Simulation of expected childhood and adolescent thyroid cancer cases in Japan using a cancer-progression model based on the National Cancer Registry: Application to the first-round thyroid examination of the Fukushima Health Management Survey

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During the 4 years following the nuclear power plant accident of 2011, 39 males and 77 females were diagnosed with or suspected of having cancer based on the first-round thyroid examination of the Fukushima Health Management Survey (FHMS) targeting residents aged <19 years in Fukushima. Prior comparisons between the observed data and Japan's National Cancer Registry (NCR) data suggested that this incidence might be excessive, but such comparisons are problematic because they need not only to adjust index unit (prevalence proportion vs incidence rate), but also examine characteristics (complete enumeration mass screening for the aged 0 to 18 years vs detections in clinical settings for all the residents) and sensitivity of the examinations. The purpose of this study is to build a common model applicable to any region in Japan under nonaccident conditions, and estimate the expected prevalence based on the numbers of subjects surveyed in the FHMS using a simulation of the sensitivity.

The cancer-progression model is an extension of Day and Walter's, the parameters of which were estimated by minimizing the weighted root mean squared error between the average age-specific thyroid incident rates from 2001 to 2010 in the NCR and those determined by the model. We estimated expected detectable prevalent cases by the model with their examination-participation proportions and simulated several sensitivities.

Median sojourn times were 34 years (males) and 30 years (females) by the model. Simulation results showed that the numbers of observed prevalent cases were within the 95% confidence intervals of the expected prevalent cases with several sensitivities in each gender.

We successfully built a cancer-progression model of thyroid cancer based on Japan's NCR data under no accident conditions. It is a tool for comparing the observed prevalence data of examinations and the NCR data, which resolved 3 issues of index unit, the characteristics and sensitivity of the examinations. Simulation results imply that the number of observed thyroid cancer cases can be detected by the FHMS first-round thyroid screening at several sensitivities under no accident conditions.

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