In a study reported in the Journal of Clinical Oncology, Pasqual et al found that patients younger than 45 years who received radioactive iodine treatment for nonmetastatic differentiated thyroid cancer had an increased risk for the subsequent development of primary solid tumors.
As stated by the investigators, “Since the 1980s, both the incidence of differentiated thyroid cancer and use of radioactive iodine treatment increased markedly. Radioactive iodine has been associated with an increased risk of leukemia, but risks of second solid malignancies remain unclear.”
The study used SEER (Surveillance, Epidemiology, and End Results) registry data through 2017 to estimate relative risks (RRs) for solid and hematologic malignancies associated with radioactive iodine treatment in patients diagnosed with nonmetastatic differentiated thyroid cancer before age 45 years. The total analysis population included 36,311 patients, with 45% receiving radioactive iodine treatment.
In addition to leukemia, radioactive iodine treatment for childhood and young-adulthood differentiated thyroid cancer was associated with increased risks of several solid cancers, particularly more than 20 years after exposure, supporting the need for long-term surveillance of these patients.— Pasqual et al
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Among 27,050 ≥ 5-year survivors (median follow-up = 15 years), radioactive iodine treatment was associated with an increased risk of solid malignancies (RR = 1.23, 95% CI = 1.11–1.37). Risk was increased significantly for uterine cancer (RR =1.55, 95% CI = 1.03–2.32) and nonsignificantly for salivary gland (RR = 2.15, 95% CI = 0.91–5.08), stomach (RR = 1.61, 95% CI = 0.70–3.69), lung (RR = 1.42, 95% CI = 0.97–2.08), and female breast cancers (RR = 1.18, 95% CI = 0.99–1.40).
Among ≥ 5-year survivors, higher relative risks for solid cancer were observed for younger vs older ages at diagnosis (P for trend = .07), with a relative risk before age 25 years of 1.60 (95% CI = 1.07–2.40) among those diagnosed at < 25 years. Relative risks increased with longer time since differentiated thyroid cancer diagnosis (P for trend = .007). After 20 years, relative risks were 1.47 (95% CI = 1.24–1.74) for all solid tumors and 1.46 (95% CI = 1.10–1.95) for breast cancer, the most common tumor type. It was estimated that excesses of 6% for solid tumors and 5% for breast cancers were attributable to radioactive iodine treatment.
Among 32,171 ≥ 2-year survivors (median follow-up = 13 years), radioactive iodine treatment was associated with an increased risk of total hematologic malignancies (RR = 1.51, 95% CI = 1.08–2.01), including leukemia (RR = 1.92, 95% CI = 1.04–3.56). It was estimated that radioactive iodine treatment was associated with an excess of 14% for hematologic malignancies.
The investigators concluded, “In addition to leukemia, radioactive iodine treatment for childhood and young-adulthood differentiated thyroid cancer was associated with increased risks of several solid cancers, particularly more than 20 years after exposure, supporting the need for long-term surveillance of these patients.”
Cari M. Kitahara, PhD, MHS, of the Division of Cancer Epidemiology and Genetics, National Cancer Institute, is the corresponding author for the Journal of Clinical Oncology article.
Disclosure: The study was supported by the Intramural Research Program of the National Cancer Institute. For full disclosures of the study authors, visit ascopubs.org.The content in this post has not been reviewed by the American Society of Clinical Oncology, Inc. (ASCO®) and does not necessarily reflect the ideas and opinions of ASCO®.