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Dr. Jorge L. Diez, Dr Nathan Lassman,
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 Endocrinology 


Cancer of the Thyroid

 

The diagnosis of cancer is terrifying for most patients because it has become associated in our minds with pain and death. But, in fact, the outlook for patients with thyroid cancer is usually excellent because:

  1. Most thyroid cancer is easily curable with surgery
  2. It causes little pain or disability
  3. Novel and effective means of diagnosis and therapy are available for several kinds of thyroid cancer.

Thyroid cancer usually presents itself as a lump or nodule in the thyroid gland. However, it should be emphasized that most thyroid nodules (98% or more) are benign. Unfortunately, it may be difficult to distinguish a benign from a malignant nodule on the basis of symptoms and physical examination, even with the help of laboratory tests including blood hormone levels and scans (images) of the thyroid gland. Experienced hands and biopsy of thyroid nodules usually provides the most valuable information in helping a physician to decide whether a surgical intervention is necessary.

 

Who is at Risk for Thyroid Cancer?

Although we do not know exactly what causes thyroid cancers, cancers are more likely to develop in patients who have received x-ray treatments in childhood for enlarged tonsils, enlarged thymus glands, acne –all these practices have been stopped several decades ago- and occasionally for other malignancies such as Hodgkin's disease.

Routine diagnostic x-rays (like chest x-rays, dental x-rays, or thyroid scans) do not cause such thyroid cancer.

 

Papillary Thyroid Cancer

A papilla is a nipple-like projection. Papillary cancers have multiple projections giving them a fern or frond-like appearance under the microscope. Tiny areas of papillary cancer can be found in up to 10% of "normal" thyroid glands, when thyroid tissue is carefully examined with a microscope. The more carefully a pathologist looks for these tiny cancers, the more commonly they are found. These microscopic cancers seem to have no clinical importance and are more a curiosity than a disease. In other words, there does not seem to be a tendency for these small cancer-like growths to enlarge and become more serious malignant tumors.

On the other hand, when papillary cancer grows enough to form a palpable lump in the thyroid gland, we consider it clinically important, for it is likely to continue to enlarge and may spread elsewhere in the body. Papillary tumors make up about 70% to 80% of all thyroid cancers, and can occur at any age. There are only about 12,000 new cases of papillary cancer in the United States each year, but because these patients have such a long life expectancy, we estimate that one in a thousand people have or have had this form of cancer.

Papillary cancer tends to grow slowly and to spread by means of the lymphatic system to lymph glands in the neck. About one third of the patients who undergo surgery for papillary cancer, the tumor has already spread to surrounding lymph glands (lymph node metastasis). Fortunately, the generally excellent outlook is usually not altered by lymph gland metastases.

The papillary cancer may also spread from one side of the thyroid to the other through the lymphatic system, again without affecting the patient's prognosis.

85% of patients with papillary carcinoma have a primary tumor that is intrathyroidal (confined to the thyroid gland itself). The 25-year mortality rate from cancer in this situation is about 1%. This means that only 1 out of every 100 such patients have died of thyroid cancer by 25 years later. By that time the vast majority of them have been permanently cured. The prognosis is not quite as good in patients over the age of 50, or in patients with tumors larger than 4 centimeters (1 1/2 inches) in diameter.

Since the outlook in patients with intrathyroidal primary tumors is so favorable, it is important that therapy not be hazardous. Radical surgery is almost never indicated for this mild type of papillary cancer. Although up to 10% of patients with intrathyroidal papillary cancer will have a recurrence of tumor, recurrences are generally due to the growth of tumor cells within lymph glands in the neck and are not life threatening. They are usually removed surgically.

The prognosis is not as good in patients where the cancer has grown through the thyroid into surrounding tissues. Specifically, this means spread through the fibrous capsule that surrounds the thyroid gland into the tissues of the neck, and not the lymph node involvement discussed above. In a very small percentage of patients (about 5%), the cancer eventually spreads through the blood stream to distant sites, particularly the lungs and bones. These distant tumor sites (metastases) can often be treated successfully with radioactive iodine (see below). Although young patients who have papillary thyroid cancer generally have an excellent outlook, patients under the age of 20 have a somewhat higher risk of spread to lungs.

 

Follicular Thyroid Cancer

The normal thyroid gland is made up of sphere-shaped structures called follicles. When a thyroid cancer contains these normal structures, the cancer is called a follicular cancer. Follicular cancer makes up about 10-15% of all thyroid cancers in the United States, and tends to occur in somewhat older patients than papillary carcinoma.

Follicular cancer of the thyroid is considered to be more aggressive than papillary cancer. In about one-third of patients with follicular thyroid cancer, the tumor is minimally invasive and tends not to spread. The prognosis is excellent in this situation. In the other two-thirds of patients, the follicular cancer is more invasive. It may grow into blood vessels and from there spread to distant areas, particularly the lung and bones. In general, the prognosis is better in younger patients than in those over 50 years of age.

 

Treatment of Thyroid Cancer

The primary therapy for all forms of clinically relevant thyroid cancer is surgery. For more aggressive papillary and follicular cancers, the generally accepted approach is to remove the entire thyroid gland, or as much of it as can be safely removed. For intrathyroidal papillary cancer and minimally invasive follicular thyroid cancer, surgeons and endocrinologists continue to debate the merits of total thyroid removal versus the removal of just one lobe and the tissue connecting the two thyroid lobes, known as the isthmus.

Since the outlook is so good for intrathyroidal papillary cancer and minimally invasive follicular cancer, independent of the extent of surgery, it has been difficult to prove which of the two surgical approaches is preferable. Therefore there are no absolute rules for the management of these cancers. Although the general characteristics of tumor behavior are understood, in any particular patient the choice of treatment is best made by physicians skilled in the management of patients with thyroid cancer.

 

Radioiodine Therapy

 

The thyroid gland normally concentrates iodine from the bloodstream at concentrations reaching 1000 times higher than other tissues. This property off thyroid tissue has been exploited for treatment. Large doses of radiation using radioactive isotopes of iodine can be directed to the cancer, without damage to surrounding tissues.

To undergo radioactive iodine therapy for thyroid cancer that has spread the entire thyroid gland must almost completely be surgically removed. Once that has been done, patients with a residual tumor in the neck or known distant metastases can then undergo a scan with a test amount of radioactive iodine (usually about 2 to 10 millicuries –a measurement of radioactivity). If a significant amount of iodine is concentrated in the areas of thyroid cancer, a larger therapeutic dose of radioactive iodine (usually 150-200 millicuries) can be administered in an attempt to destroy the tumor.

A patient who receives treatment with large doses of radioactive iodine must stay several days in the hospital, until the amount of radioactivity in the body falls to levels which will not be hazardous to other people. However, this treatment has proved to be safe and well-tolerated, and has even been able to cure cases of well-differentiated thyroid cancer after the tumor has spread to the lungs.

Because of the safety and effectiveness of radioactive iodine in patients with more aggressive thyroid cancer, many physicians also use it routinely in patients with less aggressive papillary and follicular cancers. In this situation, radioactive iodine is used to destroy tiny remnants of thyroid tissue still present after surgery. This may improve the outlook and makes it easier to monitor patients for tumor recurrence using a blood test for thyroglobulin (see below).

If well-differentiated thyroid cancer continues to spread, even after surgery and the administration of radioactive iodine, then external radiation therapy may be helpful. Chemotherapy is usually not very effective in this situation.

 

How are Thyroid Cancer Patients Followed?

Periodic follow-up examinations are essential for patients who have had surgery for papillary or follicular thyroid cancer, because recurrences sometimes occur many years after apparently successful surgery. These follow-up visits should include a careful history and physical examination, with particular attention to the neck area. Radioactive iodine scanning to obtain images of the neck and whole body may also be useful.

It is also helpful from time to time after surgery to measure the blood level of thyroglobulin. This substance is released by normal thyroid tissue and also by well-differentiated thyroid cancer cells. The blood level of thyroglobulin is very low after total thyroid gland removal, and in most patients who are taking thyroid hormone after thyroid surgery. An elevated or rising level of thyroglobulin generally implies persistent or growing thyroid cancer, but does not necessarily imply a poor prognosis. A high thyroglobulin level found in a follow-up examination alerts the physician to the possibility that other tests may be needed to be sure the tumor is not recurring. Unfortunately, in some thyroid cancer patients the presence of interfering antibodies in the blood may prevent accurate thyroglobulin measurement.

 

What about Thyroid Hormone Treatment?

Even if part of the thyroid remains, therapy with levothyroxine (e.g. Synthroid, Levothroid) not only is necessary to mantaining normal health, but is an important part of the follow-up care in thyroid cancer patients, since studies have shown that cancer is more likely to recur in those patients who do not take this medication. The thyroid hormone should be administered in sufficient quantities to suppress TSH levels.

New, sensitive TSH measurements are extremely useful for monitoring TSH concentrations and confirming that the serum TSH is just below normal in patients at low risk of cancer recurrence. Patients with more aggressive forms of papillary or follicular cancer probably should take larger doses of thyroxine in order to suppress TSH to just below normal levels.