Background: The thyroid gland is known to be sensitive to the carcinogenic effect of ionizing radiation, especially in children. The role of potential modifiers of the risk, and latency period effects, need further investigation. We examined the effect of low doses of ionizing radiation (4.5-49.5cGy) on the risk of developing thyroid cancer after long latent periods of up to 54 yr following childhood exposure.

Methods: The study population included 10,834 individuals irradiated against Tinea Capitis in the 1950s, and two matched non-irradiated groups (general population and siblings) for comparison. Cancer statistics and vital status data were obtained from national registries, updated to December 2002. Excess relative and absolute risks (ERR, EAR) were estimated using Poisson regression for survival analysis.

Results: Within the study period 159 cases of thyroid cancer were diagnosed. Total ERR/Gy and EAR/Gy/10⁴ PY for developing thyroid cancer reached 20.2 (95% CI = 11.8-32.3) and 9.9 (95% CI=5.7-14.7) respectively. The risk was positively associated with dose, and negatively associated with age at exposure. ERR/Gy was significantly elevated 10-19 yr following exposure, peaking at 20-30 yr, and decreasing dramatically (although still significantly elevated) 40 yr after exposure.

Conclusions: Our findings agree with patterns of risk modification seen in most studies of radiation-induced thyroid cancer, although risk per unit dose seems higher. Our data show that 40 yr after irradiation ERR decreases dramatically, though remaining significantly elevated. The hypothesis of different genetic susceptibility of the Jewish population deserves further exploration.