Thyroid tumors comprise 1.2-3.8% of all tumors in dogs and the majority (about 90%) are found to be malignant (known as thyroid carcinoma or thyroid adenocarcinoma). Breeds that have a higher risk of developing this type of cancer include Beagles, Boxers and Golden Retrievers although any dog can develop a thyroid tumor. Most dogs with thyroid tumors are older (>10 years).
The most common sign is the finding of a mass in the neck. In most cases, these tumors are present on only on one side of the neck (unilateral) but there is a small percentage of dogs who around found to have cancer in both thyroid glands (bilateral). If the mass grows large enough, it could cause clinical signs which may include coughing, difficulty breathing or swallowing and rarely swelling in the face.
The majority of these tumors are considered to be non-functional (this means that they are not producing any thyroid hormone). Some dogs with this type of cancer are actually hypothyroid (have low levels of thyroid hormone). If the tumor is producing thyroid hormone, dogs can have signs of hyperthyroidism, which can include drinking and urinating more, weight
loss, increased panting and restlessness.
A diagnosis of a thyroid tumor can be made either by a needle aspirate or biopsy. These tumors often have a very well developed blood supply which means that an incisional biopsy (only taking a small piece of the mass) can be challenging and is not recommended due to the high risk of bleeding.
Not only can thyroid tumors be locally aggressive and invade into surrounding tissues as it grows, but up to 35% of patients have evidence of metastasis at the time of diagnosis. The most common sites of metastasis include the local lymph nodes and lungs. To assess the extent of disease, staging tests are recommended and include:
- Chest radiographs determine if there is any evidence of disease in the lungs.
- Blood work is recommended and consists of a CBC (which checks the red and white blood cells and platelets), a chemistry panel to look at organ function, a urinalysis and a thyroid level.
- If surgery is going to be considered, a CT scan of the neck (and possible the chest) may be recommended as the next step after staging to help determine whether or not surgery will be successful and look a little closer for any evidence of metastasis. CT image shows large thyroid tumor (arrows).
Surgery is the treatment of choice whenever possible. Removal of these tumors can be complicated given the well developed blood supply and the fact that these tumors surround vital structures such as the esophagus and trachea. Whether surgery alone is enough or if radiation therapy and chemotherapy will be needed following surgery depends on the size and invasiveness of the tumor as well as the final biopsy report. Size of the tumor and invasiveness are predictors of outcome. Smaller tumors that are freely movable carry a better prognosis than larger or fixed thyroid tumors.
There are two different ways that radiation therapy can be used. The decision about what type of radiation therapy would be best is based on a number of different factors and the radiation oncologists recommendations.
For those dogs where the tumor can be removed but there are still tumor cells that remain at the surgical site, full course radiation therapy may be recommended. This type of radiation therapy involves the use of 15-20 treatments which is administered Monday through Friday for 3-4 consecutive weeks.
For those dogs whose tumors cannot be removed, we can consider using full course radiation therapy as described above although our ability to control the tumor is not as good due to the larger amount of tumor that needs to be treated.
For dogs that have tumors that cannot be removed, we can also consider a modified schedule of radiation therapy called palliative radiation therapy. This is a less intensive course of radiation therapy with fewer side effects but the trade-off is that it is not expected to control tumors for as long. Rather the goal of palliative radiation therapy is to focus on the patient’s quality of life. This type of radiation therapy involves the use of 3-4 treatments of radiation therapy with treatments being given at weekly intervals (ie. once weekly for 3-4 consecutive weeks).
Chemotherapy is frequently recommended due to the potential for metastatic disease, however, the treatment outcome is difficult to predict. Chemotherapy is most often discussed for tumors that cannot be removed, tumors that are larger than 3 cm, and/or tumors where there is invasion into the blood vessels or evidence of metastasis. Dogs that already have spread to the lungs at the time of diagnosis may also benefit from
chemotherapy. Chemotherapy is typically not used alone but is most often combined with surgery and radiation therapy.
- The most commonly used drugs for this type of cancer are Palladia, Adriamycin (doxorubicin) and Carboplatin.
- Doxorubicin and carboplatin are administered intravenously as an outpatient once every 3 weeks for a total of 4-6 treatments when combined with surgery. Palladia is an oral chemotherapy agent that is administered at home. Fortunately, the risk for side effects from chemotherapy is low and can include stomach upset (decrease in appetite, nausea, vomiting, diarrhea and/or lethargy), decrease in white blood cell counts and possible thinning of fur coat. Less than 10% of patients will need to come in to the hospital for outpatient care due to chemotherapy side effects and less than 1% will require hospitalization or experience any life threatening side effects.
- The goal of chemotherapy is to help improve your pets quality of life and we will work with you to achieve this goal.
Radioactive iodine has been used for the treatment of thyroid tumors in dogs although it is not commonly used due to technical issues. There are currently only a few facilities in the USA that are able to treat dogs with radioactive iodine so patients need to travel to one of these facilities. In addition, due to the doses that are needed for treatment, patients may need to remain in isolation for several weeks following treatment which makes this type of a treatment less practical.
The prognosis with treatment depends partly on the size of the tumor and whether or not the tumor is freely movable or is fixed to the underlying structures. The prognosis is also influenced by the rate of growth of the tumor as we sometimes see tumors that are very slow to progress even in advanced stages.
For tumors that are small and freely movable, the prognosis with surgery alone is considered to be good with a median survival time of 2 years.
Larger tumors that can still be removed by surgery may require radiation therapy and/or chemotherapy after surgery. The survival time of these patients may not be as long but in our experience, the median survival time is 18-24 months with combination treatment.
For tumors that cannot be removed, the recommended treatment would be radiation therapy (either full course or palliative radiation therapy) AND chemotherapy. Dogs that fall into this category can experience survival times of 6-12 months.
Thyroid carcinoma is not usually a cancer that is curable, but with treatment, can be controlled while maintaining your pet’s good quality of life. Your oncologist will be able to discuss the treatment options, prognosis and help guide you as to the most appropriate plan for your beloved family member.