People who were exposed to radioactive iodine (I-131) as children or adolescents after the 1986 Chernobyl nuclear disaster have a long-term increased risk of developing radiation-related thyroid cancer. In a study of more than 12,000 people in three Ukrainian states near the Chernobyl site, researchers found that the risk of radiation-related thyroid cancer doubled for every gray (Gy) of exposure to the thyroid. (A gray is an international measurement used for radiation dose.) This risk did not decrease over 9 years of follow-up during the study.

The findings were published March 14 in Environmental Health Perspectives.

Researchers led by Dr. Alina Brenner of NCI’s Division of Cancer Epidemiology and Genetics, in collaboration with Ukrainian scientists, prospectively studied 12,514 people who were younger than 18 years of age at the time of the accident and were exposed to a wide range of I-131 doses. All participants had direct measurements of thyroid radioactivity taken within 2 months of the accident. Additional data on I-131 exposure included information on diet (the greatest source of exposure was from consumption of contaminated milk) and lifestyle around the time of the accident.

Between the start of the study in 1998 and 2007, all participants underwent screening examinations every 2 years, regardless of their dose. Individuals diagnosed with thyroid cancer during the first examination were excluded from this analysis.

Sixty-five cases of thyroid cancer were discovered during follow-up. Radiation-related risk of thyroid cancer increased with increasing I-131 dose and was greatest for those individuals who were younger at the time of exposure. Risk appeared similar for men and women.

“Our results suggest that thyroid cancers attributable to I-131 exposure continue to occur two decades after exposure,” concluded the authors. In addition, they wrote, the risk per Gy of exposure does not appear to diminish with time, at least not in the amount of time that this cohort has been followed. Since radiation-related cancer risks are known to persist for many decades, the participants will need to be followed for a longer period of time to determine if an eventual decline in risk occurs.

source : NCI bulletin