Confirming the link between radiation exposure and thyroid disease, research published in the Journal of the American Medical Association shows that survivors of the atomic bombs dropped on Hiroshima and Nagasaki more than 60 years ago have an increased risk of thyroid nodules.
Malignant tumours and benign cysts of the thyroid gland were found to affect 44.8% of the survivors studied, say Misa Imaizumi, from the Radiation Effects Research Foundation in Nagasaki and Hiroshima, Japan, and colleagues.
Prevalence was significantly related to the dose of radiation exposure, with the effect particularly strong among those who were younger than 20 years old when the bombs were dropped in 1945.
"The present study revealed that 55 to 58 years after radiation exposure, a significant linear dose–response relationship existed in the prevalence of not only malignant thyroid tumours but also benign thyroid nodules and that the relationship was significantly higher in those exposed at younger ages," write the investigators.
They add: "Careful examination of the thyroid is still important long after radiation exposure, especially for people exposed at younger ages."
As thyroid disease has become a recognized tool for studying the effects of radiation exposure, Imaizumi and team assessed the incidence of thyroid disease for 4091 people aged an average of 70 years
Among the participants, 3185 were in the cities when the bombs hit, while the remainder were exposed to the radiation in utero or were outside of the city limits on the days of the attacks.
Overall, 44.8% of these people had thyroid disease. When the researchers focused on the individuals who were in the city at the time of the bombings, they found that 14.6% had solid nodules, 2.2% had malignant tumours, 4.9% benign nodules, and 7.7% benign cysts.
The scientists estimate that 28% of all solid nodules, 37% of malignant tumours, 31% of benign nodules, and 25% of cysts observed were due to radiation exposure at an average of 0.449 sieverts.
Radiation exposure did not, however, appear to be associated with autoimmune thyroid conditions such as Graves' disease.
In an accompanying editorial, John Boice, from the Vanderbilt University School of Medicine in Nashville, Tennessee, USA, says: "The risk following exposures [to radiation] in childhood apparently lasts for life, although it appears that the risk declines many years after exposure."
He suggested that this association could be because the thyroid gland in the young is particularly sensitive to radiation and proliferates in puberty.