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MALIGNANT THYROID TUMORS (THYROID CARCINOMA)
MEET THE THYROID GLANDS
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 (T4) 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 canine thyroid 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. Reported symptoms might include a voice change, difficulty swallowing, weight loss, facial swelling, or breathing difficulty but frequently there are no symptoms of any kind other than the lump, which may be fairly moveable or deeply adherent.
While 30% of dogs with thyroid tumors will show reduced thyroid hormone levels on their screening panel and 10% will show high levels, 60% of dogs with thyroid tumors will have normal levels which means there is likely nothing to detect on a screening lab report that would indicate a tumor is present. Again, the only sign of trouble is the lump.
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.
So what happens after the lump is detected?
The goal at this point is to assess the dog for general health and for potential tumor spread as well as to confirm composition of 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 withdrawn with a syringe. A sample is submitted to the lab for analysis. This is not nearly as accurate as a biopsy when it comes to distinguishing benign from malignant thyroid disease but >90% of the time this procedure will at least 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 only detects malignancy directly in about 50% of the samples where the tumor later proved to be malignant; in most cases, we can infer malignancy simply by knowing that the cytology confirmed thyroid cells.
In the dog, there is an 87% chance that a thyroid growth is malignant.
How do we treat a thyroid carcinoma after we are pretty sure that is what it is?
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
Larger tumors are more likely to spread faster than smaller tumors.
When the thyroid carcinoma spreads, it usually goes
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 months was reported in one retrospective study of such cases. If, however, the tumor is invasive and not mobile, 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 radio facility 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.
Since cats with thyroid tumors (benign or malignant) are generally hyperthyroid, it should not be surprising that radiotherapy is probably the best treatment in either situation. Thyroid tumors readily take up radioiodine regardless of whether they are benign or malignant. In one study, cats with malignant thyroid tumors who received radioiodine therapy not only were cured of their hyperthyroidism and the median survival from their cancer was approximately three years
In cats with thyroid tumors who were not also hyperthyroid, treatment is a little trickier. The pertechnetate scan is helpful in determining the tumors ability to absorb radioiodine. If the tumor is not going to take up radioiodine, then some other therapy (surgery or external beam radiation) is a better choice.
Page posted 10/20/07