Thyroid Carcinoma in Dogs
What are Thyroid Tumors?
Thyroid tumors originate in the thyroid gland located along the trachea. The thyroid gland is responsible for producing thyroid hormone which plays a role in regulating metabolic rate and other functions. Despite its important role in producing hormones, the majority of thyroid tumors are "non-functional," meaning they do not produce thyroid hormones. Occasionally functional tumors are identified and evaluation of thyroid hormone levels should be performed in any dog with a thyroid tumor. Thyroid tumors are locally invasive and have a moderate-high rate of spread to other places in the body (called metastasis).
Diagnostic tests are recommended to determine which sites in the body are involved. These tests include:
- Complete blood count (CBC)
- Serum biochemistry panel
- Urinalysis (U/A)
- Regional lymph node evaluation
- Thoracic radiographs
- +/- Abdominal sonogram (indicated for tumors on the caudal portion of the trunk or proximal hindlimbs)
Treatment for thyroid carcinomas is aimed both at controlling the tumor in the site where it originates and delaying or preventing metastasis. Controlling the tumor in the site where it originates is best accomplished with either wide surgical excision or the combination of surgery to remove obvious gross disease and radiation therapy to destroy residual microscopic disease. In one study, dogs with moveable thyroid tumors and no evidence of metastasis were treated with surgery alone with a median survival of >36 months. However, many thyroid tumors are "fixed" (deeply attached) and invasive into adjacent soft tissues. These tumors that are not freely moveable are unlikely to be removed with clean margins and the best option is a combination of radiation therapy and surgery. The recommendation of the North Carolina State University Animal Cancer Treatment Program involves pre-operative radiation therapy consisting of 19 treatments administered on a Mon-Fri schedule with a follow-up CT scan done 3 months after completion of radiation therapy to determine resectability. This is based on reports of the gradual reduction in tumor size with thyroid tumors and an attempt to optimize timing of surgery. These are difficult surgeries given the many nerves in the area and the thyroid's rich blood supply with the associated risk of significant hemorrhage. A board-certified surgeon is often recommended for these reasons.
In conjunction with surgery and radiation therapy, we recommend systemic chemotherapy (carboplatin, cisplatin and doxorubicin have reported activity) to delay or prevent metastasis. In our hospital, chemotherapy is given as an intravenous injection at 3-week intervals for a total of four-six treatments. Chemotherapy drugs work by targeting fast-growing cells; cancer cells divide faster and abnormally compared to normal cells. Because of this, chemotherapy preferentially destroys cancer and spares normal tissues. There is the potential for some normal tissues with rapid growth rates (intestinal lining, bone marrow, hair cells) to be transiently damaged by chemotherapy. However, our goal in treating cancer in animals is quality of life, so the dosages are lower than those used in people. We do not normally see significant toxicity with chemotherapy. There is always some risk when a drug is given for the first time, and the oncologist will discuss what types of side effects may develop and what to watch for. If the pet has no problems the first time they receive a drug, they should have no problems with subsequent treatments of that drug. If the pet experiences any side effects, we address them as needed and lower the dosage of the causative drug for future treatments to prevent recurrent problems.
We also recommend piroxicam, an oral Non-Steroidal Anti-Inflammatory Drug (NSAID) which has anti-inflammatory and analgesic properties as well as possible anti-tumor activity against carcinomas. This medication should be given with food and the pet owner instructed to watch for signs of gastrointestinal upset (vomiting, change in stool, etc.). If such signs develop, piroxicam should be discontinued for 3-5 days. Restarting of therapy can then be considered in conjunction with the stomach-protectant misoprostil.
If definitive treatment is not an option, palliative radiation therapy can be considered. Palliative radiation therapy consists of two consecutive twice-daily doses of radiation therapy and is aimed at alleviating pain, inflammation, and swelling associated with the tumor. We recommend that the treatment be repeated in one month if no disease or symptom progression is noted in hopes of extending the duration of response.
- Klein MK, Powers BE, Withrow SJ, et.al. Treatment of thyroid carcinoma in dogs by surgical resection alone: 20 cases (1982-1989). J Am Vet Med Assoc; 1995;206:1007-1009.
- Theon AP, Marks SL, Feldman ES, et al. Prognostic factors and patterns of treatment failure in dogs with unresectable differentiated thyroid carcinomas treated with megavoltage irradiation. J Am Vet Med Assoc 2000;216:1775-1779.
- Brearley MJ, Hayes AM, Murphy S. Hypofractionated radiation therapy for invasive thyroid carcinoma in dogs: a retrospective analysis of survival. J Sm Anim Pract 1999;40:206-210.
- Fineman LS, Hamilton TA, de Gortari A, et al. Cisplatin chemotherapy for treatment of thyroid carcinoma in dogs: 13 cases. J Am Anim Hosp Assoc 1998;34:109-1112.