection, lungworms, or collapsing trachea.
In order to accurately and definitively diagnose primary lung cancer in your dog, your vet will need to perform a physical exam as well as order several tests. A complete medical history and physical exam is important because it will establish a list of possible diagnoses. A physical exam may reveal abnormal or muffled lung sounds in your dog which may indicate dyspnea (difficult or labored breathing or shortness of breath). A vet will also listen to the heart to rule in or rule out heart disease as a likely cause of the symptoms. Many animals with fungal infections will also have enlarged lymph nodes or skin lesions.
After the physician exam, your vet may order any or all of the following tests and procedures to help determine the correct diagnosis.
Complete blood count - a CBC evaluates the red and white blood cells as well as the platelets. The results of this test are often normal in dogs with lung cancer, but it will help rule out infectious causes of the symptoms.
Biochemical profile - evaluates blood sugar, blood proteins and electrolytes, as well as providing information about liver and kidney function. It is helpful for the vet to have an overall assessment of health, and may determine the need for further diagnostic testing.
Urinalysis - is part of a complete laboratory assessment and gives a better indication of kidney function than the biochemical profile alone.
Chest radiographs - X-rays of the chest are probably the single most important tool in making a preliminary diagnosis of lung cancer. X-rays will read looking for the presence of lung tumors, fluid in the chest cavity, the size of the heart and associated blood vessels, and an evaluation of the lung tissue. What are usually found on a x-ray depend upon the type of lung cancer. Primary lung cancer are frequently located in the caudal (towards the hind end of the pet) lung lobes, however can be located in any lung lobe and are usually a single mass in the lungs, unless the tumor has spread.
Metastatic lung tumors frequently are found in multiples and invade a variety of lung lobes. However, occasionally a metastatic tumor spread from elsewhere in the body may appear as a single mass in the lungs. If your dog has a fever and has been traveling to southern United States, the masses in the lungs could be due to fungal infection and can be treated with medication.
A CT scan may be ordered in lieu of or in addition to a standard x-ray. CT scans provide greater clarity and reveal more details than regular x-ray exams. It is one of the best and fastest tools for studying the chest because it provides detailed, cross-sectional views of all types of tissue. It is often the preferred method for diagnosing many different cancers, including lung since the image allows a physician to confirm the presence of a tumor and measure its size, precise location and the extent of the tumor's involvement with other nearby tissue.
Abdominal X-rays or abdominal ultrasound exam - Imaging studies of the abdomen may not be required in every case, but evaluation of other organs is a good screening test for evidence of metastasis of a primary lung tumor to other sites. Although many tumors spread from other sites to the lungs, primary lung tumors can spread throughout the lungs, as well as to other parts of the body.
Fine needle aspirate of the lung mass - If there is a mass that is big enough and close enough to the chest wall, an aspirate of the mass may be attempted with a hypodermic needle and syringe. This involves passing a needle through the chest wall and inserting it into the mass, then gently creating suction on the syringe to remove microscopic cells for evaluation. This is a fairly safe procedure, but should be done using ultrasound guidance to determine the exact location of the mass. The dog may need to be sedated for this test.
If the dog has pleural effusion (abnormal accumulation of fluid) this can be safely and routinely removed from the chest without ultrasound guidance. Removal of fluid may help the dog breathe easier as well as provide fluid for analysis and possible diagnosis. These methods may allow the doctor to make a diagnosis without undertaking a more invasive procedure, but it should be noted that a sample from a fine needle aspirate is never as good as a piece of tissue for biopsy evaluation.
Trans-tracheal aspirate or bronchoscopy - A trans-tracheal aspirate is a procedure in which sterile fluid is introduced into the trachea and fluid and cells are suctioned out. This can sometimes be another method to obtain a diagnosis. However, this is often more useful if there is evidence of widespread lung disease rather than a single mass. This test can be performed under light sedation.
A bronchoscopy study involves placing a scope into the trachea and smaller airways. The interior of the airways can be examined by this method and samples can also be obtained for analysis. Single masses in the lung tissue cannot be seen via this method, but a mass associated with one of the airways may be sampled using this technique.
Biopsy of the lung mass - Tissue samples from a lung mass are often the only definitive way to make a diagnosis of lung cancer and help to specify which form of lung cancer it is. The most common procedure to obtain a sample of the mass is exploratory thoracotomy, which involves opening the chest cavity surgically. Often the mass can be completely removed while performing this procedure. Therefore, surgery can play both an important diagnostic as well as therapeutic role in management of the disease.
A second potential way to obtain a lung mass biopsy is by thoracoscopy. This procedure is performed using a scope, which is placed into the chest through a smaller incision than what would be used for surgery. The scope is a long tube with an attached camera, which allows visualization within the chest cavity. Depending on mass location, a biopsy may be taken using this method. However, it is unlikely that the mass could be removed using the scope. Finally, in certain cases, ultrasound guided biopsies may be performed. This is the least invasive way to obtain a biopsy, but is not always the best way to diagnose lung cancer as the sample size will be smaller than with a thoracoscopy and may be a riskier procedure for some dogs.
The diagnosis of metastatic lung cancer can be done with any of the above methods. However, it is generally accepted that if there is a known tumor somewhere in the body and masses are found in the lungs, it is presumed that these are metastatic tumors and many of the invasive procedures to diagnose the disease are not necessary. Surgical excision of the tumor is often the first step used to treat canine lung cancer if the dog is otherwise in good health. It is the treatment of choice for dogs with primary lung tumors. However, depending on the size and location of the mass, complete or even partial removal may not be possible. If there is evidence of widespread involvement, surgical removal is generally not an option.
In the majority of primary single lung tumors, the lung lobe that is involved is totally removed with the mass. This is called a thoracotomy and lung tumor removal. Usually the tumor is removed through the side of the chest cavity (lateral thoracotomy). In some cases the breast bone must be divided (median sternotomy) so that both left and right lungs can be accessed. If needed, the entire left lung can be removed at one time, as the right lung will take over for the left. Your dog may not do as well if the entire right lung (all lobes) needs to be removed.
Please be aware that this is major surgery and will require time and dedicated home care for your dog to recover fully. Post-surgical responsibilities will include:
- Limit exercise to short leash walks for 3 weeks if a lateral thoracotomy was performed; limit exercise for 6 weeks if a median sternotomy was performed
- Administer oral pain medications as needed
- Monitor respirations to make sure that these are not labored
- Monitor gums and tongue to make sure that these are pink
- Encourage eating
- Monitor the incision for signs of infection
- Do not allow licking or scratching of the incision
Depending on the type of tumor, chemotherapy or radiation therapy may be recommended in addition to surgery to prevent the spread or resurgence of malignant tissue.
If surgery is not an option, chemotherapy may be used to slow the progression of disease. Although, there is little evidence to show that chemotherapy is very effective in most primary lung cancers, there are reports of some dogs doing well (see Snuggy’s story). Both vinblastine and/or carboplatin are generally used as the chemotherapy drug. Radiation therapy is sometimes used for the treatment of lung cancer in dogs depending upon the proximity of the lung tumor to the heart.
Metastatic lung cancer can be treated with radiation or chemotherapy. Surgical intervention is not usually recommended but may be possible depending on certain clinical criteria. If there are three or fewer lung nodules affected, surgery can be done to remove these in order to increase the lifespan of your dog. This is most commonly done with osteosarcoma metastatic disease
Survival times following conventional standard treatments range from two months to two years, depending primarily on whether the cancer has spread to lymph nodes or elsewhere in the body before surgery.
Because lung cancers are highly dependent upon angiogenesis for their growth and spread, antiangiogenic therapy may be useful for this condition. Studies of lung cancer in animals have clearly shown that the greater the number of microvessels within the lung cancer, the worse the prognosis. Angiogenesis inhibition has slowed lung cancer growth significantly in animals. Researchers have shown that a drug mimicking the natural angiogenesis inhibitor thrombospondin-1 can inhibit lung cancer in animals. A similar drug, called ABT-510, is now in veterinary trials for canine cancer. In human patients, other antiangiogenic drugs have been used to treat lung cancer, including Avastin, Neovastat, TNP-470, squalamine, and endostatin.
Because Avastin is not available for use in dogs at this time (in humans the cost can run up to $100,000 per year), some vets are prescribing metronomic or low dose chemotherapy with Deramaxx (a non-steroidal anti inflammatory drug (NSAID) and an oral chemotherapy drug, cyclophosphamide (cytoxan). They believe this type of chemotherapy protocol works by inhibiting new blood vessel formation which is required for new tumor growth, similar to antiangiogenic therapy. This type of chemotherapy has not been specifically investigated for lung cancer in dogs. It has shown some benefit in delaying the recurrence of hemangiosarcoma and soft tissue sarcomas in dogs. This protocol may be worth trying if other chemotherapy agents do not prove useful in fighting this form of cancer.
The prognosis is generally good for dogs with primary lung cancer presenting with a single, small mass in their lungs that has not spread to the lymph nodes or other tissue. In this group, more than 50% are expected to live 1 year after the surgical removal of the mass. A recent study of 67 dogs diagnosed with primary lung cancer showed that the prognosis depends on several factors such as the cancer histological type, grade, how advanced the cancer is and whether the pets had symptoms at the time of diagnosis The overall median survival of these dogs was 1 year. Dogs with lower grade tumor had a median survival of 22 months whereas dogs with higher grade tumor had a median survival of 6 months. Dogs who showed symptoms had a median survival of 8 months compared to 18 months in those without symptoms.
Following wide surgical resection of the pulmonary mass, dogs with pulmonary adenocarcinomas are reported to have a longer mean survival time than animals with other types of primary lung carcinomas.