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A-bomb radiation raises risk of thyroid tumors
By Ben Wasserman
Mar 1, 2006
The Japanese survivors of the two atomic bombs face an elevated risk of thyroid tumors even 60 years after they were exposed to the radiation, according to a new study appearing in the March 1 issue of Journal of the American Medical Association.
The study, conducted by Misa Imaizumi, M.D., of the Radiation Effects Research Foundation, Nagasaki and Hiroshima, Japan, and colleagues, found that higher exposure of the thyroid gland to radiation was linked with higher incidence of certain thyroid diseases including tumors, benign or malignant, and cysts. In addition, radiation exposure at younger ages was associated with an even higher risk.
The foundation, sponsored by the Japanese and U.S. governments and its predecessor, the Atomic Bomb Casualty Commission, has studied more than 120,000 survivors since 1950. The two atomic bombs killed 200,000 people immediately and in the months afterwards.
Thyroid gland is known to be very sensitive to radiation and easy to develop tumors after radiated. High doses of radiation were used in the past to shrink enlarged thyroid gland or to treat other thyroid disorders. Previous studies have found those who received radiation at young ages had an increased risk of malignant and benign thyroid nodules.
Between 2000 and 2003, Imaizimi and colleagues surveyed 4,091 survivors (1352 men and 2739 women) at an average age of 70 of the 1945 atomic bombs in Hiroshima and Nagasaki for diagnosed cases of thyroid nodules (malignant and benign) and autoimmune thyroid diseases. They determined the radiation dose-response relationships for these thyroid diseases.
They found 44.8 percent of the total survivors suffered thyroid diseases. Among those with thyroid disorders, 436 were men (32 percent) and 1397 women (51.0%). It is a fact that the thyroid gland of females is more sensitive than that of men.
Among 3,185 survivors, excluding those who were in utero, not in the cities at the time of the atomic bombings or with radiation doses unknown, all solid nodules, malignant tumors, and benign nodules and cysts accounted for 14.6, 2.2, 4.9 and 7.7 percent, respectively.
Radiation was found to significantly attribute to the prevalence of all solid nodules, malignant tumors, benign nodules and cysts. 37 percent of malignant tumors, 31 percent of benign nodules, and 25 percent of cysts were associated with atomic radiation exposure.
There was apparently no significant dose-response relationship between radiation exposure and antithyroid antibody-positive hypothyroidism, or Graves disease - an inflammatory disorder of the thyroid gland.
Radiation exposure were particularly damaging to those who were exposed at a young age. There was a linear dose response relationship for all conditions studied among those who were younger than 20 years when exposed. The dose effects were significantly higher in those who were younger than 20 years when exposed than those who were older.
No dose response was found in those who were older than 20 when exposed to atomic bomb radiation, the authors write. In addition, autoimmune thyroid diseases were not found to be significantly associated with radiation exposure in this study.
"The risk following exposures [to radiation] in childhood apparently lasts for life, although it appears that the risk declines many years after exposure," says John D. Boise Jr. in an accompanying editorial. Dr. Boise is an epidemiologist of the International Epidemiology Institute, Rockville, MD and professor of Vanderbilt University School of Medicine, Nashville, Tenn.
"The radiosensitivity of the young thyroid gland is high and most likely relates to subsequent proliferative activity of the gland during puberty and growth, but the reasons for the absence of risk following adult exposures are not entirely clear," Boise says. "The risk of thyroid cancer seems to be enhanced if diets deficient in stable iodine result in chronic thyroid stimulation."
The results of the survey suggest that radiation-induced damage may manifest itself in one's later life. People who had received radiation in the past should tell their doctors if their doctors intend to treat them with anything radiation-based.
Radiation, mostly ionizing radiation, is commonly used to diagnose and or treat many types of diseases. Ionizing radiation has been recognized by the US government as a human cancer-causing agent. There is no safe threshold for radiation, meaning any amount of radiation can cause accumulative damages to the issue.
The US government recommends people take potassium iodide as a thyroid blocking agent in radiation emergencies. But this agent does not help protect against other tumors induced by radiation.
Source: http://jama.ama-assn.org