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Thyroid cancer incidence among atomic-bomb survivors: An update in the Life Span Study
*Kyoji Furukawa, Radiation Effects Research Foundation
Keywords: thyroid cancer, incidence, atomic-bomb radiation
Studies of Japanese atomic-bomb survivors have been providing important knowledge on the late health effects of acute radiation exposure at low-moderate doses (1, 2). As one of the earliest solid cancers linked to atomic-bomb radiation exposure, thyroid cancer has been the subject of continuing investigation in the follow-up of the Life Span Study (LSS) cohort of atomic-bomb survivors in Hiroshima and Nagasaki (3, 4). Having extended the follow-up through 2005, the present study analyzed the temporal patterns of thyroid cancer risk up to 60 years after exposure in the LSS cohort. In the present study, both clinically and microscopically diagnosed tumors were ascertained using all accessible case sources in Hiroshima and Nagasaki and histologically classified under a uniform scheme by a group of study pathologists. Among the 105,401 eligible subjects (43% of whom were alive at the end of 2005), 371 thyroid cancer cases (292 papillary carcinomas, 13 follicular carcinomas, and 66 others) were identified as a first primary between 1958 and 2005. Based on a linear non-threshold dose-response model with effect modification by age at exposure and attained age, the gender-averaged excess relative risk (ERR) and excess absolute rate (EAR) of thyroid cancer at age 60 after exposure at age 10 were estimated as 1.28 (95% CI: 0.59-2.70) at 1 Gy and 29.5 (13.8-49.6) cases per 100,000 person-year-Gy, respectively. The EAR for women was significantly higher than that for men (female:male ratio=6.3), while the sex ratio of the ERR was smaller and not statistically significant. Both the ERR and EAR significantly and rapidly decreased with increasing age at exposure, and there was no evidence for a significant excess risk among those exposed at age 20 or older. About 36% of the cases among those exposed at age < 20 were estimated to be attributable to radiation exposure, which was considerably higher than that of 4% for those exposed at age >= 20. Among those exposed as children or adolescents, the attributable fraction decreased with attained age, but remained somewhat high (15%) during the latest follow-up period of 2001-2005. The latest LSS data clearly indicated that radiation-related increase in thyroid cancer in atomic-bomb survivors was mostly derived from the increase among those exposed as children/adolescents and that radiation-related thyroid cancer risk decreased with time but persisted for at least 50 years after exposure. In addition, recent studies with the Adult Health Study participants, a clinical sub-cohort of the LSS, and tissue samples available in the LSS have provided findings about radiation effects on thyroid nodules and autoimmune thyroid diseases (5) and molecular mechanisms behind thyroid carcinogenesis (6). With a long term follow-up of large cohort of men and women of all ages, largely complete cancer ascertainment, and detailed information on lifestyle factors available to a large portion of the cohort, data of atomic-bomb survivors will continue to be an important source of information to obtain the full picture of the long-term thyroid cancer risk.
References: 1. National Research Council Committee on the Biological Effects of Radiation. Health risks from exposure to low levels of ionizing radiation (BEIR VII Phase 2). National Academy Press, Washington DC, 2006. 2. Ozasa K, Shimizu Y, Suyama A, Kasagi F, Soda M, Grant EJ, et al. Studies of the Mortality of Atomic Bomb Survivors, Report 14, 1950–2003: An Overview of Cancer and Noncancer Diseases. Radiat Res 2012; 177:229-243. 3. Preston DL, Ron E, Tokuoka S, Funamoto S, Nishi N, Soda M, et al. Solid cancer incidence in atomic bomb survivors: 1958-1998. Radiat Res 2007; 168:1-64. 4. Hayashi Y, Lagarde F, Tsuda N, Funamoto S, Preston DL, Koyama K, et al. Papillary microcarcinoma of the thyroid among atomic bomb survivors: tumor characteristics and radiation risk. Cancer 2010; 116:1646-1655. 5. Imaizumi M, Usa T, Tominaga T, Neriishi K, Akahoshi M, Nakashima E, et al. Radiation dose-response relationships for thyroid nodules and autoimmune thyroid diseases in Hiroshima and Nagasaki atomic bomb survivors 55-58 years after radiation exposure. JAMA 2006; 295:1011-1022. 6. Hamatani K, Eguchi H, Ito R, Mukai M, Takahashi K, Taga M, et al. RET/PTC rearrangements preferentially occurred in papillary thyroid cancer among atomic bomb survivors exposed to high radiation dose. Cancer Res 2008; 68:7176-7182.