Most veterinary literature describes the use of surgery or surgery combined with chemotherapy for the treatment of thyroid carcinoma. Surgery is a very important part of the therapy for differentiated thyroid carcinoma. Surgery allows confirmation of a histopathologic (biopsy) diagnosis and potentially accomplishes significant reduction in tumor size. Unfortunately more than 50% of dogs with thyroid carcinoma have detectable metastasis at the time of diagnosis. This prevents a surgical cure for these patients. Currently available chemotherapy for canine thyroid carcinoma has been disappointing.
Radioactive iodine (131I) therapy is the most commonly utilized adjuvant therapy for thyroid carcinoma in man. The basis for radioiodine therapy is the unique ability of thyroid cells to concentrate iodine. This capability allows differentiated thyroid carcinoma cells to preferentially concentrate radioactive iodine administered to dogs with persistent disease following surgical debulking. Because the radioiodine is only concentrated by thyroid cells, the remainder of the dog’s body is spared the effects of the radiation. Recent experience and numerous scientific articles suggests a good response to radioiodine therapy for differentiated thyroid carcinoma in the dog. The benefits of radioiodine therapy are maximized by the use of careful patient screening with thyroid scintigraphy to ensure adequate radioiodine uptake by the tumor. Additional benefits are obtained by surgical debulking of the tumor and the use of an iodine restricted diet prior to radioiodine administration. An iodine restricted diet ensures maximal radioiodine uptake by the iodine deprived differentiated thyroid carcinoma cells. Regulatory requirements require the hospitalization of dogs treated with radioiodine for variable periods depending on the individual dose administered as well as patient excretion rates.
Imaging Thyroid Carcinoma
Patients with thyroid carcinoma are generally best imaged with Thyroid Scintigraphy as the first imaging modality. When patients need additional anatomic information relative to possible surgical excision, tomographic imaging modalities including CT and MRI can be considered. Generally iodinated contrast agents necessary to allow delineation of soft tissues using CT should be avoided as they dramatically limit the utility of radioiodine therapy.
Figure 1: Contrast enhanced CT study demonstrating a large thyroid carcinoma. The extent of this mass is largely defined by the use of iodinated contrast administered IV at the time of the study.
Figure 2: MRI (STIR) image of another patient with a large thyroid carcinioma. Note the marked soft tissue contrast that is obtained without the use of IV contrast agents.
