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FIGURE 26-2. The effect of age at the time of radiation exposure on the development of thyroid cancer. The excess relative risk (ERR) per rad of thyroid exposure is shown over different age ranges. (Adapted from Schneider AB, Ron E, Lubin J, et al. Dose-response relationships for radiation-induced thyroid cancer and thyroid nodules. J Clin Endocrinol Metab 1993;77:362. © The Endocrine Society.)

FIGURE 26-2. The effect of age at the time of radiation exposure on the development of thyroid cancer. The excess relative risk (ERR) per rad of thyroid exposure is shown over different age ranges. (Adapted from Schneider AB, Ron E, Lubin J, et al. Dose-response relationships for radiation-induced thyroid cancer and thyroid nodules. J Clin Endocrinol Metab 1993;77:362. © The Endocrine Society.)

1-4 years 5-15 years

1-4 years 5-15 years was very long; the majority of patients were monitored for more than 30 to 40 years after receiving radiation. In the subgroup of patients observed for at least 40 years after exposure, 60% had thyroid nodules and 15% had thyroid cancer.12 The data of Schneider and coworkers indicate that the annual incidence for thyroid cancer after childhood irradiation does not reach a plateau even after a long period of time from exposure, suggesting that subjects irradiated between 1930 and 1950 who are now in their sixth and seventh decades of life continue to have an increased ERR.12

Second, there is possible selection bias because this is a non-case-control study. The potential for overestimation of thyroid disease was demonstrated in a study comparing the impact of childhood irradiation using two methods of analysis: a questionnaire versus clinical evaluation. The degree of increased relative risk was sixfold higher when the same population was studied by questionnaire as opposed to clinical examination.14 This difference was believed to be due to the increased attention paid to the potential development of thyroid nodules by patients who had received childhood radiation exposure and by the physicians who took care of them because of this well-defined risk factor. Although the data characterizing relationship to dose of exposure and age of exposure are unaffected in single institutional reviews, the most accurate analyses of true estimates of increased risk come from the case-control studies, in which all patients undergo a clinical evaluation by the same team of investigators.

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