MEDICAL DIAGNOSTIC X-RAYS AND THYROID-CANCER

Background: Diagnostic x rays are the largest man-made source of expos ure to ionizing radiation for the general population, Whether there ar e meaningful cancer risks associated with such exposures is unclear. M ost 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 p resent study used information recorded prospectively in hospital chart s to address the relationship between medical diagnostic x rays and ri sk of thyroid cancer. Methods: The Swedish Cancer Registry and the Upp sala-Orebro Regional Cancer Registry were used to identify persons wit h papillary or follicular thyroid cancer diagnosed from January 1, 198 0, through December 31, 1992, among residents of the Uppsala Health Ca re Region. After histopathologic review, there were 484 such case subj ects available for study. An equal number of age-, sex-, and county of residence-matched control subjects from the general population were r andomly selected on the basis of the Swedish Registry of the Total Pop ulation. Lifetime residential histories were compiled, and radiology r ecords were searched at all Swedish hospitals serving regions where st udy subjects ever maintained an official residence, Approximate radiat ion doses to the thyroid gland for specific types of x-ray examination s were assigned on the basis of mean values of measurements made in Sw eden in 1973-1975 and in the United States in 1970, Odds ratios were u sed to evaluate the association between diagnostic radiography and ris k of thyroid cancer, Results: A total of 3853 medical diagnostic x ray s mere ascertained among thyroid cancer case subjects and 4039 among t he matched control subjects, There was no tendency for case subjects t o 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 o r 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 ad olescence, when susceptibility to radiation-induced thyroid cancer is greatest. The relative risk of thyroid cancer was not significantly as sociated with estimated cumulative dose to the thyroid gland from diag nostic x rays (two-sided P for trend = .80). Conclusion: These data in dicate that the risk of thyroid cancer due to medical diagnostic x ray s, if any, is very small.