, including breast cancer, according to a new analysis of the United States SEER cancer registries.
The study included data from 36,311 patients between 1975 and 2017. Among 5-year differentiated thyroid cancer survivors, over a median follow-up of 15.6 years, radioactive iodine treatment was linked to a 23% increased risk of solid tumors. Among 20-year survivors, there was a 47% increased risk in solid tumors and a 46% increased risk of breast cancer. Two-year survivors had a 51% increased risk of hematologic malignancies, including a 92% increased risk of leukemia. The researchers estimate that 6% of all solid tumors, 5% of breast tumors, and 14% of hematologic malignancies among differentiated thyroid cancer patients who have survived at least 1 year are attributable to radioactive iodine (RAI).
“Our study is not the first to show an increased risk of leukemia or solid cancer after RAI therapy, although some may be surprised about the increased risk of breast cancer, which was not observed in some earlier studies on this topic. The large size of our study, our focus on younger patients (who are more susceptible to the late effects of radiation therapy than older patients), and more than 40 years of follow-up, enabled us to provide more precise estimates of these risks. Our findings were not surprising given current understanding of the long-term, carcinogenic effects of radiation exposure,” said lead author Cari Kitahara, PhD, senior investigator in the division of cancer epidemiology and genetics at National Cancer Institute. The risk estimates also are similar to previous studies of exposure to medial and nonmedical radiation sources, she added.
Although radioactive iodine has seen an increase in use for treatment of differentiated thyroid cancer, there is little evidence that it improves outcomes in localized differentiated thyroid cancer, and the American Thyroid Association guidelines recommend against radioactive iodine therapy for low-risk differentiated thyroid cancers smaller than 1 cm, and lower radiation levels for larger tumors. The pediatric guideline suggests a similar approach, except that it doesn’t discourage use of RAI in low-risk differentiated thyroid cancers.
“Physicians should discuss the overall balance of risks and benefits of RAI therapy with their patients. Although RAI has been used in the management of thyroid cancer for many decades, clinical practice guidelines have changed over time and now encourage avoidance of unnecessary or excessive use of RAI therapy for low-risk tumors. Our results suggest that even greater caution and more consideration of the late effects of RAI therapy are needed for younger patients, who are more vulnerable to the carcinogenic effects of radiation exposure and are more likely to experience these long-term effects than older adults,” Dr. Kitahara said.
The study was funded by the National Cancer Institute. Dr. Kitahara has no relevant financial disclosures.