Survey of Radioiodine Therapy Safety Practices Highlights the Need for User-Friendly Recommendations

Abstract

Fetuses whose mothers are treated with radioiodine after approximately 10–12 weeks of pregnancy are at high risk of developing iatrogenic hypothyroidism (1). Other than this well-documented occurrence, there is no evidence of harm to others from radiation originating from patients treated with radioiodine. Given that radioiodine is concentrated in breast milk and radioiodine has been documented to be taken up by the thyroid in nursing newborns whose mothers were given diagnostic activities of radioiodine (2), discontinuing lactation before radioiodine therapy and avoiding breastfeeding after radioiodine treatment is justified despite the lack of a case report of infantile hypothyroidism ascribed to radioiodine ingestion from breastfeeding. Aside from circumstances relating to pregnancy and lactation, the harm that a radioiodine-treated patient could inflict upon another person while following common sense instructions appears to be low. Patients, who themselves receive a much higher dose of radiation because they ingest the full radioiodine treatment, suffer relatively few side effects. For the most part these occur in tissues that actively take up the radionuclide, and the adverse effects occur in a dose-dependent manner (3). This provides some reassurance that the small amount of radiation exposure to the public from those who receive radioiodine treatment is unlikely to cause harm, even if the treated patients ignore nearly all of the radiation safety instructions they receive. On the other hand, it is known that thyroidal exposure to higher levels of radiation, especially in children, can result in harm (4) and by extrapolation with a linear no-threshold dose–response relationship, one may assume that exposure to low levels of radiation might result in some harm. Thus, this theoretical possibility of increased harm at any increase of radiation exposure beyond background radiation, combined with no evidence of benefit of radiation exposure to the public, has led to the practice of keeping radiation exposure to others As Low As Reasonably Achievable (ALARA). High levels of radiation exposure are dangerous. It has been estimated that half of the people receiving a dose to the whole body over a few minutes to a few hours of between 3500 and 5000 mSv would die within 30 days (multiple mSv by 100 to convert to mrem). Similarly, high-dose exposure (starting somewhere between 100 and 1000 mSv) over a relatively short period of time, is associated with the development of a number of malignancies. Conversely, the average yearly radiation exposure from natural sources to an individual in the United States is approximately 3 mSv. Radon gas accounts for two thirds of this exposure, while cosmic, terrestrial, and internal radiation account for the remainder. No adverse health effects have been demonstrated from these levels of natural radiation exposure. In addition, artificial sources of radiation from medical, commercial, and industrial activities contribute another 0.6 mSv, for a total average yearly radiation exposure of 3.6 mSv. Doses (in mSv) from common medical imaging procedures include the following: bitewing dental x-ray, 0.004; chest x-ray (posterior-anterior), 0.02; lateral lumbar spine x-ray, 0.3; mammography, 0.7; lung ventilation/perfusion scan, 1.5; barium swallow, 1.5; technetium-99m bone scan, 4.4; barium enema, 7; 2-deoxy2[F-18]fluoro-d-glucose positron emission tomography scan, 7; chest or abdominal computed tomography scan, 8-10; and coronary angiogram, 5–16. A personalized annual radiation dose estimate can be calculated at the website http:// www.epa.gov/radiation/understand/calculate.html. Although radiation may cause cancers at high doses and dose rates, currently there are no data that unequivocally establish the occurrence of cancer following exposure to low doses or dose rates (e.g., below about 100 mSv). People living in areas with high levels of background radiation (>10 mSv per year) such as Denver, Colorado, have shown no adverse biological effects. Keep these millisievert values in mind as you read the next paragraph. Effective May 29, 1997, and updated on July 29, 2009, the Nuclear Regulatory Commission (NRC) revised Federal Regulation 10 CFR 35.75, which permits NRC-licensed facilities to release a patient treated with radioiodine from their control if the total effective dose equivalent to any other individual from exposure to the released individual is not likely to exceed 5 mSv. Further, a licensee must provide the released individual, or the individual’s parent or guardian, with instructions (including written instructions) on actions