Thyroid Tumors (Adenocarcinoma) in Dogs

What are thyroid tumors?

Thyroid tumors in dogs are relatively uncommon. Dogs have two thyroid glands, which are located alongside the windpipe in their neck. This gland produces thyroxin, a hormone that regulates the body’s metabolism. Golden retrievers, beagles, and boxers between the ages of 9 and 11 are most common breeds that develop this tumor. These tumors are almost always malignant, meaning that they tend to spread to the lungs and lymph nodes and can grow into nearby structures such as the windpipe, blood vessels, and esophagus. At the time of diagnosis, 35 – 40% of affected dogs will have visible evidence of the tumor’s spread to other parts of the body (especially the lungs). As many as 60% of affected dogs will have the cancer that affects both thyroid glands, which causes the glands to malfunction in 50% of these cases. In about 10% of cases, the tumor will produce excessive amounts of thyroid hormone, which can have systemic side effects. 

 

Signs and diagnosis

The most common clinical sign of a thyroid tumor is a mass in the neck. Other signs usually are due to the tumor affecting the nerves of the voice box, and may include a change in the tone of the dog’s bark, harsh breathing sounds, difficulty breathing, and rapid breathing. If the nerves that control swallowing have been affected or the tumor is pressing on the esophagus is, difficulty in swallowing may be noted. Other less common findings include bruising under the skin and swelling of the head. If the tumor produces excessive amounts of thyroid hormone, weight loss, marked increased appetite, muscle wasting, increased thirst, and increased urination may be seen.

For patients with a suspected thyroid tumor, a complete blood count, chemistry profile, and urine testing are completed in the first stages of diagnosis. Blood tests can then be run to see if the thyroid glands are either functioning normally or are hyperactive (overactive). A fine needle biopsy (sample) of the tumor provides a diagnosis about 50% of the time. Neck ultrasounds can often show the extent of the tumor and may find that the tumor has spread to lymph nodes in the neck. Chest X-rays are important to identify the visible spread of tumor to the lungs. It is important to note that no imaging test can identify the microscopic spread of cancer to the lungs and other internal organs.

It has been estimated that only 25 – 50% of patients that have a thyroid tumor are candidates for surgery. Only tumors that are not attached to the underlying tissues in the neck should be removed. Surgery involves making an incision in the underside of the neck, separating the muscles that cover the thyroid gland and then removing the thyroid tumor. Both thyroid glands are inspected, as some patients have tumors in both glands.

Radiation therapy is used to treat thyroid cancer that is invasive into underlying tissues.Typically, one daily radiation treatment is administered from Monday to Friday with the weekends off for a total of 18 to 21 treatments. By administering multiple small doses of radiation, radiation’s side effects can be decreased.

Thyroid tumors tend to be moderately responsive to chemotherapy; therefore, there is value in this mode of treatment. Unlike humans, most dogs that receive chemotherapy do not lose their hair and usually have only mild side effects, which may include an occasional loss of appetite and vomiting.Treatment with radioactive iodine can be successfully used to treat some thyroid tumors. Since this treatment requires a high dose of radioactive iodine, most hospitals do not offer it due to staff safety concerns.

 

Prognosis

Negative prognostic factors for patient survival include a large-sized tumor (tumors larger than 100 cm have a 100% spread rate), the presence of measurable metastasis, bilateral tumors, immovable tumors, and tumors that cause paralysis of the voice box. 

One study with 20 patients that received only surgery for the treatment of freely removable thyroid tumors that had not spread to the lungs or lymph nodes found average survival times of 20.5 months. Of these patients, 8 died of non-tumor related diseases. With the removal of these patients from the study, the average survival time was more than 36 months. Therefore, surgery is the treatment of choice in patients with movable tumors. 

Another study included 13 patients that were treated with cisplatin chemotherapy. Most of the patients had tumors that were invasive and not freely movable. One dog had complete remission and 6 dogs had partial remissions; 3 dogs had stable disease and 3 did not respond at all. The average survival time was 191.8 days.

One study included a group of dogs with infiltrative thyroid carcinomas that were solely treated with radiation therapy. These tumors shrunk to undetectable disease in all dogs. Spreading to the lungs was seen in 3 dogs and to the bone in another dog. Overall average survival time was 24.5 months.

Another study with 13 dogs that had invasive thyroid carcinoma showed an average survival time of 96 weeks. Four dogs died of metastatic disease and 4 from progression of the disease. It is concluded that radiation therapy is a better mode of treatment than chemotherapy alone. Although the studies have not been published, chemotherapy combined with radiation therapy may be more beneficial than a single mode of treatment.