Background: The potential risk of subsequent malignant neoplasms (SMNs) after radioactive iodine (RAI) treatment of thyroid cancer (TC) is an important concern. Methods: A systematic review was updated comparing the risk of SMNs in TC patients treated with RAI to TC patients without RAI. Six electronic databases were searched (up to March, 2018), supplemented with a hand search. Two reviewers independently screened citations, reviewed full-text papers, and critically appraised/abstracted data. Random-effects meta-analyses were conducted using crude data and data statistically adjusted for confounders. The outcomes were any SMN and specific SMNs for which sufficient data were available. Results: In total, 3506 unique electronic search citations and 93 full-text papers were examined, including 17 studies (3 systematic reviews and 14 original studies). Published knowledge syntheses were limited by inclusion of small numbers of studies, with two systematic reviews suggesting an increased risk of any SMN and one meta-analysis suggesting a reduced risk of breast SMN after RAI treatment. In a meta-analysis of crude data, the risk ratio of any SMN in RAI-treated TC patients was 0.98 ([confidence interval (CI) 0.76-1.27]; n = 10 studies of 65,539 individuals, heterogeneity Q = 64.26, degrees of freedom [df] = 9, p < 0.001, I2 = 85.99). The pooled risk ratio for any SMN, adjusted for confounders, was 1.16 ([CI 0.97-1.39]; n = 6 studies, data from at least 11,241 TC patients, Q = 10.86, df = 5, p = 0.054, I2 = 53.96). In secondary analyses examining specific SMNs, although relatively rare, the risk of subsequent leukemia was increased, but the risk of multiple myeloma was reduced in RAI-treated TC patients. There was no significant increased relative risk of breast cancer, salivary cancer, or combined hematologic malignancies according to RAI treatment status. Conclusions: The body of evidence on whether 131I treatment of thyroid cancer is associated with the primary outcome of any SMN is highly heterogeneous and complex. More research examining the long-term risk of specific SMNs after 131I treatment is needed.
Keywords: iodine radioisotopes; meta-analysis; radioactive iodine; subsequent malignant neoplasms; systematic review; thyroid cancer.