Thyroid Cancer After Diagnostic Doses of Iodine-131: A Retrospective Cohort Study

Abstract

The incidence of thyroid cancer was evaluated in 35,074 patients examined for suspected thyroid disorders between 1951 and 1969 with an average of 1.92 megabecquerel [(MBq) μCi] of ^131I. The radiation dose to the thyroid gland was, on the average, approximately 0.5 Gy. The mean age at the time of examination was 44 years; 5% were under. Record linkage with the Swedish cancer Register identified ^131I examination, in contrast to 39.4 expected based on general population rates [standardized incidence ratio (SIR)=1.27,95% confidence interval=0.94‐1.67]. Risk was highest among males (SIR=2.22,(i)n=23), and patients receiving more than 74 ⊸ or 2.74 MBq of ^I (SIR=2.04,(i)n=17). However, these observations were suspected thyroid tumor received the highest ^131I exposures and were at highest overall risk (SIR=2.77, (i)n=34). Patients given ^131I for reasons other than a suspected tumore were not at increased risk (SIR=0.62, (i)n=16). Patients anticipated to be at highest risk, i.e., women (SIR=1.12, (i)n=40) and those observed for 10 years or more (SIR=0.93, (i)n=27), showed no evidence of a dose response. Overall, these data provide little proof that ^131I is carcinogenic potential of internal ^131I beta particles might be as low as four times less than external x rays or γ rays. [J Natl Cancer Inst 1988;80:1132-1138]