Radioactive iodine therapy for hyperthyroidism may be associated with an increased risk of death from cancer, according to a longitudinal cohort study published in the July 1 issue of JAMA Internal Medicine.
The study followed 18,805 individuals whose hyperthyroidism was treated with radioactive iodine in the United States and United Kingdom between 1946 and 1964.
Researchers found positive dose-response relationships between radioactive iodine therapy and most of the solid cancers that were evaluated. However these only attained statistical significance in the case of female breast cancer – where there was a 12% increase in the risk of death from breast cancer from a 100-mGy tissue- or organ-absorbed dose – or for all solid cancers combined, where a 100-mGy dose to the stomach was associated with a 6% increase in death from all solid cancers.
Based on this, the authors estimated that 8% of solid cancer deaths, including 14% of breast cancer deaths, could be attributed to the radiation. When combined with current US mortality rates, that translated to around 13 excess solid cancer deaths, including three deaths from breast cancer, for every 1000 patients receiving a 100 mGy absorbed dose to the stomach or breast at age 40 years.
However they noted that patients with Graves disease are now recommended to receive higher doses, and calculated that for 150-mGy, 200-mGy and 250-mGy dosages there would be 19-32 excess solid cancer deaths per 1000 patients treated at age 40 years.
“To our knowledge, this is the first study to characterize the dose-response relationship between RAI treatment and site-specific cancer mortality in patients with hyperthyroidism using reliable estimates of absorbed dose to exposed organs or tissues,” wrote Cari M. Kitahara, PhD, from the Division of Cancer Epidemiology and Genetics at the National Cancer Institute, and co-authors.
Radioactive iodine therapy did not appear to be associated with an increased risk of death from leukemia, non-Hodgkin lymphoma or multiple myeloma.
The authors noted that this was unexpected given previous findings of an elevated risk of leukemia in patients with thyroid cancer who received higher levels of radiation. They suggested that the greater uncertainty in calculation of red bone marrow exposure compared to that of other organs and tissue, as well as the relatively small number of leukemia deaths, may have limited their ability to detect a dose-response relationship.
The study was funded by the National Cancer Institute. One author declared membership of a consortium supported by the pharmaceutical sector.
SOURCE: Kitahara C et al. JAMA Internal Medicine 2019, July 1. .