| Location: | Thyroid |
| Behaviour: | Mass in the neck, possible disturbed thyroid function, high risk of metastases |
| Diagnostics: | Blood and urine analysis, X-ray lungs, tissue sample surrounding lymph nodes, ultrasound and possibly CT-scan or scintigraphy |
| Treatment: | Surgery, radiation, radioactive iodine therapy, (chemotherapy) |
| Prognosis: | Depending on fixation and size of tumor, presence metastases (between 6 months and 3 years) |
In the thyroid, cells can degenerate and form a tumor. The thyroid is responsible for the production of thyroid hormones who play an important part in the metabolism. Usually, benign thyroid tumors are very small and do not cause abnormalities of the metabolism. When one can feel a lump at the thyroid and/or notice symptoms of a disturbed metabolism, there is a big chance it concerns a malignant tumor.
The original (primary) tumor will originate at the thyroid and in 60% of patients, both thyroid lobes are involved. If both lobes are affected, there are greater chances for metastases. Furthermore, the larger the tumor is, the higher the chances are for metastases. Up to 35-40% has metastases at diagnosis and eventually up to 80% of these tumors will lead to metastases (towards the lungs and nearby lymph nodes).
These tumors are most prevalent in older dogs, with a median age of 9-11 years.
Thyroid tumors are more often found in Beagles, Boxers, Golden retrievers and Siberian Huskies.
Symptoms can consist of a lump in the neck (which can lead to rapid breathing, coughing, difficulties swallowing, change of bark tone), but also a too low or too high function of the thyroid. When the function is too low, the dog can be languid, have a thinner undercoat and gain weight. When the function is too high, the dog can eat a lot and still lose weight, be hyperactive, drink and urinate a lot and/or have diarrhea.
Taking a tissue sample is often tricky as there are many blood vessels at this location. A standard staging is recommended. Via blood analysis (including the measurement of the thyroid hormone thyroxin), urine analysis, X-rays of the lungs and tissue samples of nearby lymph nodes, the veterinarian can form a better idea of the behavior of the tumor and therefore how to approach it. High thyroxin values can indicate a deviation of the thyroid function. The blood and urine analysis show how other organs such as the heart, kidneys and liver are doing. Via ultrasound or CT scan of the neck, the veterinarian can confirm whether the tissue mass originates from the thyroid, how far this tumor spreads and how perfused it is. Scintigraphy can also show whether there are metastases present, where they are situated and indicate if the tumor can be treated via radioactive iodine.
The treatment choice depends on the size of the tumor, its distribution towards other tissues, the presence of metastases and whether there are symptoms present of a disturbed thyroid function.
Surgery is advised when the tumor is not attached onto the surrounding tissues (which is the case in 25-50% of thyroid tumors) and there are no metastases. Risks linked to surgery are possible bleedings, accidentally removing the nearby parathyroid glands (causing a disturbance of the calcium metabolism), larynx nerve damage and a too low thyroid function when both thyroid lobes have to be removed.
Radiation is often used after surgery when residual tumor tissue is suspected or when surgery is not a good option (e.g. when the tumor is fused to the environment). Possible side effects of radiation consist of damage to the larynx, trachea and esophagus, although in general, radiation is well tolerated. It is possible that after a while (months to years after treatment) insufficient thyroid hormones are produced. On the other hand, these hormones can be supplemented.
if the tumor takes up iodine well, radioactive iodine therapy is an interesting treatment option. Radioactive iodine is administered to the dog and will build up in the thyroid tissue. There, it locally radiates the thyroid tissue, including the tumor. Depending on the amount of thyroid tissue that gets destroyed, it might be necessary to supplement thyroid hormones after the treatment. The advantage of this treatment option vs radiation is that through radioactive iodine both the metastases of the thyroid tumor as the thyroid tumor itself can be very precisely treated, at least if both take up iodine. Moreover, if necessary, the treatment can be repeated without much side effects.
Chemotherapy can be considered for dog with large, non-resectable tumors or many metastases, although the results for chemotherapy are thus far not as good as for the treatments mentioned above. In some dogs with tumor type good results were obtained (up to a year survival), but at this point it is unclear which dogs benefit from this.
Overall, the prognosis is very good after surgery (3 years if the tumor is loose (not attached on the surrounding tissue), 6-12 months when spread in the surrounding tissues). For radiation and radioactive iodine therapy good results were obtained as well.
For dogs in whom the thyroid tumor could not be removed surgically and thus received radiation therapy, 80% was still alive after 1 year and 72% after 2 years. The average duration of survival for radiation varies between 24.5 to up to more than 45 months.
The survival times after radioactive iodine therapy is 30 (if only radioactive iodine therapy) to 34 (if radioactive iodine therapy is combined with surgery) months.
The composition of the tumor tissue and the tumor size have an influence on the prognosis. The higher the tumor grade and size, the worse the prognosis is. For malignant tumors with a size between 21-100 cm3, metastases were reported in 74% of cases, for tumors larger than 100 cm3 this was 100%. If caught early and the tumor isn’t attached to the surrounding tissues, the prognosis is good.
Centre Hospitalier Vétérinaire Frégis
Arcueil, France
