Canine Thyroid Tumors

Thyroid tumors are uncommon tumors in dogs accounting for about 1-3% of all tumors. They often occur in middle-aged dogs and no sex predilection has been found. The breeds commonly seen with this cancer include Boxers, Golden Retrievers, and Beagles, but it has been found in a number of other breeds. The cause of this tumor is unknown, but there has been an increase in tumor formation after exposure to radiation therapy in nearby areas. Additionally, hypothyroidism is a risk factor in forming these tumors.

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The common presentation for most dogs is the palpation of a mass in the area of the neck. The mass may be tightly adhered to the underlying tissue or freely movable. Some dogs may have coughing problems or swallowing difficulty. Some dogs may have signs of hyperthyroidism, which means their tumor is functional and secreting thyroid hormone. This is very rare though (<10%) and most cases have a nonfunctional tumor.

Staging for this cancer involves the following: routine bloodwork (CBC, serum chemistry), thyroid levels, chest radiographs, lymph node aspiration and tumor aspiration or biopsy. Factors that have been shown to play in role in how well dogs will do include: size of the tumor, histologic cell type, evidence of metastasis, and the amount of new blood vessels growing into the tumor. Therapy is based on several factors: size of the tumor, spread of tumor, the presence of disease in other organs and whether the tumor is functional or not.

Surgery to remove the entire tumor is a viable option when the tumor is small and freely movable. Studies have shown dogs can have survival times of over 2-3 years with surgery alone. If surgery is complete, meaning the entire tumor is removed, many dogs can simply be watched or continued with chemotherapy for high grade and invasive tumors. If surgical removal is narrow to incomplete, we recommend radiation therapy to remove any residual cancer. Chemotherapy could also be used as an alternative to radiation therapy, although may be slightly less effective at preventing local tumor recurrence. In cases where the tumor is too large to remove, many dogs will do very well with radiation therapy. Radiation therapy in these circumstances has yielded survival times of > 3 years for over 60-70% of the cases depending on the grade and stage of the tumor. Metastasis is a negative prognostic factor.

There are additional treatments that may be used in conjunction with surgery, radiation therapy, and chemotherapy. Non-steroidal anti-inflammatory drugs (NSAIDs) have been shown to have anticancer effects against some types of carcinomas. Also, starting dogs on low-dose thyroid supplementation may be beneficial. A newer form of therapy that combines an NSAID with low-dose chemo is known as metronomic therapy. This form of treatment is designed to be anti-angiogenic, or against new blood vessel formation. Cancer cells must acquire new blood vessels for oxygenation and nourishment in order to grow into a sizable mass. The goal of starting metronomic chemo is to prevent these blood vessels from forming to feed any microscopic cancer cells. This may thereby delay disease recurrence and improve the long-term prognosis.