Diagnostic X-ray exposure and female papillary thyroid cancer: a pooled analysis of two Swedish studies

The aim of the present study was to evaluate the risk for female papillary thyroid cancer from occupational and medical low level radiation exposure. The analyses are based on data from two Swedish case-control studies on determinants for thyroid cancer. One hundred and eighty six thyroid cancer cases, diagnosed during 1977-89 and aged 20-70 years, were collected from cancer registers. Twice as many population controls were selected. Questionnaires were mailed in 1990-91 to living cases and controls. A high risk was found for the occupational group of dentists/dental assistants, odds ratio (OR) = 13.1, 95% confidence interval (CI) = 2.1-389. For all occupational exposure to X-rays OR = 2.1, 95% CI = 1.0-4.4 was obtained. Diagnostic X-ray exposure was associated with increased risk, with a dose-response tendency yielding OR = 2.6, 95% CI = 1.5-5.1 for the highest absorbed thyroid dose (> 1.0 mGy). If only females of 50 years or less at diagnosis were considered, higher ORs were obtained. Increased risks were also found for some site-specific examinations, some of them giving very low radiation dose to the thyroid; more than 10 dental X-rays gave OR = 3.5, 95% CI = 1.6-7.6. A potentiated risk for prior X-rays was seen among women with three or more parities, with OR = 4.7, 95% CI = 1.5-14.8. Exposure to visual display units yielded OR = 2.3, 95% CI = 0.9-5.6. As in all questionnaire-based case-control studies possible recall bias must be considered but is unlikely to cause dose-response patterns and interaction as indicated in this study.