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Individualized radioiodine (I-131) therapy for nonresectable thyroid carcinoma in dogs

Thyroid tumors in dogs are relatively uncommon, accounting for 1-4% of all tumors diagnosed in the species. This tumor type is more commonly diagnosed in middle-aged to older dogs with a median age of 9-10 years. There is no sex predilection and predisposed breeds include boxers, beagles, and golden retrievers. In dogs, 80-90% of thyroid tumors are malignant, and <10% are functional (producing thyroid hormone). They are locally invasive and up to 30% of dogs will have detectable metastasis at the time of diagnosis, usually to the lungs or regional lymph nodes. Recommended diagnostics include CBC, chemistry, urinalysis, thyroid hormones, high-detail thoracic radiographs (3 views), and abdominal ultrasound. If the thyroid tumor is functional, scintigraphy of the thyroid is recommended to find out the size, local invasion, and the possibility of spreading disease to the local nodes or lungs. Cytology can also be done but thyroid carcinomas are very vascular and blood contamination may occasionally preclude diagnostic cytology. In many cases, thyroid tumors are suspected based on physical exam findings. Poor prognostic indicators include invasion of the mass into surrounding tissues and tumor volume greater than 20 cm3. Treatment options include surgery, radiation therapy, and chemotherapy, alone or in combination. Another alternative treatment modality for functional thyroid tumors, is I-131 radiation therapy. 


Surgery is the treatment of choice for thyroid tumors that are small, superficial, freely movable, and not invading surrounding structures. If this is the case, complete surgical resection results in the best prognosis, and these animals often have a local cure. Incompletely resected tumors should be treated with adjunctive radiation therapy. Unfortunately, surgery is not an option for many dogs with thyroid carcinomas due to invasiveness, size, or metastasis (spread disease). Radiation therapy, curative fractionated or palliative (coarse fractions), often results in a good response, with survival times around 22 months in recent studies. Chemotherapy with doxorubicin or platinum agents may be used as adjunctive therapy, with a response rate approaching 50%. 


Based on this, we recommended thyroid scintigraphy to confirm the diagnosis and find out the size of the tumor as well as local invasion and spreading disease to the lungs and local nodes. If there is spread disease, your pet would not be a good candidate for surgery. In that case, I-131 therapy can be performed and has the advantage of not only treating the thyroid tumor itself but also treating any spread disease. 


Dogs often require high doses of I-131 and are hospitalized for 2-4 weeks after I-131 therapy due to radioactivity. It is also imperative to make sure that the dog's overall health is good aside from the thyroid tumor. If something abnormal is found, we may recommend its treatment before I-131 can be done. This is to try to avoid health deterioration from other reasons while the dog is in the hospital. If the patient's health deteriorates after I-131, we are very limited in our ability to treat as done in an ICU setting or perform other imaging and treatments because of the high radioactivity and laws preventing personnel exposure. Therefore, we try to look for hidden conditions before treatment to try to avoid health deterioration while in the hospital.
















The scintigraphy above shows is a large, irregular, and invasive mass present in the ventral neck. There is also a small uptake in the cranial mediastinum. It is unknown if the smaller uptake is a continuation of the neck mass or a solitary nodule in the cranial mediastinum. The size of the tumor is 6x7x14cm which classifies this disease at least at stage III out of IV thyroid tumor. Tumors larger than 5cm and with spread disease have a worse prognosis.


After I-131 treatment, dogs remain in the hospital until their radiation levels decrease to a certain level determined by law. This may take 2-4 weeks. Dogs require special care for 4 weeks after hospital discharge, including minimal handling to minimize radiation exposure. With high doses of I-131 in dogs, there is an 8% risk of myelosuppression (suppression of the bone marrow, which produces blood cells and platelets), which can be fatal. Most dogs will require thyroid supplementation post-treatment. Advantages of I-131 treatment include specific targeting of abnormal thyroid tissue in spread thyroid disease, as well as the ability to repeat treatment if thyroid levels remain high after treatment or if there is tumor recurrence. In a study, among 30 dogs with thyroid carcinoma that underwent I-131 therapy, 5 dogs (16%) needed additional 1-2 treatments with I-131 due to persistent symptoms and/or persistently elevated thyroid levels and/or lack of complete resolution of the neck mass. Thyroid scintigraphy is usually repeated 1-3 months after the I-131 treatment to document improvement and see if additional doses are needed. If there is not a complete response to I-131 treatment, the tumor may shrink enough to allow for surgical removal. However, studies comparing benefits of I-131 treatment alone vs. I-131 in addition to surgery, do not show clear benefits or increased survival time when compared to each other. 

Based on a study of dogs with non-resectable thyroid carcinomas that were treated with I-131, the median survival time for dogs with no gross evidence of metastasis (spread disease) was around 800 days, and 366 days for animals with metastatic disease (spread disease). If radiation-associated myelosuppression develops, death is usually within a few months.

The T4 before therapy for this dog was 10.2 ug/dl. The dog received 100 mCi of I-131 and was hospitalized for 22 days. The T4 was <0.5 ug/dl at the time of discharge.  The patient went home with thyroid supplementation.  The scintigraphy on the left is 1 month post-I131. The T4 at the time was 1.6 ug/dl and TSH 0.18 ng/ml while on thyroid supplementation.   The scintigraphy on the right was 5 months post-I-131. The T4 at the time was 0.9 ug/dl and TSH was increased to 1.55 ng/ml. We can notice an increase in uptake of the Tc99 despite the normal (low T4).  The Tc99 thyroid uptake went from 11.9% before treatment to 2.9% at 1 month and 3.7% at 5 months. 
























Individualized I-131 dose determination:

Each patient receives an individualized I-131 dose according to the characteristics of his or her hyperthyroidism. This method takes into consideration several patient-specific variables, including the degree of hyperthyroidism, the size of the thyroid tumor, and several specific physiologic parameters such as the biologic half-life and iodine uptake. The dose is calculated based on:

VCA Arboretum View Animal Hospital

Internal Medicine,  and Nuclear Medicine Department

Iodine-131 Therapy


Telephone: (630) 963-0424


Joao Felipe de Brito Galvao, MV, MS, DACVIM (SAIM)

Cristiane Campos Otoni, MV, DACVIM (SAIM)

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