Background : Diagnostic x rays are the largest man-made source of exposure to ionizing radiation for the general population. Whether there are meaningful cancer risks associated with such exposures is unclear. Most previous case-control studies have relied on recalled histories of x rays, and there is concern that completeness and accuracy of recall might differ between cancer case and control subjects. Purpose : The present study used information recorded prospectively in hospital charts to address the relationship between medical diagnostic x rays and risk of thyroid cancer. Methods : The Swedish Cancer Registry and the Uppsala-Örebro Regional Cancer Registry were used to identify persons with papillary or follicular thyroid cancer diagnosed from January 1, 1980, through December 31, 1992, among residents of the Uppsala Health Care Region. After histopathologic review, there were 484 such case subjects available for study. An equal number of age-, sex-, and county of residence-matched control subjects from the general population were randomly selected on the basis of the Swedish Registry of the Total Population. Lifetime residential histories were compiled, and radiology records were searched at all Swedish hospitals serving regions where study subjects ever maintained an official residence. Approximate radiation doses to the thyroid gland for specific types of x-ray examinations were assigned on the basis of mean values of measurements made in Sweden in 1973–1975 and in the United States in 1970. Odds ratios were used to evaluate the association between diagnostic radiography and risk of thyroid cancer. Results : A total of 3853 medical diagnostic x rays were ascertained among thyroid cancer case subjects and 4039 among the matched control subjects. There was no tendency for case subjects to have had more of the types of x-ray procedure associated with higher radiation dose to the thyroid gland (i.e., those involving the head or neck area). This finding was true even when analysis was restricted to x rays occurring before 1960, when doses likely were higher than in more recent years, and for examinations occurring in childhood and adolescence, when susceptibility to radiation-induced thyroid cancer is greatest. The relative risk of thyroid cancer was not significantly associated with estimated cumulative dose to the thyroid gland from diagnostic x rays (two-sided P for trend = .80). Conclusion : These data indicate that the risk of thyroid cancer due to medical diagnostic x rays, if any, is very small. [J Natl Cancer Inst 1995;87:1613–21]

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