MALIGNANT THYROID TUMORS
The two thyroid glands are located in the throat, nestled just behind the larynx on either side of the windpipe. These glands are responsible for making and storing thyroid hormones, in particular thyroxine, also called “T4.” This hormone is released and then activated by the body’s tissues into triiodothyronine, or T3. Triiodothyronine (T3) acts as a sort of a volume dial for metabolism, controlling how fast our cells burn fuel, while thyroxine is an inactive form. There is actually quite a long story involving how these hormones work, how an assortment of inactive forms are made and shifted back to activity again, but that story is for another time. Today’s story is not about the healthy thyroids and their activity but about thyroid malignancy. The situation is a little different between dogs and cats so we will discuss them separately.
The thyroid tumor in the dog generally does not affect production of thyroid hormones. This means there is nothing to detect on a screening lab report that would indicate a tumor is present; the dog with the thyroid tumor is usually neither hypothyroid nor hyperthyroid. The tumor is usually detected when someone, be it the owner at home or the veterinarian during examination, notices a lump around the dog’s throat. The lump may be deeply adhered, potentially altering the dog’s voice or ability to swallow, or the lump may be fairly moveable. The average age at tumor detection is 9 years and there is no gender predilection. Some studies have suggested that the following breeds have a predisposition to the development of thyroid carcinoma: Boxer, Beagle, and Golden Retriever.
In canine thyroid carcinomas, approximately one dog in five
will be HYPERTHYROID.
The goal at this point is to assess the dog for general health and for potential tumor spread as well as to identify the lump. The initial assessment of the dog with the throat lump should include a physical examination, basic database of blood and urine testing including a T4 level, and a set of chest radiographs to check for obvious tumor spread. Testing will be needed beyond this to identify the lump/growth.
There are several procedures that could be performed to identify the growth. If the growth is loose and fairly moveable in area, the easiest procedure may be to simply remove and biopsy it. This provides the most accurate means of identifying the growth and may even prove curative. The problem is that often the thyroid carcinoma is extremely vascular and deeply adhered to the deeper structures of the throat which means that surgery may not be so simple.
A less invasive option is “fine needle aspirate.” Here, a needle is inserted in the growth and cells are sucked out with a syringe. A sample is submitted to the lab for analysis. This is not nearly as accurate as a biopsy but >90% of the time this procedure will identify the growth as thyroid in origin. Since statistics indicate a thyroid growth is malignant 87% of the time this goes a long way to identifying the growth as a thyroid carcinoma. Cytology (i.e. analysis of the fine needle aspirate) is able to identify malignancy about 50% of the time when it is present.
In the dog, there is an 87% chance that a thyroid growth is malignant.
Treatment depends on several factors: how deeply invasive the tumor is and whether there is already measurable tumor spread. There are three imaging techniques that can be used (and sometimes combined) to answer these questions: ultrasound, scintigraphy (nuclear medicine), and MRI.
Ultrasound uses sound waves to image the deeper structures of the throat and determine the feasibility of removing the primary tumor. This method is non-invasive and many hospitals have the necessary equipment available. If surgery is being considered, ultrasound might be an especially helpful choice as it will indicate whether surgical resection is even possible.
nuclear medicine scan showing not only the primary tumor but also
areas of tumor spread in the lung
(tumorous areas are in green and red)
Scintigraphy is especially helpful at detecting tumor spread. Here, a radioactive material called “pertechnetate” is given intravenously. It follows a similar course as iodine would, iodine being an important component of thyroid hormone. This means that the nuclear medicine image can reveal areas of thyroid tissue in all sorts of inappropriate areas indicated the extent of tumor spread. Scans are reasonably priced but not necessarily available in all areas; one has to be lucky enough to live near nuclear medicine facility.
For a list of facilities click here.
MRI (magnetic resonance imaging) is another way of detecting the depth of a tumor and possibly tumor spread. A highly detailed image is produced but imagining requires general anesthesia and is expensive. Again, not every area has a facility that can perform this kind of imaging but potentially one could be in an area with MRI access but without scintigraphy access so MRI might come to be a helpful tool in tumor assessment.
Studies show that 16-38% of dogs with thyroid carcinoma
have evidence of tumor spread at the time of diagnosis.
Larger tumors are more likely to spread faster than smaller tumors.
When the thyroid carcinoma spreads, it usually goes
to the lymph nodes of the throat and neck and to the lungs.
After the tumor has been assessed for spread and for depth, an appropriate therapy can be selected.
If the tumor is freely moveable and no tumor spread can be detected, surgery is probably the best choice. A median survival time of 20.5 was reported in one retrospective study of such cases. If the tumor is invasive, excision will probably not be complete and will likely be fraught with complications such as nerve damage or hemorrhage. It is best in this situation to pursue a different therapy.
EXTERNAL BEAM RADIATION THERAPY
This form of therapy requires referral to a special radiation oncologist and such facilities may not be readily available. In this therapy, the primary tumor in the throat is irradiated typically three times weekly. The tumor responds slowly (peak effect is usually seen 8-22 months after the treatment course has completed). In one study of 8 dogs, a median survival time of 24.5 months was found. Side effects include hoarse voice, cough, and difficulty swallowing (usually complications were self limiting and resolved in 2-3 weeks). Later complications include baldness at the radiation site and dry cough in fewer than 20% of patients.
As in hyperthyroid cats with benign thyroid nodules, iodine 131 can be used to treat thyroid carcinoma in the dog. The advantage of this therapy is that the iodine can travel to areas of tumor spread and be sequestered there. Iodine 131 emits beta particles (high speed electrons) penetrating and destroying tissue for only a fraction of an inch. This means the normal tissue is spared around the tumor and only the tumor is treated. In one study, this therapy yielded a median survival time of 30 months. Much higher doses of iodine 131 are used in this situation than in the feline treatment of hyperthyroidism. This translates to most dogs requiring thyroid supplementation at the end of therapy as there will probably not be enough normal thyroid tissue left. There is a less than 10% incidence of fatal bone marrow suppression associated with this therapy. Dogs must stay at the radiofacility during their course of therapy and afterwards until their radiation emissions drop to a safe level. A special diet is sometimes recommended prior to therapy to maximize the uptake of iodine 131 so as to enhance the tumor’s response.
Chemotherapy is still under investigation as a treatment for thyroid carcinoma but protocols are still being worked out. There is some evidence that treatment with non-steroidal anti-inflammatory drugs (NSAIDs) suppress progression of many types of carcinomas. Use of these medications may be a helpful adjunctive treatment.
While canine thyroid carcinomas do not usually produce excess hormone, the opposite is true in cats; cats with thyroid carcinomas are usually hyperthyroid. Hyperthyroidism is an extremely common situation in older cats and the thyroid growth involved is benign in over 95% of cases. This makes finding the less than 5% of cats with malignancy all the more difficult to distinguish. A pertechnetate scan is needed to make the distinction. If the owner is considering radiotherapy for treatment of hyperthyroidism, the malignancy should be identified as the scan normally precedes radiotherapy. If the owner is opting for another treatment (surgery, or oral medication), the scan may not happen and the diagnosis will be missed.
scintigraphy scan of a cat
with a thyroid carcinoma
In the cat, the carcinoma tends to spread to local lymph nodes and is less commonly found in lung. Again, scintigraphy can assess the degree of tumor spread as well as confirm whether the tumor is benign or malignant.
Radiotherapy with iodine 131 can be used to control tumor spread (median survival of 20 months has been reported). The primary tumor in the throat can often be surgically removed and is not as likely to be deeply invasive as in the dog.
Page posted 10/20/07