Thyroid cancer? While few people devoted much thought to it before, this disease has been thrust into public consciousness by the tragic news about Japan's recent nuclear catastrophe. After a giant quake and huge tsunami crippled the Fukushima plant, radioactive iodine was released into the atmosphere, contaminating areas nearby and carried by winds aloft for hundreds of miles. Radioactive iodine can cause thyroid cancer, especially in children and teenagers who breathe it in, and particularly if they ingest it from milk and produce. This was proven true, for example, with an increase in the incidence of thyroid cancer among the young exposed to radioactive iodine in the Soviet-era Chernobyl nuclear calamity.
But recent reports exaggerating the potential harms of the Japanese accident - with projections that part of the plume of radioactive iodine from it would reach the United States - needlessly panicked many Americans, especially those living on the West Coast; see my prior blog posts, "A U.S. Health Meltdown Over Japan's Nuclear Crisis" and "A Nuclear Accident Prescription: Why Japan Has Distributed Potassium Iodide Doses." While the actual amounts of radiation from the accident reaching our shores will be far below levels that might pose health hazards, this nuclear incident -- and the new public awareness that it has brought to the association of radioactive iodine and thyroid cancer -- offer a timely opportunity to review the facts about the disease.
Increasing Incidence of Thyroid Cancer
The thyroid, a butterfly-shaped gland beneath the Adam's apple in the neck, is responsible for producing and regulating some of the hormones that control metabolism. The incidence of cancer in it has more than doubled over the last three decades, increasing to almost 12 cases per 100,000 in 2007 from 4.85 cases per 100,000 in 1975. Much of this rise can be attributed to the increased detection of small tumors. There is increased utilization of imaging technology such as neck ultrasounds, CT scans, MRIs and PET scans that has resulted in the discovery of incidental thyroid nodules. Studies have shown that when doctors feel the neck during a physical examination, about 5 percent of patients will be found to have thyroid nodules. However, when the thyroid is studied by an ultrasound examination, it turns out that half of all adults actually have thyroid nodules and 5 percent or so of these represent low-grade cancers. When doctors detect these "incidental" thyroid nodules, they will subject some of them to a fine needle aspiration biopsy to determine if they are cancerous. Many, but not all of the increased number of thyroid cancers that are found are small (called microcarcinomas), measuring less than 1 centimeter (less than ½ inch).
The increase of observed thyroid cancer, however, is not fully explained just by the disease's improved detection through greater use of imaging procedures. There also has been a real increase in the incidence of these tumors.
There are two known causes of well-differentiated thyroid cancer, the most common variety. The first is exposure to therapeutic x-rays. Indeed, x-ray therapy once was used to treat children with enlarged tonsils and adenoids, birth marks and even ringworm, as well as for teens with acne. This led to an increase in the development of both benign and malignant thyroid growths. Well-differentiated thyroid cancer is the type that was caused by the Chernobyl nuclear disaster. The second known risk factor is a genetic predisposition, with 5 percent or so of the occurrences of this most common form of thyroid cancer running in families.
However, these two risk factors alone also do not account for the rising incidence of thyroid cancer. It is likely that some of the thyroid cancer increase can be attributed to other environmental factors, such as the amount of iodine people ingest; environmental toxins, such as materials used to make plastics, fire retardants and pesticides; or other unknown causes.
Thyroid Cancer Treatment
Close to 90 percent of these thyroid cancers fall into the group of well-differentiated tumors; women are three times more likely than men to develop these cancers. More than 95 percent of patients younger than 45 who develop one of these tumors will survive, even if there is evidence of spread to the lymph nodes in the neck, which is found at the time of diagnosis in many patients. Those older than 60, especially men with large tumors, do not fare as well, but their overall prognosis is still good.
Most patients with differentiated thyroid cancer undergo surgery to remove their thyroid. Depending on their age and the degree of involvement of surrounding tissues and lymph nodes, or if there is evidence of the disease's spread to distant tissues such as lung or bone, radioactive iodine may be used for treatment. It is a paradox that relatively low levels of radioactive iodine can cause thyroid cancer by injuring DNA in thyroid cells, while large doses of radioactive iodine can kill both normal and cancer cells in the thyroid.
All patients with thyroid cancer are placed on thyroid hormone. Doctors very rarely treat these patients with external beam radiation therapy and even less frequently use some new medications that target the molecular abnormalities found in thyroid cancer. After a patient's initial treatment, the standard follow-up includes periodic ultrasound exams of the neck as well as blood tests to measure thyroglobulin, a protein that is an excellent tumor marker for differentiated thyroid cancer.
Other Forms of Thyroid Cancer
Medullary thyroid cancer is a less common form of the disease, accounting for 5 to 10 percent of patients. This tumor is more aggressive than the more common form and has a higher tendency to run in families. It can be detected through a fine needle aspiration biopsy of the suspicious thyroid nodule or through the measurement of calcitonin, a blood protein that is produced by the cells that form the tumor.
Fortunately, the least common form of thyroid cancer is anaplastic cancer, which progresses rapidly and almost always is a fatal form of the disease. Less than 1 percent of patients with thyroid cancer have this form.
The management of thyroid cancer has evolved greatly over the last several decades and a number of centers have been developed with multidisciplinary teams to treat the disease. Further information about thyroid cancer can be obtained online from the Cedars-Sinai Thyroid Cancer Center, The American Thyroid Association and The Endocrine Society's Hormone Foundation.