A systematic review on thyroid cancer and diseases from radiation exposure in adulthood

The thyroid regulates many vital functions in the body, including growth, metabolism, reproduction, heart rate, blood pressure and body temperature. The thyroid gland is vulnerable to ionizing radiation, with cancer being the primary health effect of concern following low to moderate exposures. A linear dose-response relationship between childhood radiation exposure and thyroid cancer has been well documented, and evidence indicates that exposure is also associated with non-malignant thyroid diseases, such as hypothyroidism. However, the literature focused on the risk of thyroid cancer or non-malignant diseases following adulthood exposure is more variable and could be better delineated.

The relationship between ionizing radiation and thyroid diseases is an important regulatory issue. During radiological or nuclear emergencies, public or occupational exposures could result in uptake of radioactive iodine to the thyroid. As such, the International Commission on Radiological Protection has recommended reference levels for emergencies, while Health Canada has established generic criteria that include guidance on when to take protective actions, such as stable iodine thyroid blocking. Additionally, accidental spills (in a laboratory setting, for example) could result in occupational doses with radioactive iodine uptake. While these occurrences are uncommon, there remains a gap in understanding the risks in these settings specifically for an adult population, that could have important regulatory implications in emergency management. For instance, some countries do not distribute potassium iodide to older populations, while others have chosen to do so for risk communication purposes.

Given the uncertainties that remain, the CNSC has undertaken a systematic review to improve the level of understanding of the risk of thyroid exposure to ionizing radiation in adulthood, that can in turn inform the radiation protection framework. The purpose of this presentation will be to outline the review process, progress made, challenges faced, and results to date.