In the recent article by Tsuda and colleagues, the authors performed two types of comparisons, internal and external. In the internal comparison, no difference among nine areas in Fukushima was observed. In contrast, in the external comparison, extremely high incident risk ratios—between 20 and 50 (except in one area)—compared with the rates from national cancer registries in Japan were reported.For valid external comparison, comparability should be discussed. In this case, the system of case finding in Fukushima and other area is not the same, and therefore a direct comparison could be misleading.In the study, the participation rate was as high as 81%, and this rate affects the incidence rate (i.e., a screening effect). For example, age-standardized incidence rate of thyroid cancer per 100,000 in Japan was 2.2 among men and 7.9 among women in 2007.[2] In Korea, however, it is 18.3 among men and 87.4 among women in 2010.[3] Kweon et al.[4] described how the high age-standardized incidence rate is mainly explained by enhanced detection (screening effect) and the changes in medical practice patterns rather than by specific factors. Even higher age-standardized incidence could be observed if the participation rate is extremely high, as in the study by Tsuda et al.For valid causal inference, more detailed data on age and sex distribution are needed.