As with almost any large nuclear reactor accident, the risk of thyroid cancer caused not only by external exposure, but by exposure to iodine-131 (131I) is a key concern. Experience, and some epidemiological studies from the Chernobyl accident demonstrate that large exposures to 131I can cause thyroid cancer in children (0 to 18 years of age), with younger children having more risk than older ones. External exposures that occurred in Hiroshima and Nagasaki also contribute to the current understanding of these risks.

Because of the release into the atmosphere of significant quantities of 131I from the Fukushima Daiichi reactor accident – about 100 to 500 PBq of 131I, although approximately 1760 PBq in the case of the Chernobyl accident (1 PBq = 1015 Bq) – it was expected that there would be a risk of thyroid cancer in the exposed populations, in particular in exposed children. To address this risk, as well as the risk of other cancers and leukaemia, the Japanese government put a medical surveillance programme in place, shortly after the accident, for the 2 million people living in the Fukushima Prefecture, which includes approximately 360 000 children. According to the Fukushima official report of November 2013, about 239 000 children to date have had preliminary thyroid screening exams, and 1 148 of these children have undergone secondary examinations as part of this programme. The initial ultrasound screening of all children is to be performed within the first three years after the accident, followed by complete thyroid examinations from 2014 onwards. Residents will be monitored regularly thereafter.

As of November 2013, these thyroid examinations of 238 765 children in Fukushima have resulted in thyroid cancer (papillary carcinoma) being diagnosed in 26 children, all of whom have been cured through surgical intervention. In addition, the examinations identified 59 children with suspected thyroid malignancy.

The thyroid cancer incidence rate identified thus far in the Fukushima Prefecture seems, at a glance, to be higher than previously reported in the general population of Japan. However, further study is being done on the cancer rate in Japan, not only through the National Cancer Registry, but also using the same ultrasound screening method and protocol used for the residents of the Fukushima Prefecture.

So far in the Fukushima Prefecture, there is no specific concern about doses from external exposures, even for the thyroid gland. This situation is reflective of the fact that the scientific understanding of thyroid cancer, and in particular the differentiation between spontaneously-occurring and radiation-induced thyroid cancer, is not complete in some areas. Some remaining scientific questions include:

• The latency period of thyroid cancer, as estimated from the Chernobyl experience, is in the order of four or five years. Russian scientists have reported not seeing any increase in childhood thyroid cancer between 1986 and 1990. The Japanese childhood thyroid cancers in excess of what was seen as the background rate started appearing in 2011, less than one year after the Fukushima Daiichi nuclear accident. As such, the thyroid cancers that have been clearly identified in the children of Fukushima do not seem to fit with the current understanding of the latency period for radiation-induced thyroid cancer in children observed around Chernobyl, but it cannot be completely ruled out at this time that radiation exposure may be the cause of these cancers. More scientific understanding is needed.
• The World Health Organizati on (WHO) noted in its health risk assessment report that the health statistics dated from 2007 indicated that childhood thyroid cancers were increasing in Japan, such that the current baseline incidence rate of childhood thyroid cancer incidence in Japan may need to be assessed in more detail to be correctly employed in epidemiological studies of the children exposed in the Fukushima Prefecture.
• To date, approximately 115 000 children from the Fukushima Prefecture have not yet had their thyroids examined. These examinations may change the current statistics.
• The accident occurred only two years and nine months ago before this data was compiled. According to current knowledge, the risk of contracting thyroid cancer is highest for children exposed at younger ages. As such, all the children of the Fukushima Prefecture, in particular those who were under five years old at the time of the accident, should be closely followed to ensure that any thyroid cancers that do appear will be quickly and appropriately addressed, and that this information can be used to better understand the overall risks to the population.
• It is most important to clarify the thyroid dose estimates when discussing the cause and effect of radiation-induced thyroid cancers. There is a large difference in such doses between those exposed by the Chernobyl accident and those exposed by the Fukushima Daiichi nuclear accident.

The objective of this workshop was to develop a state-of-the-art scientific understanding of radiation-induced thyroid cancer, and to share knowledge and experience in this area in order to support the efforts of the Japanese government and the Fukushima Prefecture to enhance public health. Experience in holding effective social dialogues, in order to best understand and appropriately address social concerns, was also a workshop focus.

The workshop began with a half-day tutorial session, followed by two days of plenary presentations and discussion, including panel sessions summarising the results of each session. A closing panel provided overall results and conclusions from the workshop. A rapporteur provided a workshop summary report and assisted the session co-chairs in summarising key points.

Co-organised by the Japanese Ministry of the Environment (MoE), Fukushima Medical University (FMU) and the OECD Nuclear Energy Agency (NEA)

The working languages of the workshop were in English and Japanese, with simultaneous Japanese to English and English to Japanese translation.