Nuclear Radiation and Thyroid Cancer; A Systematic Review

Background: The increasing prevalence of thyroid cancer among the survivors of atomic bomb attacks in Japan, nuclear fallout in the Marshall Islands, and Chernobyl nuclear power plant accident indicate a strong relationship between nuclear radiation and the occurrence of thyroid cancer. Materials and Methods: Systematic researches were conducted in the medical database of the American National Library of Congress, PubMed, using keywords of thyroid cancer, radiation, papillary thyroid cancer. After screening the titles and abstracts of 1791 citations, a total of 77 citations were identified to include in the current study. Results: The latency period for developing nuclear radiation-induced thyroid cancers was 5 to 10 years to decades after exposure. The most important risk factor for developing radiation-induced thyroid cancer was the received radiation dose. A strong dose-response relationship between radiation dose and the incidence of thyroid cancer has been reported (P value<0.001). Also, a strong relationship between the age of exposure to nuclear radiation and the incidence of thyroid cancer has been reported among the survivors of atomic accidents. Although the incidence rate of thyroid cancer in girls was 2-3 times more than boys in some studies, in other studies from the Chernobyl accident, no gender effect was observed. The risk of radiation-induced thyroid cancer was estimated to be higher in areas with iodine deficiency. The most common route of internal radiation was through absorption of radioactive iodine isotopes via consumption of the contaminated milk. The main mechanism in the pathogenesis of radiation-induced thyroid cancer was activation of the MAPK pathway through genetic alterations (e.g., gene rearrangements in RET family and NTRK1) or point mutations (e.g., in BRAF). Among these genetic alteration, RET/PTC3 and RET/PTC1 rearrangements were more common in radiation--induced thyroid cancer. The clinical behavior of radiation-induced thyroid cancer was a non-invasive type similar to the sporadic one at the same age Conclusion: Nuclear radiation, especially at a young age (under 20 years old), directly or indirectly in a dose-dependent manner via reactive oxygen species production induces gene alterations that underlie the increasing incidence of thyroid cancer after radiation exposure. Preparedness to deal with nuclear radiation including prescription of stable iodine prophylaxis, evacuation of the contaminated regions and restriction of access to contaminated foods are recommended in power plant accidents or atomic bomb explosions.