CONCLUSIONS AND RECOMMENDATIONS
From the data presented in this report, it is apparent that exposure of susceptible populations to radioiodine from a radiation incident poses an increased risk of thyroid cancer and other thyroid conditions. It is also clear that KI is a useful agent for protection against thyroid-related health effects of exposure to radioiodine. However, evidence is sparse or absent on a number of important issues related to KI distribution and radioiodine exposure. Most important, there is a need for more-effective KI distribution and predistribution strategies and methods that ensure timely availability of KI to appropriate populations in the event of a radiological incident involving radioiodine. Further studies of the influence of perceived risk on public reaction to various KI distribution programs could inform decisions about whether and how most effectively to provide KI to appropriate populations. In addition, a number of issues related to disease risk posed by radioiodine exposure after a radiological incident need further study. Subjects of particular need include the
effect of dietary iodine in modifying the carcinogenic risk posed by radioiodine; the type, frequency, and clinical course of thyroid tumors in those exposed to radioiodine as children; the risk of thyroid carcinogenesis in adults exposed to radioiodine in fallout; the risk and mechanisms of autoimmune disease and hypothyroidism after radioiodine exposure; and the risk of tumor development at other sites after radioiodine exposure.
On the basis of information presented in this report, the committee concentrated on three main subjects for assessing the five issues posed in the statement of task: risks and benefits of potassium iodide distribution; implementation issues related to potassium iodide distribution and stockpile programs; and additional research needed. A summary of the background and rationale is provided for each recommendation, and the text is organized to present the link between the various elements of the committee’s charge and the findings and recommendations.
Benefits of and Risks Posed by Potassium Iodide Distribution
On the basis of its assessment, the committee reached the following conclusions and offers a number of recommendations.
Conclusion 1: Exposure of susceptible populations to radioiodine from a radiation incident increases the risk of thyroid cancer and other thyroid disorders.
Radioactive iodines (radioiodines, such as 131I) are produced during the operation of nuclear power plants (NPPs) and during the detonation of nuclear weapons. Radioiodine is one of the contaminants that could be released into the environment in the event of a radiation incident that involves a disruption of the integrity of the fuel assembly and containment structures of a nuclear power plant (NPP), because of an accident or terrorist activity. Because iodine concentrates in the thyroid gland (it is essential for the synthesis of
thyroid hormones) and because the thyroid cannot distinguish between radioactive iodine and nonradioactive iodine, exposure to radioiodine by inhalation of contaminated air or ingestion of contaminated milk or other food can lead to radiation injury to the thyroid, including increased risk of thyroid cancer and other thyroid diseases. The risk of thyroid cancer resulting from exposure to radioiodine is strongly age-related; fetuses, infants, and children are at highest risk. Fetuses are at risk through their pregnant mother’s exposure and breast-feeding infants are at risk through breast-feeding milk from their exposed mothers, or through inhalation or ingestion from another source. Reports of radiation-epidemiology studies indicate that those exposures to radioiodine caused excess cases of thyroid cancer years later in the sensitive groups within the exposed population. See Chapters 2, 3, and 4 for details.
Conclusion 2: Potassium Iodide (KI) is an important agent for protection against thyroid-related health effects of exposure to radioiodine, if taken shortly before or after exposure.
Radiation doses to the thyroid from radioiodine can be limited by taking nonradioactive (stable) iodine in such a form as KI. KI has been widely used as a safe means to block uptake of radioiodine in diagnostic or therapeutic nuclear medicine or in the event of a radiological incident. KI is highly effective in blocking uptake of radioiodine if taken shortly before or shortly after exposure, and side effects after short-term use have been minimal. KI is inexpensive to manufacture and is stable for long periods if properly packaged and stored. Fetuses, infants, children, and pregnant women (to protect the fetus because iodine readily crosses the placenta), and nursing mothers (to protect breast-feeding infants because iodine is concentrated in breast milk) are most in need of protection from radioiodine exposure and most likely to benefit from KI. For the general population, there is little benefit in providing KI to adults over 40 years old. KI does not protect other organs or tissues from external exposure to radiation or from internal exposure to other radioactive isotopes, such as strontium, cesium, and cobalt. See Chapters 2 and 5 for details.
Conclusion 3: In planning for responses to nuclear incidents in the United States, the likelihood and possible magnitude and extent of a release in the United States cannot be extrapolated from the Chornobyl accident, because of substantial safety and other facility-design features in US reactors.
The accident at Chornobyl was precipitated by an explosion and a fire in the graphite-moderated core. The persistent fire resulted in the release of large quantities of fission products, including radioiodine and noble gases, into the environment. That series of events would not occur in US reactors, because of fundamental differences in reactor design. Reactor designs in the United States are different from the Chornobyl design in several ways. The choice of moderators (material used to slow down neutrons) is different in US nuclear power plants (NPPs). In the United States, water is used; Chornobyl-type reactors use graphite (graphite is combustible, and water is not). US NPP designs prevent sudden, difficult-to-control increases in power level (sudden increases in the fissioning process). US NPPs use multiple layers of barriers to ensure that nuclear fuel and fission products cannot escape from the core. In the United States, NPPs have a pressure vessel with thick walls; the Chornobyl reactor had no such containment vessel. US NPPs are designed on the basis of full containment, the complete enclosure of all reactor and primary support systems for the reactor in the event of a design-basis accident. In NPP licensing, the US Nuclear Regulatory Commission subscribes to the "defense-in-depth" (multiple-layer) safety strategy, which includes accident prevention, redundant safety systems, containment, accident management, siting, and emergency planning. See Chapters 3 and 5 for details.
Recommendation 1: Potassium iodide (KI) should be available to everyone at risk of significant health consequences from accumulation of radioiodine in the thyroid in the event of a radiological incident. KI should be available to infants, children, and pregnant and lactating women. There is little benefit in
providing KI to adults over 40 years old. To be most effective, KI must be taken within a few hours before or after exposure to inhaled or ingested radioiodine.
Radioactive iodines (radioiodines, such as 131I) are fission products present in nuclear power plants (NPPs) and released from detonated nuclear weapons. In the event of nuclear accidents or as a result of nuclear terrorism, radioiodine could be released to the environment. Because iodine concentrates in the thyroid gland, exposure to radioiodine by inhalation of contaminated air or ingestion of contaminated milk and other foods can lead to radiation injury to the thyroid, including increased risk of thyroid cancer and other thyroid diseases. Thyroid radiation exposure from radioiodine can be limited by taking stable iodine. KI is a chemical compound that contains iodine and can be used to protect the thyroid gland from possible radiation injury by reducing the amount of radioiodine concentrated by the thyroid after inhalation of radioiodine. KI is also effective for protection against the harmful thyroid effects of radioiodine ingested in contaminated milk and other foods, but food testing and interdiction programs in place throughout the United States are more effective preventive strategies for ingestion pathways.
The risk of thyroid cancer from exposure to radioactive iodine is strongly age-related. Fetuses, infants, and children are at highest risk. Pregnant and lactating women should take KI to protect their unborn or breast-feeding children. Among older adults, there is little risk of thyroid cancer and a higher risk of complications from KI; therefore, for the general public, there is little benefit in providing KI for adults over 40 years old. To be most effective, KI must be taken within a few hours before or after exposure to inhaled or ingested radioiodine.
The existing Food and Drug Administration (FDA) guidance in the United States addresses seven risk groups with three different thresholds of thyroid radiation exposure for the administration of KI. Developing a distribution program that addresses every risk group will be too complicated to administer. Thus, to provide a conservative and simple protective action during a nuclear incident, a single directive for the entire public should be adopted at an
intervention level that is protective for the most susceptible population groups: 50 mGy (5 rad) of predicted thyroid exposure for infants, children, adults under 40 years old, and pregnant and lactating women.
Recommendation 2: KI distribution should be included in the planning for comprehensive radiological incident response programs for nuclear power plants. KI distribution programs should consider predistribution, local stockpiling outside the emergency planning zone (EPZ), and national stockpiles and distribution capacity.
Emergency preparedness for potential radiological incidents involves extensive planning and development of a comprehensive response program capable of responding to various credible scenarios. Such response programs require involvement and cooperation of local, state, and federal agencies and organizations, with periodic review and evaluation of programs’ effectiveness. Currently, for example, the US Nuclear Regulatory Commission and the Federal Emergency Management Agency (FEMA) require and regularly evaluate emergency preparedness at and around NPPs. Availability of KI for administration in the event of a radiological incident involving radioiodine is an important component of such preparedness programs. See Chapters 5 and 7 for details.
Recommendation 3: FDA should re-evaluate current dosing recommendations and consider extending the shelf-life for KI tablets stockpiled or distributed for use in response to a radiological incident involving radioiodine.
Current FDA-approved KI formulations generally range in shelf-life from 2 to 3 years. It is known that KI is relatively stable and should have a much greater shelf-life when stored under optimal conditions. It would be desirable to have a longer shelf-life for KI that is to be stockpiled or used in a KI distribution program to limit the need for frequent replacement of outdated product. Given the known stability of KI when properly packaged and stored, the current
shelf-life determinations appear to be unreasonable for a product that would be used only in an emergency situation.
Implementation Issues Related to Potassium Iodide Distribution and Stockpile Programs
On the basis of its assessment, the committee reached the following conclusion and offers a number of recommendations regarding potassium iodide distribution programs.
A strategy is needed whereby local planning agencies could develop geographic boundaries for a KI distribution plan based on site-specific considerations because conditions and states vary so much that no single best solution exists.
KI distribution planning in the United States has focused on the Nuclear Regulatory Commission’s early-phase Emergency Planning Zone (EPZ) of a 10-mile radius. However, the EPZ provides only a basis for planning. A specific incident might call for protective actions to be restricted to a small part of the EPZ or require that they be implemented beyond the EPZ as well. See Chapters 5 and 7 for details.
Recommendation 1: A better understanding of the strengths and weaknesses, short-term and long-term successes and failures, and resource requirements of different KI distribution plans implemented in the United States and abroad would be extremely helpful for designing and implementing effective future KI distribution programs.
KI distribution plans have been implemented for decades. However, their long-term success in terms of community awareness
(both of the plan and of the location of distributed KI tablets), continuing availability of KI to all appropriate populations, and impact on perceptions of risk have not been well studied. A more thorough assessment of these programs’ successes and limitations is necessary.
Recommendation 2: State and local authorities should make the decision regarding implementation and structure of a KI distribution program. The choice of program should be based on how well specific plans would perform on decision objectives, given features of the local region. The decision regarding the geographical area to be covered in a KI distribution program should be based on risk estimates derived from calculations of site-specific averted thyroid doses for the most vulnerable populations.
In the United States, public health and safety are state responsibilities. However, states generally have delegated some authority for incident response to local public officials and local responders. Local responders are generally the first to arrive at the scene of an accident or incident and have the best knowledge of local conditions that might affect response decisions. Similarly, state and local authorities are in the best position to assess site-specific characteristics that will influence the selection, development, and implementation of the public-health protection strategies that will be most effective in meeting their objectives, including the availability of KI for appropriate populations. A major objective might be to “minimize radioactive iodine risk to thyroid” with subobjectives of maximizing KI availability, especially for the most vulnerable population; optimizing ability to take KI on time; and minimizing harm from inappropriate KI administration. Another major objective might be to “minimize harm from other aspects of incident” with subobjectives of ensuring that KI procedures do not impede evacuation, averting mortality and morbidity from radiation or accidents (beyond thyroid risks), and avoiding excessive resource use in KI procedures. Specific localities may have added objectives, such as minimizing legal liability.
For fixed facilities, NPPs, it is possible to calculate anticipated radiation exposures after an incident, and there is now a sufficient medical and scientific literature to estimate dose-related thyroid-cancer risks after exposure to radioiodine. The established dose-modeling methods approved for each site make it possible to construct site-specific thyroid dose estimates based on local conditions. The radiological dose to the thyroid can then be used to determine site-specific risk estimates, taking into account demographics and distance from the radioactive iodine source for use in incident-response planning to protect the vulnerable population in geographic areas determined to be at significant risk for radioiodine exposure.
The existing FDA guidance in the United States addresses seven risk groups with three different thresholds of thyroid radiation exposure at which the administration of KI would be advised. Developing a distribution program that addresses every risk group will be too complicated to administer. Thus, to provide a conservative and simple protective action during a nuclear incident, site-specific risk estimates should be based on recommending KI administration for the entire vulnerable population if the highest-at-risk members of that population reached the recommended threshold (for example, advising administration of KI to all infants, children, pregnant and lactating women and adults under age 40 when the predicted thyroid dose without KI prophylaxis in children under 4 years of age reaches 50 mGy (5 rad) ). The committee recommends that the scientific information that is becoming available from epidemiology studies of the Chornobyl populations and from other radiation incidents be carefully monitored to determine if the FDA recommendation on threshold dose needs to be reconsidered.
The choice of the best plan will depend on the site-specific features and the decision objectives. For example, predistribution of KI to households (with secondary postdistribution at reception centers) might be the preferred option in a local area with a large population and limited evacuation routes, whereas stockpiling KI at reception centers might be preferred if evacuation of the population could occur within a few hours.
Recommendation 3: KI distribution and administration plans developed at the state and local level should receive federal resources for implementation and maintenance.
Notwithstanding the need for state and local authorities to make the decision regarding the most appropriate public-health protection strategies for a comprehensive incident-response program, implementation and maintenance of such programs requires substantial resources. The potential scope of such incidents may extend beyond local or state jurisdictions, and costs may vary widely depending on local conditions and population demographics. In addition, the cost of resupplying depleted reserves and outdated stocks is significant. For all those reasons, it would be appropriate for the federal government to provide resources for these programs.
Recommendation 4: The federal government should maintain stockpiles and a distribution system as a supplement to states’ programs to assure availability of KI to affected populations in the event of a major radiological incident involving radioiodine.
Radiological incidents have the potential for widespread contamination and population exposure. Like any large-scale disaster, such an incident may quickly outstrip the response capability of local and state authorities and resources. The federal government already has in place a number of mechanisms to provide assistance in the event of a natural disaster, accident, or other incident through such organizations as the Centers for Disease Control and Prevention and FEMA. It is appropriate that KI remains in the vendor managed inventory provisions available to local and state authorities in the event of a radiological incident involving radioiodine. In addition, KI could also potentially be included in the US government’s push package program.
Recommendation 5: The federal government should ensure an adequate supply of KI tablets in suitable dosages for use by the target populations of infants, children, adults under 40 years old, and pregnant and lactating women of all ages.
Several organizations, including the World Health Organization, have developed recommendations for KI dosage according to age group for protection against thyroid cancer in the event of a radiological incident involving radioiodine. Available FDA-approved tablet formulations of KI include 65-mg and 130-mg scored tablets. The recommended dosage for neonates is 16 mg, which requires administration of one-fourth or one-eighth of a tablet. FDA provides detailed instructions on how to achieve and administer that dosage, but this is a formidable task, particularly in an emergency situation. Therefore, there is a need for development of practical dosage forms (for example, 32-mg scored tablets, so that a half-tablet could be administered to a neonate) that are readily available.
In addition, FDA maintains two guidance documents that provide different KI dosage recommendations. FDA should review its guidance documents and develop a single guidance strategy for administration of KI in the event of a radiological incident involving radioiodine, with particular attention to dosage for infants and children.
Additional Research Needed
On the basis of its assessment of the current state of information regarding KI distribution programs, the committee reached the following conclusion and offers a number of recommendations for further studies that will improve the base of knowledge on which to make related public-health decisions.
Although questions remain regarding long-term health risks from radioiodine, particularly among potentially high-risk subgroups, there is now sufficient medical and scientific literature to estimate dose-related thyroid cancer risks following exposure to radioactive iodine.
KI distribution planning in the United States has focused on the Nuclear Regulatory Commission’s early-phase Emergency Planning Zone (EPZ) of a 10-mile radius. However, the EPZ provides only a basis for planning. A specific incident might call for protective actions to be restricted to a small part of the EPZ or require that they be implemented beyond the EPZ as well. See Chapters 4 and 5 for details.
Recommendation 1: KI distribution plans should include a carefully developed and tested public education program with continuing evaluation to ensure effectiveness and continued access to KI by the appropriate population.
Implementation of a KI distribution plan is only the first step in providing protection against thyroid cancer from radioiodine exposure. The plan should include structuring, implementing, and sustaining a public-awareness and public-education campaign to support any distribution programs and to inform people about radiation risks and the use of KI. In addition to a public-education program, health-care professionals, the mass media, and other groups may need tailored communication processes. There is a continuing need for resupply of KI stocks (including stocks that become outdated) and for assurance of access by appropriate populations. In addition, the overall effectiveness of the distribution program should be periodically assessed.
Recommendation 2: A national program should be developed for follow-up of all individuals to whom KI was administered following a radiological incident, to assess short- and long-term health effects of KI administration.
There is little information on the long-term health effects of KI administered for public protection from radiological incidents. Such information would be important for planning of incident
preparedness programs. To be effective, such a program would need to be national in scope and in the form of a registry to follow affected people for many years.
Recommendation 3: Research is needed in a number of areas to provide better information to inform policy-makers and health-care providers about the risks posed by radioiodine exposure and methods to minimize long-term health effects. An evaluation of the strengths and weaknesses, successes and failures (short-term and long-term), and resource requirements of the different KI distribution plans implemented in the US and abroad should be conducted by a federal agency to aid states and local regions in designing and implementing effective KI distribution programs.
Research is needed to develop more effective KI distribution programs that address the critical issues of KI availability to appropriate at-risk populations, predistribution approaches to improve long-term public awareness of the location of KI tablets, and effective risk-communication strategies to maximize information transfer and minimize public anxiety. Research is also required to improve understanding the effect of dietary iodine in modifying the carcinogenic risk posed by radioiodine; of the type, frequency, and clinical course of thyroid tumors that develop in those exposed to radioiodine as children; of the risk of thyroid carcinogenesis in adults exposed to radioiodine in fallout; of the risk and mechanisms of autoimmune disease and hypothyroidism after radioiodine exposure; and of the risk of tumor development at other sites after radioiodine exposure.