Revision 1
March 19, 1998
by Radiation, Science, and Health, Inc.,
Edited by J. Muckerheide

1.2.3
Medical

1.2.3.2
Medical Patients

For patients given I-131 in therapeutic applications:

"When the observed cancer incidence in a prospective study of 1,005 I-131 treated patients with thyrotoxicosis (mean follow-up of 15 years ) was compared to that of 2,141 surgically treated patients, the risk was 9.1-fold greater (p<0.05) in the irradiated group (Hoffman 1984). When compared with the Connecticut Cancer Registry, however, the relative risk from I-131 (3.8) was not statistically significant. After a mean follow-up of 8 years, the thyroid cancer incidence ratio of 21,714 I-131 treated patients to 11,732 surgically treated patients was 2.6, and was not significant (Dobyns 1974). A relative risk (insignificant) of 1.01 was found when the cancer incidence in 4,557 I-131 treated patients followed for an average of 9.5 years was compared with data from the Swedish Cancer Registry (Holm 1984)."

For patients given I-131 in diagnostic applications:

"An initial follow-up study of 10,133 subjects who received diagnostic doses of I-131 at the Radiumhemmet, Stockholm, Sweden, yielded no evidence of an increase in thyroid cancer risk (Holm 1980b, Holm 1980c). A more recent study included 35,074 Swedish subjects (28,180 women and 6,884 men) from six institutions who had survived 5 years or more after a diagnostic dose of I-131 (Holm 1988). The mean dose was 1.92 MBq of I-131 (range: 0.04-35.52 MBq), the mean radiation dose was approximately 0.5 Gy, the mean age at exposure was about 44 years (range: 1-74 years), the mean follow-up was 20 years during the period 1951-1984, and the data were compared with the Swedish Cause-of-Death Registry. A total of 50 thyroid cancers were found in the I-131 group compared with an expected number of 39.37 cases, yielding an overall standardized incidence ratio of 1.27 observed to 1.0 expected cancers (95% confidence interval, 0.94-1.67). Of the 50 observed cancers, 10 were either medullary or poorly differentiated and 1 was a sarcoma. Medullary carcinomas have not been seen to be associated with radiation exposure. Six thyroid cancers occurred among men who were 50-74 years old at the time of exposure; this subgroup yielded the only significantly increased standardized incidence ratio 3.14 (95% confidence ratio, 1.15-6.84). Sixty-eight percent of the cancers occurred among 31% of the subjects who had received a diagnostic dose of I-131 because of suspected thyroid cancer. Of these 34 cases, 15 cancers (44%) became clinically apparent 5-9 years after exposure, suggesting that they were occult at the time of the I-131 diagnostic procedure. In summary, the results of these studies do not support the conclusion that diagnostic doses of I-131 significantly increases the risk of thyroid cancer. (Holm 1988)"