This article is all about lung cancer in dogs. We'll cover which dogs are more likely to develop lung cancer, the signs of lung cancer, types of cancer that affect the lungs, how we determine primary vs metastatic lung cancer, the recommended work-up, as well as treatment options.
Which Dogs Develop Lung Cancer (Pulmonary Neoplasia)?
The average age of dogs diagnosed with primary lung tumors (lung tumors that start in the lungs) is approximately 11 years. Breeds that might be at increased risk include: Boxer, Doberman, Australian shepherd, Irish setter, Bernese Mountain dog.
Second-hand smoke and urban living are thought to be potential causes of lung cancer in dogs.
An increased risk of lung cancer was noted in dogs with increased amounts of anthracosis (increased carbon in the lungs due to increased exposure to polluted air or inhalation of smoke or coal dust particles), which suggests a link between the development of lung cancer and the inhalation of polluted air. An experiment involving training dogs to smoke cigarettes (it was a long time ago and awful, I know) through a small opening in their neck leading to their trachea (tracheostoma) led to an increased development of lung cancer.
Signs of Lung Cancer
What might be surprising is that many primary lung tumors are diagnosed incidentally (up to 30%) on routine screening with chest x-rays (thoracic radiographs). This means that the patient doesn't have any signs of the tumor, we just find it because we're taking chest x-rays as part of a geriatric screen or routine pre-op work up for something else.
Dogs usually need to have a great deal of cancer in their lungs before they show clinical signs (behavior changes such as lethargy, decreased appetite, cough).
Signs and the respective likelihood of their occurrence include: coughing (52-93% of dogs), difficulty breathing (6% to 24%), lethargy (12% to 18%), loss of appetite (13%), weight loss (7% to 12%), coughing up blood (3% to 9%), and lameness secondary to a condition called hypertrophic osteopathy (4%).
Many of the signs are non-specific. Almost anything can cause lethargy, weight loss, and decreased appetite. This highlights the importance of a very thorough work-up if your dog isn't feeling well.
Recommended Work-up - Which Tests Are Needed?
Routine blood work (CBC, chemistry panel, urinalysis) are needed to help establish the overall health of the patient, and ensure that there aren't underlying conditions that might influence/affect treatment going forward (kidney or liver disease, urinary tract infection, etc.). Note that these tests are unlikely to tell us anything about the presence of a lung tumor.
The first step in identifying a lung tumor is to take chest x-rays (thoracic radiographs). This is typically how we first learn about the condition.
The radiographs should help us determine if the changes in the lungs are due to a primary lung tumor (cancer that started in the lung) or metastasis (cancer that started somewhere else in the body and spread to the lungs).
In each case (primary lung tumor vs metastasis), the size of the mass or masses is typically very different. When we have a primary lung tumor, we usually have a large lung mass. This tumor may spread to other lung lobes, in which case we will see a large lung tumor accompanied by smaller lung masses/nodules. If cancer started in another organ (the liver, a skin tumor) and spread to the lungs, we will see one or more smaller lung nodules.
Medicine is not black and white and determining whether a lung tumor is primary or metastatic is not always easy. We also have to take into consideration that there are other conditions that can sometimes look like lung cancer such as fungal disease or a benign granuloma. Your clinician will determine if either of these are a possibility based on what the radiographs look like and what area of the country you live in.
Look at the difference between a primary lung tumor (large mass on left, white arrows) and metastatic lung cancer (many smaller nodules/spheres on right) below.
When we have pulmonary metastasis (spread of cancer from somewhere else to the lungs), we have to figure out where the primary tumor is (in which organ did cancer start). In order to do that, we look throughout the body in places that commonly harbor highly metastatic tumors. This includes the oral cavity (perform a good oral exam, which may require sedation), anal sacs (perform a thorough rectal exam), abdominal organs (perform a thorough abdominal ultrasound; this should be done by a skilled ultrasonographer with a high quality machine and may require sedation; if an abnormal organ is noted a sample will need to be procured using ultrasound-guided fine needle aspiration and submitted to the lab for cytology).
The goal is to diagnose the primary tumor so that we understand which chemotherapy agent is best to help slow the cancer down; some cancers are more sensitive to chemotherapy treatment than others.
If radiographs suggest that we have a primary tumor (and there is no evidence that it has spread to other lung lobes or lymph nodes in the chest), the best treatment is surgical removal the affected lung lobe (if possible). Patients that undergo surgery will have the best outcome. In some cases, chemotherapy will be recommended after surgery.
If a family is interested in pursuing surgery, we recommend abdominal ultrasound (or abdominal CT scan) to rule out the presence of occult diseases that would limit life expectancy. We also need to perform thoracic (chest) CT. This test is critical in helping to ensure that the lung tumor is only present in one lung lobe. If CT shows obvious evidence of cancer in other lung lobes or lymph nodes, or shows that the tumor is too large for surgery, chemotherapy would be recommended instead of surgery.
If surgery is not being pursued, we would then want to sample the lung mass (if possible) to obtain a diagnosis and direct selection of the most appropriate chemotherapy agent. Aspirating a lung mass involves heavy sedation and ultrasound-guided aspiration.
A chest x-ray is not sensitive enough for pre-op planning for a lung lobectomy- CT must be used for the best outcome (one study showed that only 9% of CT-detected lung metastases were noted on thoracic radiographs and radiographs only detected 57% of lymph node metastasis, while CT detected 93%).
If your dog has a solitary lung tumor and you're pursuing surgery, I don't believe it makes sense to aspirate the tumor. Aspirating the tumor (in this case) runs the risk of spreading cancer cells from the lung lobe, through the thoracic cavity (this is not an issue if you are not doing surgery and need a diagnosis to guide chemotherapy selection...there are trillions of cancer cells in a large mass, spreading a few will not change life expectancy). When a surgeon removes a lung tumor, they take great care not to disturb the tumor and dislodge cancer cells. We want the cells contained in the single lobe as much as possible. Once the tumor is processed by the pathology lab (a lymph node should be submitted as well), we then find out more specific information about the tumor.
Surgery (partial or full lung lobectomy) is the treatment of choice for primary lung tumors. This procedure is usually performed by a board-certified surgeon. At the time of lung lobectomy, the surgeon should also biopsy/remove a hilar lymph node; if cancer has spread to the lymph node, it affects the patient's prognosis.
Patients are usually hospitalized for a few days after surgery for monitoring, comfort care, and to monitor their chest tube. They're usually sent home once the chest tube is removed.
If the CT scan shows that metastasis has occurred, chemotherapy is typically recommended instead. The type of cancer (adenocarcinoma, squamous cell carcinoma, histiocytic sarcoma) guides the type of chemotherapy recommended.
Stereotactic radiation therapy (treating the lung tumor with three treatments of highly targeted radiation) is being used more commonly with anecdotal evidence for improved survival. We don't yet have peer reviewed published studies describing outcome for this treatment for primary lung tumors; if you are interested, your oncologist can refer you to a radiation oncologist that provides SRT.
Lung Tumor Prognostic Factors - How Long Do Dogs Live?
NOTE: Median survival times (MST) are reported below. Remember, this means half of dogs lived longer and half didn't live as long in the research study.
Studies have shown that dogs that have lung tumors that are found incidentally live longer than those that have clinical signs for their disease at diagnosis such as lethargy, decreased appetite, and cough (MST 545 days vs 240 days).
Metastasis to the lymph nodes (in the chest) is associated with a shorter survival, which is why it's very important to biopsy these at surgery. Studies show a MST 30 days to 60 days if lymph node involvement is present (depending on the study) vs. 126 days to survival not reached for dogs without lymph node involvement (according to multiple studies). NOTE: Survival not reached means that the dogs were still alive at the time the study was published.
Dogs with well-differentiated tumors (MST 790 days) live longer than those with moderately (MST 251 days) or poorly differentiated (MST 5 days) tumors. *This is something we find out from the biopsy report when the tumor is removed. For patients that have moderately or poorly differentiated tumors, chemotherapy is recommended, which may improve survival.
In cases where surgery is able to remove all visible disease (we call this macroscopic or gross disease), dogs live longer than if gross disease was left postoperatively (MST 330 days vs 28 days).
Dogs with adenocarcinoma live longer than dogs with squamous cell carcinoma (MST 19 months vs 8 months). To complicate things further, certain types of adenocarcinoma have different prognoses (your oncologist would give you this information if relevant).
Dogs with small tumors (tumor volume less than 100cm3) live longer than those with large tumors (tumor volume larger than 100 cm3).
Obviously there are MANY prognostic factors. Most dogs have some combination of good and bad. Of course we cannot predict exactly how long an individual patient will live, but we use these studies and this information as a guideline in an attempt to better inform a family of how well a patient will do overall, whether surgery should be considered (does the family want to proceed with surgery knowing that their dog's very large tumor suggests a poor prognosis), and whether we should recommend chemotherapy.
Dr. Lori Cesario
Board Certified Veterinary Oncologist
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