Exposure to Ionizing Radiation in Adulthood and Thyroid Cancer Incidence

Department of Epidemiology, School of Public Health, University of North Carolina, Chapel Hill, NC, USA.
Epidemiology (Cambridge, Mass.) (Impact Factor: 6.2). 03/2009; 20(2):181-7. DOI: 10.1097/EDE.0b013e318196ac1c
Source: PubMed


Recent reviews of the radiation epidemiology literature have concluded that there is little evidence that exposure to ionizing radiation in adulthood causes thyroid cancer. This paper examines the association between radiation dose and thyroid cancer incidence among Japanese survivors who were adults at the time of the atomic bombings of Hiroshima and Nagasaki.
Analyses were conducted using data on cancer incidence during the period 1958-1998 among 59,687 members of the Life Span Study of atomic bomb survivors who were 20 years or older at time of bombings of Hiroshima and Nagasaki. Poisson regression methods were used to derive estimates of associations between thyroid dose in gray (Gy) and thyroid cancer incidence by sex, age at exposure, and time-since-exposure.
The number of thyroid cancer cases among women (n = 241) was nearly 5-fold the number of cases among men (n = 55). Estimated thyroid dose was positively associated with thyroid cancer incidence among women A-bomb survivors (excess relative rate/Gy = 0.70; 90% confidence interval = 0.20-1.46). In contrast, a negative association was observed between thyroid dose and thyroid cancer among men (excess relative rate/Gy = -0.25; <0 to 90% confidence interval = 0.35). The association among women was positive for all time-since-exposure periods examined but tended to diminish in magnitude with time-since-exposure.
Exposure to ionizing radiation in adulthood was positively associated with thyroid cancer among women A-bomb survivors, although the risk seems to be lower than for those exposed to radiation in childhood.

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    • "A recent analysis indicated that thyroid cancer risk in children exposed to head and neck radiation is inversely correlated to the age, decreasing to a nonstatistically significant level by age 15 [43]. However, a carcinogenic effect of radiation on the adult thyroid population cannot be excluded, as indicated by the increased incidence of thyroid cancer among female survivors of the atomic bomb that were older than 20 years at the time of the explosion [44]. Moreover, a recent report indicates that dental X-rays may increase thyroid cancer risk also in adults [45]. "
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    ABSTRACT: Background. In the last decades, thyroid cancer incidence has continuously and sharply increased all over the world. This review analyzes the possible reasons of this increase. Summary. Many experts believe that the increased incidence of thyroid cancer is apparent, because of the increased detection of small cancers in the preclinical stage. However, a true increase is also possible, as suggested by the observation that large tumors have also increased and gender differences and birth cohort effects are present. Moreover, thyroid cancer mortality, in spite of earlier diagnosis and better treatment, has not decreased but is rather increasing. Therefore, some environmental carcinogens in the industrialized lifestyle may have specifically affected the thyroid. Among potential carcinogens, the increased exposure to medical radiations is the most likely risk factor. Other factors specific for the thyroid like increased iodine intake and increased prevalence of chronic autoimmune thyroiditis cannot be excluded, while other factors like the increasing prevalence of obesity are not specific for the thyroid. Conclusions. The increased incidence of thyroid cancer is most likely due to a combination of an apparent increase due to more sensitive diagnostic procedures and of a true increase, a possible consequence of increased population exposure to radiation and to other still unrecognized carcinogens.
    Journal of Cancer Epidemiology 05/2013; 2013(1):965212. DOI:10.1155/2013/965212
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    • "Known risks factors for testicular cancer include cryptorchidism, testicular atrophy and maternal exposures [10,11]. Exposure to ionizing radiation, particularly in childhood, may influence the incidence of thyroid carcinoma later in life [12]. However, the sex differential observed among AYAs in this study is not clear, although it may imply a specific susceptibility gene hormone receptor in the pathogenesis of thyroid carcinomas or possibly due to a greater medical surveillance in young women. "
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    BMC Cancer 04/2012; 12(1):151. DOI:10.1186/1471-2407-12-151 · 3.36 Impact Factor
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    • "To illustrate the comparability of the proposed approach for estimation using the AMFIT program for background stratified Poisson regression analysis, the data from a recent analysis of the association between radiation dose and thyroid cancer incidence among female Japanese atomic bomb survivors who were aged 20 years or older at the time of the bombings in August, 1945, are used (Richardson 2009). The study included 241 thyroid cancers ascertained during the period 1958–1998 among women in the LSS. "
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    ABSTRACT: Background stratified Poisson regression is an approach that has been used in the analysis of data derived from a variety of epidemiologically important studies of radiation-exposed populations, including uranium miners, nuclear industry workers, and atomic bomb survivors. We describe a novel approach to fit Poisson regression models that adjust for a set of covariates through background stratification while directly estimating the radiation-disease association of primary interest. The approach makes use of an expression for the Poisson likelihood that treats the coefficients for stratum-specific indicator variables as 'nuisance' variables and avoids the need to explicitly estimate the coefficients for these stratum-specific parameters. Log-linear models, as well as other general relative rate models, are accommodated. This approach is illustrated using data from the Life Span Study of Japanese atomic bomb survivors and data from a study of underground uranium miners. The point estimate and confidence interval obtained from this 'conditional' regression approach are identical to the values obtained using unconditional Poisson regression with model terms for each background stratum. Moreover, it is shown that the proposed approach allows estimation of background stratified Poisson regression models of non-standard form, such as models that parameterize latency effects, as well as regression models in which the number of strata is large, thereby overcoming the limitations of previously available statistical software for fitting background stratified Poisson regression models.
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