To the Editor:
Toshihide Tsuda and his colleagues1 report that the annual incidence rate of thyroid cancer observed from September 2011 to March 2014 among ca. 300,000 children in Fukushima ages 18 years or less as of March 2011 is approximately 30-fold higher than the mean annual incidence rate in Japan reported by the National Cancer Center of Japan. However, the formula they use in Table 2 for deriving the incidence rate ratio, in which the prevalence is approximately equal to the product of “incidence rate” and “latent duration of disease,” has not been validated to date, although they cite 2nd edition of Rothman2 probably for authorizing their formula.
Rothman2 presents two formulas involving incidence rate: (1) Risk equals incident rate × time, and (2) prevalence, if small, is approximately equal to incidence rate × mean duration of the disease. They coined the above-mentioned formula by replacing risk in (1) with prevalence and by replacing the mean duration of disease in (2) with latent duration of the disease, which corresponds to time in (1). Since Tsuda et al. also use formula (2) in the previous presentation,3 I wonder why they abandon it in the present article.1 One possible reason may be that they noticed the following statement in Saunders and Rothman4 regarding the formula in (2): “the formula presenting the three quantities does not apply to age-specific prevalence.” The prevalence of thyroid cancer is indeed age specific.
Incorrect citation is also seen in the reference (numbered 31), which they refer to for defending their assertion against the criticism that 4 years are too short for radiation exposure to induce thyroid cancer. The author of the cited reference (numbered 31) is “World Trade Center (WTC) Health Program Administrator” and we do not read “Centers for Disease Control and Prevention” therein. This source is for policy making, not for scientific research. The author states “However, the National Academy of Sciences has reported that childhood cancers have a period of 1 to 10 years. Therefore, based on the best available scientific evidence and the following methodology presented in this revised White Paper on Minimum Latency and Types or Categories of Cancer, the administrator selected a minimum latency of 1 year for use in the evaluation of cases of childhood cancer for certification in the WTC Health Program.…”
I would be happy if the present letter might help readers correctly understand the results presented in the last column of Table 2.
1. Tsuda T, Tokinobu A, Yamamoto E, Suzuki E.. Thyroid cancer detection by ultrasound among residents ages 18 years and younger in Fukushima, Japan: 2011 to 2014. Epidemiology. 2016;27:316–322
2. Rothman KJ. Modern Epidemiology: An Introduction. 2002 New York, NY Oxford University Press
3. Tsuda T.Executive Committee of “Meeting of Nationwide Pediatricians,”. . On the results of the thyroid screening carried out by Fukushima Prefecture. In: Nationwide Pediatricians Shall Protect Children from Radiation Exposure from the Scientific Viewpoint. 2013 Osaka, Japan Executive Committee of “Meeting of Nationwide Pediatricians”:27–40 (in Japanese)
4. Greenland S, Rothman KJ.Rothman KJ, Greenland S, Lash TL. Measures of occurrence. In: Modern Epidemiology. 20083rd ed Philadelphia, PA Lippincott Williams & Wilkins:47–48