Researchers from National Cancer Institute (NCI) has found that treating patients with hyperthyroidism using radioactive iodine (RAI) show an association between the dose of therapy and long-term risk of death from solid cancers, including breast cancer.
“We identified a clear dose–response relationship between this widely used treatment and long-term risk of death from solid cancer, including breast cancer, in the largest cohort study to date of patients treated for hyperthyroidism,” said the lead author, Cari Kitahara, Ph.D., of NCI’s Division of Cancer Epidemiology and Genetics, in a news release.
“We estimated that for every 1,000 patients treated currently using a standard range of doses, about 20 to 30 additional solid cancer deaths would occur as a result of the radiation exposure.”she added.
RAI is one of three commonly used treatments for hyperthyroidism. The other two are anti-thyroid drugs, which have been rising in popularity, and surgical treatment, which is used least often.
The Cooperative Thyrotoxicosis Therapy Follow-up study conducted a long-term follow-up of a large cohort of people with hyperthyroidism (mainly Graves’ disease) who were treated with radiation between 1946 and 1964.
In the new analysis — which included nearly 19,000 people from the original cohort, all of whom had received RAI and none of whom had cancer at study entry — the researchers used a novel, comprehensive method of estimating radiation doses to each organ or tissue.
Most of the radiation was absorbed by the thyroid gland, but other organs like the breast and stomach were also found to be exposed during the treatment.
The researchers observed positive dose–response relationships between the dose absorbed by an organ and mortality from cancer at that site.
According to the research, the relationship was statistically significant for female breast cancer, for which every 100 milligray (mGy) of dose led to a 12% increased relative risk of breast cancer mortality, and for all other solid tumours considered together, for which relative risk of mortality was increased by 5% per every 100 mGy.
Based on these findings, the researchers estimated that for every 1,000 patients aged 40 years with hyperthyroidism who were treated with the radiation doses typical of current treatment, a lifetime excess of 19 to 32 radiation-attributable solid cancer deaths would be expected.
“We found the increased risks of death from solid cancer overall and from breast cancer more specifically to be modest, but RAI is still a widely used treatment for hyperthyroidism,” said Kitahara.
The researchers wrote that additional research is needed to more comprehensively assess the risk–benefit ratio of radiation versus other available treatment options for hyperthyroidism. Furthermore, because the types of anti-thyroid drugs administered to patients in the original cohort were different from those prescribed more recently, the researchers wrote that more studies are needed to evaluate long-term health effects of current anti-thyroid drugs, including in comparison to RAI treatment.
The study is published in JAMA Internal Medicine.