Treatment options for thyroid tumors include surgery, radiation therapy, chemotherapy and I-131 therapy.
Surgery is the preferred option, if possible. Freely movable tumors that are excised have reported survival times greater
than three years (Klein 1995).
For tumors that are grossly excised but have invasion beyond the capsule, follow-up radiation therapy should be considered.
Ideally, radiation therapy should be started two to four weeks after surgery. Protocols for radiation therapy may vary between
facilities, but typically involve the use of 15 to 19 treatments over three to four weeks. Control rates for radiation therapy
in the adjuvant setting have not been well documented, but clinical experience suggests that long-term control (i.e., more
than two years) can be achieved with the combination of radiation therapy and surgery.
Definitive or full-course radiation therapy typically is reserved for patients with microscopic disease. However, definitive
radiation therapy still may provide long-term control, even in dogs in which surgery is not an option. In one study, eight
dogs treated with definitive radiation therapy had a median survival time of two years, and all eventually had complete resolution
of the tumor. In a second study, 25 dogs with measurable disease were treated with a definitive course of radiation therapy.
The one-year and three-year progression-free survival times were 80 percent and 72 percent, respectively.
Interestingly, in both studies the tumor did not achieve a maximum response for six to 22 months following treatment. The
slow regression of these tumors following radiation therapy is most likely why radiation therapy previously was not considered
effective as part of the treatment of these tumors.
Palliative radiotherapy may be effective in controlling these tumors for prolonged periods. This involves the use of several
larger doses of radiation over one to four weeks. For thyroid tumors, a better term would be hypo-fractionated or coarsely
fractionated radiation therapy, because the goal of this treatment is palliation and tumor control.
In an early study, 13 dogs were treated with four weekly treatments of radiation therapy. Ten out of the 13 achieved a complete
or partial response, and the overall survival time was 96 weeks. Benefits of this type of radiation therapy would be lower
cost, fewer treatments and minimal side effects. However, it is expected that the response rate and duration will be inferior
to definitive radiation therapy protocols.
Thyroid tumors have a moderate rate of metastasis, so that chemo-therapy is indicated in the treatment of these tumors with
possibly the exception of smaller movable tumors. Chemo-therapy can be used either alone or in combination with surgery and/or
The most commonly used drugs include doxorubicin, cisplatin and carboplatin. Both cisplatin and doxorubicin have been shown
to have measurable responses when used to treat gross disease; as a primary treatment, their response rates have been reported
to be as high as 40 percent to 50 percent.
Chemotherapy is recommended currently as an adjuvant to surgical excision radiation therapy, as well as in combination with
radiation therapy for non-resectable tumors. The typical treatment schedule for any of these drugs is once every three weeks
for a total of four to six treatments. Potential side effects include gastrointestinal toxicity, myelosuppression, nephrotoxicity
(secondary to cisplatin) and cardiomyopathy (with cumulative doses of doxorubicin); however, the risk of a significant side
effect is only 5 percent to 10 percent.
Radioactive iodine therapy can be considered to treat dogs with both functional and non-functional thyroid tumors. In a recent
study, 39 dogs with measurable disease were treated with radioactive iodine. Those with non-functional tumors were fed a low
iodine diet for the three weeks preceding treatment to enhance the uptake of I-131. The median survival time for all was 27.6
Twelve dogs had surgery three to six weeks after I-131 therapy, but this did not improve survival times. I-131 may provide
an advantage over external-beam radiation therapy for dogs that have ectopic thyroid tumors or metastatic disease. Drawbacks
of I-131 therapy include travel, given the limited facilities that can provide this service, prolonged isolation times, cost
and potential for significant bone-marrow suppression.
The dose of radioactive iodine that is required to treat dogs is significantly higher compared to hyperthyroid cats. This
means there are few facilities capable of treating dogs and the holding time is significantly longer — in some cases up to
In the above study, three dogs died of complications from bone-marrow suppression. There is the potential for radiation pneumonitis
for those dogs that have pulmonary metastasis or ectopic tumors in the thorax.
Dr. Cronin earned her DVM degree from Cornell University in 1990. She completed an internship at the Animal Medical Center
in New York and a medical oncology residency at North Carolina State University. She is a diplomate of the American College
of Veterinary Internal Medicine in the specialty of oncology. After completing her residency, she was lecturer at the University
of Pennsylvania Veterinary Teaching Hospital and medical oncologist at Angell Memorial Animal Hospital in Boston. In 2001,
she co-founded the New England Veterinary Oncology Group in Waltham, Mass.