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The aim of this study was to evaluate the association between thyroid cancer and diagnostic X-ray radiation based on medical records. By using the Swedish Cancer Registry, 187 living cases with thyroid cancer (81%), aged 20–70 years at the time of their diagnosis in 1980–1989 were identified in the Northern Health Care Region of Sweden. Seven cases were reclassified as having a disease other than thyroid cancer and were excluded. The investigation included 180 living cases and 360 controls from the National Population Registry. In 132 cases and 251 controls data from X-ray records on earlier investigations > 5 years prior to diagnosis and corresponding years for the controls were analysed. The mean calculated thyroid dose for the cases was 7.0 mGy (median 1.1) and for the controls 7.4 mGy (median 1.0). This study showed no difference in the total material between calculated absorbed thyroid dose of medical diagnostic X-ray in cases versus controls. In younger women (≤ 50 years at diagnosis) with papillary thyroid cancer an association was found, however it was not significant. The results could be related to selective bias and should be treated with caution. Future studies require analysis of diagnostic medical X-ray investigations involving the thyroid gland including more specific data on sex and age of exposure in the whole study group.