Objectives Providing sufficient information about potential benefits and harms is essential in decision-making about participation in screening programs. Thyroid cancer screening (CS) with ultrasound is generally considered harmful and is not currently recommended. However, after nuclear accidents, thyroid CS has historically been performed as part of radiation effect surveys and post-accident response. Influencing factors must be considered when providing decision-making information to people in the area surrounding the Fukushima nuclear accident regarding the local government-led present opt-out screening. We aimed to identify factors associated with the decision to participate in thyroid CS in Fukushima using a scenario-based questionnaire survey among young people of the same age as the subjects of this screening.

Methods We presented a scenario to female university students as if they were offered thyroid CS after the nuclear accident, and asked whether they would participate. We then provided a handout currently used in Fukushima regarding the potential benefits and harms of the screening, and asked respondents again about their willingness to participate. Next, we provided a leaflet outlining specific examples of overdiagnosis and its harms, and gave lectures on radiation health risks, radiation exposure doses in Fukushima, and CS. We again asked about respondents’ decisions about participation at each step. We assessed risk perception regarding cancer onset caused by radiation exposure and estimation of the potential benefits and harms of thyroid CS.

Results 16% of the 76 questionnaire respondents included the real subjects for thyroid CS in Fukushima. Immediately after reading the scenario and after providing the existing handout, 75% and 60% of respondents said they would participate in the screening, respectively. The most common reason for participation was seeking reassurance. After providing the leaflet about overdiagnosis and its harms and after giving lectures, the proportions of respondents who chose to participate decreased to 19% and 11%, respectively. In contrast, the proportions of respondents who chose not to participate increased to 15%, 24%, 53%, and 66% at each step. Decision-making at each step was not related to cancer risk perception of radiation, but to the estimation of potential harms.

Conclusions Many young residents participated in the local government-led thyroid CS in Fukushima because they were designated as targets. The current findings suggest that individuals chose to undergo screening to relieve anxiety when benefits were estimated to exceed harms. The handout currently used for thyroid CS does not adequately address overdiagnosis as the most significant harm of screening. Providing a leaflet with explanations of the harms of overdiagnosis changed the decision-making process and increased the number of respondents who chose not to participate because of potentially significant harmful effects. Furthermore, the decision to participate in the screening was influenced by explaining scientific evidence regarding low radiation exposure and low cancer risk from radiation in Fukushima, and the magnitude of overdiagnosis in thyroid CS. To develop adequate decision-making aids regarding CS with a high risk of overdiagnosis, specific tools explaining the harms of overdiagnosis are needed, in writing and through dialogue.