2ND PRIMARY TUMORS FOLLOWING THYROID-CANCER - A SWEDISH RECORD-LINKAGE STUDY

The incidence of second primary tumors was studied through record-linkage in 2 968 thyroid cancer patients reported to the Swedish Cancer Registry during the period 1958-1975. The cohort was matched with the Swedish Cancer Registry between 1959 and 1984. A total of 283 second primary tumors were reported more than one year after thyroid cancer diagnosis, and the standardized incidence ratio (SIR) was 1.18 (95% confidence interval = 1.03-1.31). A significantly elevated risk of cancer of the kidney, endocrine glands, and nervous system was noted. Men had a higher risk (SIR = 1.37; 95% CI = 1.06-1.70) than women (SIR = 1.11; 95% CI = 0.96-1.28). Patients who were 36-45 years at the time of the thyroid cancer diagnosis were at highest risk of developing a second primary tumor (SIR = 1.35; 95% CI = 0.99-1.81). Significantly elevated risks were seen 5-9 years after the thyroid cancer diagnosis (SIR = 1.44; 95% CI = 1.14-1.69), and the SIR was close to unity after greater-or-equal-to-15 years of follow-up. Previously described elevated risks of subsequent leukemia and breast cancer were not confirmed in this study. Close medical surveillance, thyroid cancer treatment, hereditary factors, and a high frequency of autopsy could all contribute to the elevated risk of a second primary tumor in these patients.