Commentary

Screening for Thyroid Cancer After the Fukushima Disaster

What Do We Learn From Such an Effort?

On March 11, 2011, the Fukushima Prefecture of Japan north of Tokyo experienced a massive 9.0 magnitude earthquake. The Fukushima-Daiichi nuclear power plant nestled along the coast withstood the earthquake as it was designed to do, and remained structurally intact. Then the unthinkable occurred: a tsunami from the earthquake slammed the coast at a height that exceeded the protective wall between the sea and the power plant. The impact of the tsunami caused catastrophic damage to the reactor and surrounding plant, and resulted in the release of large amounts of a mixture of radionuclides into the atmosphere. It was the largest civilian nuclear accident since the Chernobyl accident in April 1986.

Dr. Tsuda and colleagues report in this issue of the journal the results of the first of two rounds of screening exams for thyroid cancer, including the use of thyroid ultrasound, among persons living in the Fukushima Prefecture at the time of the event, and who were ≤18 years old. This required an enormous logistical effort and excellent coordination with other ongoing activities. However, there are some substantial limitations remaining that affect what can be learned from this effort. This commentary is intended to provide a broader context within which these results can be examined, understood, and interpreted.¹

To better understand why things were done the way they were, consider the following points. First, the most immediate concerns are to treat the injured, contain the release of radioactive material, and evacuate persons most heavily exposed or who will be at risk of further exposure if they do not leave the area. At Fukushima, more than 80,000 people were evacuated within a 20-km radius of the plant, and over 100,000 within a 30-km radius. Such an event demands a complex set of actions, some linked with others, usually in the absence of adequate resources (particularly in this case with widespread damage from the earthquake) and under severe time pressures.

Second, once the most acute needs are addressed, it is important to also consider the longer-term health outcomes of an accident. This can be a formidable task. The ideal might be to establish an infrastructure to identify and follow populations of potential interest, to monitor their health on a regular basis, and to estimate the radiation dose they could have received, but this requires substantial resources and expertise. Particularly difficult is estimating an individual person’s radiation dose, which is needed to conduct the most meaningful dose–response analyses. One needs information regarding at least the following: (1) how much radioactive material was released and what are the primary components of the release; (2) how was the material dispersed and what is (are) the primary pathway (s) of exposure for humans; (3) were measurements taken and, if so, how might they be used; (4) where was the individual at the time of the event and since; (5) how old was the individual at the time of the event; and (6) what are the individual’s personal habits that might be relevant as modifying factors when considering a dose–response (e.g., diet).

Finally, it is important to specifically state what the primary objectives of the project are in advance. Is there a scientific component? Are there important scientific questions that this project could address or answer? Are there people qualified to carry out the scientific research being considered? There are a number of possible objectives for putting this screening program in place. They include the following: (1) to determine medical care needs in the population(s) most likely exposed, (2) to establish a strong basis to plan for future medical needs in this population, (3) to improve our understanding of radiation-induced (or radiation-associated) thyroid cancer, (4) to investigate further the dose–response relationship between radiation dose to the thyroid and the development of thyroid cancer, and (5) to learn more about the shape of the radiation dose–response at low doses.

Given this web of challenges, the lack of resources for this kind of activity that usually exists, and consequently the lower priority it typically receives, it should be of no surprise that a number of study designs and approaches have been tried, largely driven by the data available.

What we know about the human health effects of exposure to ionizing radiation comes from three sources of information: (1) persons exposed during a medical procedure, for either therapeutic or diagnostic purposes; (2) persons exposed in occupational settings; and (3) people exposed directly from the environment as the result of an accident or natural disaster of some kind. Exposures from environmental sources often can provide the most relevant information about risk because the circumstances of such exposures are more similar to the kinds of exposures that people are most concerned about. In fact, we have considerable knowledge about radiation and cancer: the types it causes, the nature of the dose–response at middle and higher doses; and the impact of a number of factors that modify risk (e.g., age at exposure). The items of primary interest today are more sophisticated and are more difficult to answer. They include the nature of the dose–response at low doses and the impact of uncertainty in radiation dose on the estimation of disease risk.

A number of large-scale studies have been conducted of persons exposed to ionizing radiation in an environmental setting. The most well-known and comprehensive of these is the long-term follow-up of persons who survived the atomic bombings of Hiroshima and Nagasaki in Japan. The other large studies include the following: (1) exposures from atmospheric testing of nuclear weapons in the Marshall Islands and in Utah and Nevada, site of atmospheric testing from the Nevada Test Site; (2) exposures from atmospheric release of radionuclides from the Hanford Nuclear Site in eastern Washington State; (3) exposures from atmospheric releases of radioactive material to residents living along the Techa River near the Mayak facility in the southern Urals of the Russian Federation; (4) exposures to workers at the Mayak facility; and (5) exposures from the release of radioactive material to the atmosphere from a nuclear power reactor at the Chernobyl Power Station in Ukraine. Exposure circumstances among these studies are very different. This enables us to learn multiple things about radiation effects within a single study, but also makes it difficult to compare findings across studies.

This broader context provides a framework for considering how the findings of Tsuda and colleagues fit into the overall picture, and what scientific contributions they make in improving our understanding of radiation-induced thyroid cancer and/or the health aftermath of the 2011 great east-Japan earthquake and tsunami. This program is different from other follow-up efforts in that it was not initiated as a research project to follow a cohort of people living in the area, but rather as a clinical service to the population potentially exposed. It is a large population-based screening program that was planned and conducted by the Fukushima Prefectural Government. Nearly all of the exposure was from radioactive iodine in the form of ¹³¹I and cesium in the forms of ¹³⁴Cs and ¹³⁷Cs. Dose to the thyroid from ¹³¹I comes mostly from ingestion of contaminated food and drink and the concentration of the radioactive iodine in the thyroid. It has a relatively short half-life (a little over 8 days); so most of the dose is received in the first few weeks after the event. In contrast, ¹³⁷Cs has a longer half-life (about 30 years) and delivers a dose externally.

Measurements were not available during the first few days after the event, largely because of the widespread destruction caused by the earthquake. Existing infrastructure had been destroyed and power was not available. These harsh conditions impeded the data-collection process greatly, and consequently the ability to estimate individual radiation doses for the people screened. Instead, group doses were estimated for nine geographic areas in proximity to the plant, making extensive use of computer models and models based on past experience.

This screening program was large and complex. The project identified 367,686 individuals eligible to be screened and a little over 80% (298,577) underwent a thyroid ultrasound screening exam by December 2014 (i.e., the first round). There were 2,251 individuals with positive ultrasound findings. From this group, 109 thyroid cancer cases were found based on cytology after fine needle aspiration, of which 86 were histologically confirmed (83 as papillary carcinoma and three as poorly differentiated carcinoma). One person initially considered a case was determined to have a benign tumor. A second round of screening is currently underway. Results from preliminary analyses including eight new cases were not materially different.

As the focus of attention moved from the acute phase to the longer term, questions and concerns became more centered on the impact of the radiation exposures on the health of those exposed, including psychological and mental health, the environment, and the social, economic, and political character of the population. Of major concern is whether and when can people who were evacuated return to their homes.

Unfortunately, the screening program can only provide some guidance regarding the most important practical and scientific questions. One of the most important limitations of this study is that there are no individual-level data available that can be used to estimate radiation dose to the thyroid. Instead, direction and distance are used to approximate dose within nine smaller areas making up Fukushima prefecture. All persons living in a particular area would be assigned the same dose (or perhaps within subgroups of the area population, such as gender or age at the time of the event). Although it is not possible to address questions regarding risk of thyroid cancer in a quantitative way, it was possible to look at thyroid cancer incidence across the nine areas to see if there is any correspondence (correlation) with the area dose estimates. The early reports after the Chernobyl accident used similar geographic methods to demonstrate increases in thyroid cancer associated with the contamination levels of small geographic units (raions) around the Chernobyl Power Station.^2,3

The World Health Organization has published a comprehensive review⁴ of what occurred at Fukushima and what has been done since the event in terms of estimating doses, monitoring persons with the higher doses, and planning for more cases arising over the next several years. The United Nations Scientific Committee on the Effects of Atomic Radiation has also published a similarly comprehensive account of Fukushima.⁵ Both of these reports predict that cancer rates in the population of the Fukushima Prefecture will remain stable, and that we would not expect to observe significant changes in cancer statistics that could be attributed to radiation exposure from Fukushima releases. Given the preliminary geographic dose estimates, any excess of cases due to radiation from the Fukushima plant would be too small to detect using epidemiologic methods.

As stated at the beginning, this commentary is intended to broaden the context or perspective from which the report by Tsuda and colleagues can be evaluated. I have highlighted some of the major challenges faced in a large-scale disaster of the kind experienced in Fukushima, particularly regarding the collection of data from individuals. In this respect, it was not possible to collect the detailed data needed to estimate an individual radiation dose. Therefore, the findings cannot contribute to the two most urgent scientific questions: the characteristics of the dose–response curve at low doses, and the details of the role of other factors that might modify the risk of thyroid cancer associated with radiation exposure. Similarly, these findings do not add anything new regarding radiation-induced (or related) thyroid cancer. The current status of our knowledge in this area has been nicely summarized elsewhere.⁶

These results will probably be of most use to the regional government that planned and carried out this project. It may help them direct resources now and plan for future medical needs more accurately. These results also serve to inform the public of what the government has done and continue to do in response to the Fukushima disaster. They also demonstrate the need to maintain a long-term medical follow-up program for the screened population. Although the overall impact of the radiation exposure from Fukushima is small on a population basis, and likely not to be detected using standard epidemiologic techniques, there are likely to be subgroups of the population with considerably higher doses. These people clearly warrant continued follow-up.

The reporting of this project emphasizes that things that cannot fail, do. The unthinkable happened. Nobody could imagine that a tsunami would ever be taller than the protective wall. Health professionals must therefore always be prepared for what seems to be the impossible.