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While radioactive iodine is preferentially taken up by the thyroid, it’s not perfectly specific and can end up in some degree in several other organs including bone marrow, salivary glands, lung and even breast tissue. This SEER analysis of tens of thousands of survivors of well-differentiated thyroid cancer diagnosed at age <45 suggests a significant association between receipt of radioactive iodine (approximately half of cases) and development of both heme (RR 1.51) and solid (RR 1.23) secondary malignancies. This would mean an estimated 14% of heme and 6% of solid malignancies in survivors of thyroid cancer may be attributable to their treatment. What’s more, these relative risks increased with time out to >20 years from treatment. | Pasqual, J Clin Oncol 2022