Thyroid tumors arise from the thyroid tissue located in the neck. The majority (80 to 90 %) of thyroid tumors are malignant (carcinomas). The majority of dogs with thyroid tumors have normal thyroid function. Decreased thyroid gland production can occur due to tumor destruction of normal thyroid tissue. Thyroid carcinomas have a moderate potential for metastases (spreading of the cancer), with approximately 33% of patients having evidence of metastases at the time diagnosis. The most common organ for this tumor to spread (metastasize) to is the lungs.
Most dogs will present for a mass or swelling in the neck. Difficulty swallowing, eating or a dry cough are also common. Tumors may be discrete and freely moveable or may cause a large diffuse swelling of the neck. Diagnosis may be made with either a fine needle aspirate or incisional biopsy. Because these tumors have a large blood supply, blood contamination may prevent identification of cancer cells in samples with limited tissue or fine needle aspiration. For small, moveable tumors, removal with biopsy will yield a definitive diagnosis. Ultrasound evaluation is often helpful with large masses to determine the degree of invasion and possibility of surgical removal. Once a thyroid tumor has been diagnosed, chest x-rays are recommended to evaluate for possible metastases (spread) to the lungs. Routine blood work, including a thyroid level is also recommended to evaluate your pet’s overall health, including thyroid function.
Treatment recommendations for thyroid tumors are dictated by the size of the mass, degree of invasion, and whether the tumor is functional. For small, moveable masses, surgical removal is recommended. If the tumor is contained within the capsule of the gland, is completely removed, and low grade, no further therapy may be needed. For tumors where surgery is unable to remove all of the tissue or there is evidence of invasion into blood and lymphatic vessels, additional therapy is warranted. A board-certified veterinary surgeon can determine the optimal surgical treatment plan for your pet.
Definitive (curative intent)- Definitive course radiation therapy is recommended for patients whose tumors have been “debulked” with surgery, but surgery fails to remove all microscopic tumor tissue. Radiation therapy is administered in order to kill any remaining tumor cells at the surgical site. Chemotherapy is often recommended in combination with radiation to try to prevent or delay metastatic disease.
Chemotherapy is recommended for patients whose tumors exhibit characteristics of aggressive behavior such as invasion into blood or lymphatic vessels. Chemotherapy is also used in combination with radiation therapy for patients whose tumors have been removed or for patients undergoing palliative radiation for large unresectable tumors. Chemotherapy without surgery or radiation is useful for controlling thyroid carcinoma, even in cases of metastatic disease. Most patients can be managed well for months to up to two years. Carboplatin and doxorubicin chemotherapies appear to have some efficacy in thyroid carcinomas. Chemotherapy is administered intravenously every 3 weeks for 4 to 5 treatments. Low-dose oral chemotherapy (metronomic) is another option which is cost effective, provides excellent quality of life and delays regrowth and metastasis by interfering with the tumors ability to form new blood vessels.
The prognosis for benign thyroid tumors (adenomas) is good, as most benign tumors can be cured with surgery. The prognosis for thyroid carcinomas is dependent on size of the tumor, grade (differentiation), degree of local invasion, and presence of lymphatic and/or vascular invasion. For small tumors, surgery alone may yield survival times of 2 to 3 years or longer.
For tumors that cannot be completely removed with surgery or tumors that have evidence of lymphatic and/or vascular invasion, additional therapy (radiation and/or chemotherapy) is recommended. Ultimately, most patients with thyroid carcinomas will develop metastatic disease; however, with combination therapy, many patients will enjoy cancer-free, good quality of life for 1 to 2 years.
For large, invasive tumors that cannot be removed with surgery, combination chemotherapy and radiation therapy will often result in decrease of the size of the tumor and a significant improvement in your pet’s quality of life.