Abstract
Recently, James C. Sisson and The American Thyroid Association (1) examined the radiation safety component related to the use of I-131 in the treatment and release of patients with thyroid diseases. While the guidelines accurately address the issue of compliance with the Nuclear Regulatory Commission (NRC) regulations (2), we find that for many circumstances the examples provided leave a casual impression about the duration of the restriction period necessary to comply with those guidelines, and further do not fully address the in-depth counseling that is required for infants' exposures, even though infants are mentioned as a population at risk (3). Five circumstances render the example recommendations insufficient: (i) dosages >15 mCi are often administered, (ii) uptakes in hyperthyroidism frequently exceed 50%, (iii) the effective half-life often exceeds 5 days, (iv) exposure to others after the restriction period is not sufficiently taken into account, and (v) their guidelines insufficiently address the protection of the class of vulnerable very young children (e.g., infants, pre-schoolers, or those in limited educational activities such as kindergarten) who are in the immediate care of the treated patient (3 –7). These omissions and the implied inclusion of infants in their Table 2 are likely to lure many practitioners into a false sense of security about the safety net provided for small children who are held, carried, fed, clothed, diapered, bathed, and otherwise tended to by the patient. These small children may well receive exposure to radiation levels in excess of the limit of 5 mSv and exposures are likely to be well beyond 1 mSv, which is the level at which patients must be counseled with written instructions on how to keep exposures to others as low as reasonably achievable (ALARA) (3) and is the limit recommended for children by the National Council on Radiation Protection and Measurements (NCRP) (7). Barrington et al. (4), for example, found two children of ages 1 and 3 years who received doses of 3.3 and 7.2 mSv, demonstrating the vulnerability of small children. A child's physical and emotional needs will not be met by instructing the treated parent to remove themselves from these daily activities for a minimum of 5 days. Exploring options on how to maintain doses ALARA while continuing abbreviated care activities should have been included by the authors.
Young children and infants typically spend much more time at considerably closer proximity to their parent's thyroid than do grade-school children and adolescents. For this class of dependent child, use of exposure models that apply to independently active children and adults is inappropriate. Many articles that have addressed the issue of patient release have noted that the safety of small children represents a particular concern (4 –6). Some recommend that children be placed in the care of others for a week or more, depending on dosage and uptake. All proscribe close contact for a period of weeks, not 5 days, depending on the dosage and uptake.
Sisson et al. (1) chose to use an example that does not represent more challenging situations. For example, counseling to protect an at-home infant of a hyperthyroid patient with 80% 24-hour uptake and a 15 mCi dosage requires far more consideration than that implied in their examples. Additionally, Sisson et al. (1) cite only the effective thyroid half-life of 5 days when other researchers (5) have measured effective half-lives of 6.2±1.2 days. Thus, a 5-day effective half-life, rounded down from the 5.2 days derived by the NRC (8), is at the very short end of measured effective half-lives. The hyperthyroid models used in NRC regulatory guide 8.39 (9) use an uptake of 80% and NCRP Report 155 Appendix B.2 (7) uses a much longer 7-day effective half-life. Our experience indicates that a 50% uptake with a 5-day effective half-life renders a dose rate after 7 days that is only half that of an 80% uptake and 6-day effective half-life. Finally, the exposure to small children is greatly increased due to two other factors—the influence of distance and doses from post-restriction behavior. Radiation exposure increases exponentially as the distance from the treated thyroid decreases. This effect, embodied in the inverse square law, will significantly impact radiation exposure to children who are held or provided care in close proximity to the thyroid of the patient. Infants are often carried in halters or in the arms of mothers for extended periods. Infants are also rocked, carried, and burped in close proximity to the thyroid. These activities elevate exposure rates to this class of dependant by a factor of 8–10 over that at 1 meter from the thyroid.
The final issue that disproportionately affects children is radiation exposure after the restricted contact period. The recommendations of Sisson et al. (1), like others, do not adequately address exposure from a treated patient past the restriction period. After the restriction periods outlined in Table 2A-1, the dose accumulated from a hyperthyroid treated patient to an infant during close contact activities will be significant and must be accounted for using simulated unrestricted behavior to reasonably quantify the total effective dose received, as required by regulation (2). Once the patient is released from restrictions, he or she may feel compelled to compensate for lost quality time by spending initially an unusual amount of time in close contact and later returning to close-contact behavior as normal (10). After a 2-day daytime restriction and an 8-day sleeping restriction for a patient with 50% 24-hour uptake and a 15 mCi dosage, exposure rates from the thyroid using a more realistic effective half-life of 6 days are sufficient for the dose to an infant to accumulate to >7.5 mSv from unrestricted behavior. The dose from sleeping 8 hours per day with the child after the 8-day restriction period can by itself substantially exceed 5 mSv. It is very important that the parent not sleep or nap with the child for at least 2 weeks. Even 4 weeks may be appropriate to be conservative about the dose from this activity. Should the dosage to the patient exceed 15 mCi, the circumstances become even more problematic.
We recognize that practical issues related to implementing postiodine treatment guidelines and the ALARA principle are substantial when small children are in the home. We also recognize that clinically important effects of relatively low levels of radiation exposure are controversial. However, if the goal, as stated in the article, is to comply with NRC public/family exposure guidelines, we believe that the task force's recommendations as summarized in their Table 2A of the article do not provide adequate guidance to practicing physicians who may counsel these patients before or after I-131 treatment for hyperthyroidism.