What Is the “Screening Effect” Six Years After the Fukushima Nuclear Power Plant Accident?

Six Years After the Fukushima Nuclear Power Plant Accident

One such example illustrating the complexity surrounding screening is the impact of childhood and adolescent thyroid cancer diagnosis in Fukushima prefecture, Japan, which was discussed during the 5th International Expert Symposium on Radiation and Health held on September 26–27, 2016, in Fukushima. A large-scale Thyroid Ultrasound Examination program was launched in October 2011 as a part of the Fukushima Health Management Survey and has been ongoing for more than five and a half years since the natural disaster in East Japan that caused the subsequent Fukushima Nuclear Power Plant (NPP) accident (3). Accumulated health examination data, repetitive examinations using sophisticated equipment, and a standardized diagnostic protocol have led to the detection of an increasing number of childhood and adolescent thyroid cancers. More than 140 papillary thyroid carcinomas were operated among the cohort of about 300,000 individuals <18 years of age at the time of the accident in Fukushima (4,5).

Although the thyroid doses from potential exposure to radioactive iodines after the accident were very low compared with those around Chernobyl (6), misunderstanding and rumors suggesting a scenario similar to the Chernobyl accident evoked distress and anxiety about a rapid increase in thyroid cancer in Fukushima. This has led to an epidemic of fear (7) that exaggerates a wrong interpretation of an increase of thyroid cancer in Fukushima attributing it to radiation. Therefore, it is important for physicians and public-health workers first of all to understand correctly the reasons of the increased detection rate of thyroid cancer in the Fukushima area.

What Is the “Screening Effect”?

The international expert meeting addressed not only the benefits but also the disadvantages of thyroid cancer screening in Fukushima, despite its primary healthcare purpose, and issued several important recommendations, including a cautionary remark about the potential overdiagnosis due to large-scale screening (8). While the concept and methodology for the estimation of the magnitude of overdiagnosis of thyroid cancer has been described before (9), the term “screening effect” is ambiguous and not well-defined.

Screening itself is “the presumptive identification of unrecognized disease or defect by the application of tests, examinations, or other procedures which can be applied rapidly. Screening tests sort out apparently well persons who probably have a disease from those who probably do not. A screening test is not intended to be diagnostic. Persons with positive or suspicious findings must be referred to their physicians for diagnosis and necessary treatment” (10,11). After the introduction of screening, a rapid increase in incidence may be expected because both incident and prevalent cases are detected. Once screening continues for a long period of time, only incident cases will be detected, and the incidence will likely display a stable pattern (12). Thus, an increase in disease incidence during screening is time-dependent and can be described in terms of fold-change over the baseline, including an uncertainty estimate, at any time point or over a period of time.

To quantify the increase due to enhanced assessment, the term “screening effect” is proposed, which can be defined as the ratio between age- or age/sex-specific incidence in the target group and the reference incidence in an otherwise similar or appropriate group in the absence of screening. If a population/group submitted to screening has been exposed to certain environmental, life-style, or non-modifiable factors whose effects can be quantitatively measured and which are known to be causatively associated with a disease/condition under consideration, the size of the screening effect should be adjusted for these factors in order to determine correctly the increase attributable to screening per se. It also should be borne in mind that the sensitivity and specificity of different screening tests or tools may affect the number of cases detected, which need to be accounted for if screening effects are intercompared.

To understand and avoid misinterpretation of the results of the Thyroid Ultrasound Examination program in Fukushima appropriately, we need to realize the existence of the screening effect and urgently make proper investigations to evaluate its magnitude and temporal pattern using different approaches (13 –15). Finally, the risk of possible overdiagnosis and overtreatment should be further discussed and evaluated based on the available evidence on cancer biology and known prognostic factors because of the necessity of individual-level analysis in the clinical setting.

In summary, the important instances of the large-scale Thyroid Ultrasound Examination program in Fukushima on the brink of the sixth year since its beginning are:

• The program implemented after the Fukushima NPP accident demonstrates a high prevalence of thyroid cancer in patients aged 0–18 years at the time of the accident in March 2011.