Cancer Research Lab

Childhood exposure to ionizing radiation to treat tinea capitis increases the long-term risk of thyroid cancer, according to findings published in the December issue of the Journal of Clinical Endocrinology and Metabolism.

“The thyroid gland is known to be sensitive to the carcinogenic effect of ionizing radiation, especially in children,” Dr. Siegal Sadetzki, of Chaim Sheba Medical Center, Tel Hashomer, Israel, and colleagues write.

The researchers assessed the effect of low doses of ionizing radiation on the risk of developing thyroid cancer up to 54 years after exposure in childhood. The study population included 10,834 subjects irradiated against tinea capitis in the 1950s, an equal number of matched nonirradiated subjects taken from the general population, and 5392 nonirradiated siblings. National registries, updated to December 2002, were used to obtain cancer statistics and vital status data.

The subjects were followed for a median of 46 years. The mean age at irradiation was 7.1 years and the mean age at the end of follow-up was 52.1 years. A total of 159 cases of thyroid cancer were diagnosed during the study period (103 in irradiated individuals and 56 in nonirradiated subjects).

The investigators found that the total excess relative risk per gray (ERR/Gy) for developing thyroid cancer was 20.2 per 10,000 person-years. The risk was inversely associated with age at exposure.

“Significantly elevated ERR/Gy was first noticed for latent periods of 10 to 19 years after exposure, reaching about 29 in the 20 to 30 years after exposure,” Dr. Sadetzki’s team reports. “A dramatic decrease in the ERR was observed 40 years after exposure,” they note. “No difference in the ERR was found relative to the number of irradiations.”

The investigators say the results are in line, generally, with patterns of risk modification seen in other studies of radiation-induced thyroid cancer. “The carcinogenic effects of low-level radiation,” they conclude, “must be considered in the planning of safety measures against public health hazards.”