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The aim of this study was to investigate exposure to diagnostic and therapeutic ionizing radiation as a risk factor for thyroid cancer. This case-control study encompassed 180 women and men with thyroid cancer, aged 20-70 years at the time of diagnoses in 1980-89. From the National Population Registry 360 controls were selected. The response rate was 95% for the cases and 90% for the controls. Thyroid radiation dose from medical and dental X-ray examinations was divided into three categories. The highest exposure category (> 0.59 mGy) yielded an odds ratio (OR) for women ≤ 50 years of age of 2.7 with 95% confidence interval (CI) of 1.2-6.6. OR increased to 4.9 (CI = 1.6-16) if papillary cancer alone was considered in this age group. For women aged > 50 years and for men no significantly increased risks were seen. External radiotherapy yielded OR = 3.1 (CI = 0.9-12) and for women only OR = 4.9 (CI = 1.1-24). In papillary thyroid cancer, logistic regression gave increased risk for X-ray examinations with a thyroid dose >0.59 mGy (OR = 2.7, CI = 1.0-6.9) and for external radiotherapy (OR = 2.7, CI = 0.4-16). No increased risk was found for exposure to131 I. The findings in this study indicated an association between diagnostic or therapeutic radiation and thyroid cancer in women, but may have been influenced by recall bias.