Radiation-Induced Thyroid Cancer
Assessing the risk after Japan's nuclear disaster.
An expert in radiation-induced thyroid disease, Robert McConnell, MD, Co-Director of the Columbia Thyroid Center, discusses one of the many health concerns raised after the Japanese nuclear disaster one year ago: radiation-induced thyroid cancer.
After the Chernobyl accident in 1986, Dr. McConnell made 40 trips to the Ukraine and Belarus where much of the fallout landed. There he helped to care for and study the effects of radiation exposure on tens of thousands of people exposed to nuclear fallout, including approximately 25,000 children. About 6,000 people developed thyroid cancer, and that number is expected to increase.
What can then be expected after the Japanese reactor meltdown? Will radiation-induced cancer pose a threat to the Japanese, or to others beyond Japan's shores?
The thyroid is one of the most radiosensitive of all organs, meaning that it is particularly vulnerable to developing cancers after radiation exposure. This is especially true in children. In treatment, very high doses of radioactive iodine are administered therapeutically to treat an overactive thyroid by intentionally killing the thyroid cells. In uncontrolled exposure and lower doses, however, radioactive iodine can damage the thyroid rather than kill the cells, leading instead to an increased risk of cancer.
When large amounts of radioactive iodine are released into the environment, as happened in both Chernobyl and Japan, the thyroid will easily take it up. According to Dr. McConnell, "The thyroid cannot tell the difference between radioactive iodine and nonradioactive iodine, so if it is present in the environment, the thyroid will take it in."
Nonetheless, the risk of thyroid-induced thyroid cancer after Japan's crisis is fairly minimal, says Dr. McConnell. First, although large amounts of radioactive iodine were released into the environment, the Japanese people were evacuated from the area quickly, and afterward, they were instructed to restrict their intake of green leafy vegetables, milk, and water from potentially contaminated sources. That did not happen in 1986.
Second, the types of radiation exposure differed in the two incidents. In 1986, people were exposed to high concentrations of radioactive cesium, strontium and iodine. Cesium and strontium have long half lives, meaning that radioactive contamination will remain for hundreds of years. Potassium iodine, a safe form of iodine, that can be used to saturate the thyroid and block it from absorbing radioactive iodine, does not protect the thyroid from these two radionuclides.
In Japan, cesium and radioactive iodine were released. Although cesium increases the risk for cancer in general, it poses no specific risk to the thyroid since it is not concentrated within the gland. Radioactive iodine, with its half life of eight days, was virtually gone in about two months. Since the Japanese largely evacuated the area around the nuclear power plant, their overall exposure was much lower than those living near Chernobyl 25 years before. Residual radioactive iodine that might have spread to surrounding areas still would have degraded and disappeared relatively quickly, making it very unlikely that people in distant areas would have been impacted.
Finally, thyroid cancer is largely treatable, says Dr. McConnell. The most common type of radiation-induced thyroid cancer is papillary thyroid cancer, and the great majority of cases are slow growing and curable. At the Columbia Thyroid Center, where patients are treated by a multidisciplinary team of endocrinologists, surgeons, nuclear medicine experts, and other specialists, the survival rate is approximately 99% at ten years among men under 40 and women under 45.