Cancer Risk in Population Examined With Diagnostic Doses of 131I

Abstract

Previously, we conducted a study of 35,074 patients receiving diagnostic doses of 131I for suspected thyroid disorders between 1951 and 1969. We reported that, between 1958 and 1984, the incidence of thyroid cancers in these patients was insignificantly greater than the incidence expected in the general population. This increase was attributed to the underlying condition that prompted the examination and not to the administration of 131I. The purpose of the present study was to analyze the total cancer risk in the same cohort of patients examined with diagnostic doses of 131I. To further evaluate the underlying risk of disease in these patients, we compared the incidence of all cancers with that expected in the general population. The average radiation dose was approximately 500 mGy to the thyroid and less than 10 mGy to other organs. In the 35,074 patients, 3,746 cancers occurred following the first 5 years after examination, and the resulting standardized incidence rate (SIR) was 1.01 (95% confidence interval = 0.98-1.04). SIRs were significantly increased for endocrine tumors other than thyroid cancer (1.93) and for lymphomas (1.24), leukemias (1.34), and nervous system tumors (1.19). The risk of leukemia was similar for chronic lymphocytic leukemia (CLL) (SIR = 1.30) and non-CLL (SIR = 1.34). SIR was significantly decreased for cancers of the female genital organs (0.86). The risk for cancer of all sites and types combined was highest 5-9 years after examination (SIR = 1.07) and did not differ from unity thereafter. With greater than or equal to 10 years of follow-up, risk was not statistically associated with the dose of 131I. Overall, the data exclude cancer risk increments greater than 5% (SIR = 1.05) with 95% confidence. The significant increase in the risk of non-CLL, a prominent radiogenic malignancy, however, warrants special attention. We are continuing our study to determine the possible factors involved in the significant increase in the risk of leukemia.

Radiation exposure and thyroid cancer: a review

“…In the past, it was thought that the dose rate (Gy/time unit) was an important parameter because radiation-induced thyroid tumors were observed after exposure to external radiation at high dose rates, but the study of subjects exposed to 131 I for medical conditions in Sweden did not reveal any increased risk of thyroid cancer (22). Also, the occurrence of thyroid tumors in the Marshall Islands was attributed to the exposure to short-lived radioisotopes of iodine.…”

Analysis of thyroid malignant pathologic findings identified during 3 rounds of screening (1997‐2008) of a cohort of children and adolescents from Belarus exposed to radioiodines after the Chernobyl accident

“…Previous studies of mostly adult patients exposed to I-131 from diagnostic and therapeutic procedures had largely negative findings 3,4 Four-five years after the accident, 5 ecological 6,7 and then analytical epidemiological studies 8,9 reported increased risks of thyroid cancer among those living in Chernobyl-contaminated areas. It was not clear, however, whether the observed increases were due to radiation doses or to intensive and wide-spread screening for thyroid abnormalities.…”

Perspective: National Cancer Institute summary report about estimated exposures and thyroid doses received from iodine 131 in fallout after Nevada atmospheric nuclear bomb tests

“…In Sweden, 35,074 patients receiving diagnostic 131 I (estimated total thyroid dose, 110 rad per scan) have been studied carefully, and based on experience with external x‐ray radiation, a substantially higher rate of thyroid cancers was anticipated. No increase in thyroid cancers was detected, however 4–6 . A German study of almost 14,000 patients similarly treated also failed to detect any carcinogenic effect associated with such a dose 7…”

“…The relative biologic effectiveness (RBE) differs significantly for different types of radiation (e.g., neutron radiation has a much higher RBE than does gamma radiation). Also, type of dose (i.e., continuous versus intermittent), frequency of exposure, and dose per exposure can alter the RBE of a given total dose dramatically 4 …”

“…The impact of lower doses, particularly their effect on children, remains undetermined. Age at exposure may significantly modify the carcinogenic effect of 131 I 4 …”