Types of Thyroid Cancer
Papillary and Follicular
Although there are several types of thyroid cancer the vast majority are papillary, follicular, or a mixture of these two types of cancer. This cancer is extremely slow growing compared with most other cancers; therefore, it offers a very excellent chance of cure. So good was the "cure rate" that for many years it was not felt necessary to follow patients after their surgery: "The patient with thyroid cancer will generally outlive his surgeon". However this was partially true because the average age of the thyroid cancer patient is twenty years less than the average age of a surgeon. If a patient is found to have thyroid cancer today, then he is followed forever!
Unlike most other cancers, thyroid cancer seldom recurs during the first five years. In a study of almost 600 thyroid cancer patients done in 1977, it was found that after twenty years, patients who had only half their thyroid gland removed were five times more likely to have a recurrence of their disease than those in whom a total or near total thyroidectomy was done. It was shown that patients who had both sides of their thyroid gland removed, followed by a dose of radioactive iodine, and who took thyroid medication had a recurrence rate of less than 5% after twenty years. Today this is a standard treatment for thyroid cancer.
At surgery it is necessary to remove at least 90% of the thyroid gland (total, near total, or subtotal thyroidectomy). If only half of the thyroid was removed, a second operation is necessary to remove most of the opposite side. An exception to this rule is the very small (1.5 cm or less) cancer with no evidence of metastasis.
No thyroid replacement is given after surgery and the patient is allowed to become hypothyroid which usually takes 3-6 weeks as indicated by a rise in the TSH. A scan of the neck will show if thyroid tissue is still present and a dose of radioactive iodine is given to "ablate" this tissue. The patient is then put on thyroid replacement taking at least 1 microgram of L-thyroxine per pound of patient weight. TSH is then checked. If the TSH is less than 0.5, the patient remains on this dose of thyroid replacement indefinitely.
A follow-up scan is done at the end of the first year. This follow-up scan is done while the patient is off thyroid replacement. Because L-thyroxine has a half life of 6.7 days, it requires approximately six weeks for this medication to completely "clear" the body. The patient is put on Cytomel (which has a half life of 48 hours) six weeks prior to his scan. Then Cytomel is stopped after four weeks. This means that the patient is only off his thyroid replacement for approximately two weeks before his scan. If any remaining thyroid cancer is detected on a follow-up scan, it is treated with surgery or radioactive iodine 131.
Recently, a blood test which indicates a recurrence of thyroid cancer in patients has been developed. This test is simple and has the advantage of not having to stop a patient's thyroid medication prior to performing it, as in the case with thyroid scan.
Thyroid tissue (both normal tissue and cancerous tissue) produces a protein called thyroglobulin which is measured by performing a blood test. Patients with thyroid cancer have about the same amount of thyroglobulin as normal people; however, after removal of all the thyroid gland by surgery and radioactive iodine ablation, the thyroglobulin level falls to near zero. By monitoring the thyroglobulin (TG) level on these patients with a yearly blood test, a recurrence of the thyroid cancer can be detected if the patient's blood shows a rise in the thyroglobulin level. In the past thyroid scans were performed at periodic intervals of one to five years in order to check for recurrent cancer. This necessitated stopping thyroid hormone. Now scans are not done after the first year unless the thyroglobulin level rises or another lump occurs in the neck.
Other types of Thyroid Cancer
A rare type of thyroid cancer (medullary cancer) produces a hormone called calcitonin. Medullary cancer is more malignant than papillary or follicular cancer just described. When elevated levels of calcitonin are found in the blood, it is indicative of medullary cancer. Calcitonin disappears from the blood after the cancer is removed, and its level should be periodically checked to detect early recurrence. Many of the cases are hereditary, and these patients have a unique genetic mutation in the RET proto-oncogene. This mutated gene can be detected from birth, many years before the cancer occurs. Therefore, relatives of patients having medullary cancer must be checked for this mutation in the RET proto-oncogene to see if they will develop this type of cancer. This will allow the cancer to be "cured" before it develops.
A fourth type of thyroid cancer called anaplastic cancer is very malignant, but it is less common than the other types and occurs primarily in elderly people.
Occasionally, other types of cancer will involve the thyroid gland such as lymphoma or metastatic cancer from another organ (i.e. kidney). The treatment depends upon the type of primary cancer. Needle biopsy is usually able to differentiate the type of cancer involved and allows the surgeon to plan his surgery appropriately before the operation.