Estimated prevalence of thyroid cancer in Fukushima prior to the Fukushima Daiichi nuclear disaster
A medical intervention for children, being conducted in Fukushima, Japan, deserves to be discussed in this “Too much medicine” campaign.(1)
Since October 2011, Fukushima prefecture has been conducting a thyroid ultrasound examination program for residents aged <=18 years. As of June 30, 2014, 296026 children have been tested (screening rate, 80.5%), of whom 104 were either found to have or suspected to have thyroid malignancy.(2) While the absence of a control group makes it difficult to interpret this result,(3) forming a large-scale control group can raise ethical concerns, given the possibility of over-diagnosis and over-treatment. Here, as a historical control we estimated the prevalence of juvenile thyroid cancer in Fukushima prefecture before the start of the thyroid examination program, using population-based statistical data.
The estimation targeted the prevalence of thyroid cancer in prefectural residents aged <=40 years in 2010 (a year before the Fukushima Daiichi nuclear disaster). By using the life table method based on thyroid cancer incidence and overall mortality rates, we calculated the age-specific cumulative probability of thyroid cancer incidence,(4,5) and multiplied it by the number of residents aged 0 in each calendar year. Summing the age-specific prevalence for each birth cohort to a given attained age produced the age-specific cumulative prevalence.
Figure shows that, in 2010, the prevalence of thyroid cancer in children aged <=18 years in Fukushima was estimated to be 2.1 persons (0.5 male and 1.6 females). The age in which the prevalence of thyroid cancer exceeds 50 persons in both sexes combined was estimated to be 31 years and that exceeding 100 persons was estimated to be 35 years. By using the same method, the cumulative probability of thyroid cancer mortality in persons aged 40 years is one in approximately 300,000 persons (0.00027% in males and 0.00032% in females).
Compared with these results, the fact that as many as approximately 100 thyroid cancer cases aged <=18 years have currently been found suggests over-diagnosis. As of 2013, the age-standardized mortality rate of thyroid cancer in Japan has been decreasing for both males and females. In South Korea, widespread adoption of thyroid cancer screening in adults led to a rapid increase in the incidence of thyroid cancer, but the mortality rate remained unchanged.(6) Surgery for thyroid cancer requires most patients to take lifelong medications and can cause complications in a few cases. A false-positive result leads to unnecessary, sometimes invasive, additional procedures. In fact, in the thyroid examination program in Fukushima, 2237 children (0.8%) proceeded to the additional examination, of which 485 underwent aspiration biopsy. A balance between the potential risks and benefits must be considered for the implementation of the thyroid examination program.
1. Moynihan R, Glasziou P, Woloshin S, Schwartz L, Santa J, Godlee F. Winding back the harms of too much medicine. Bmj 2013;346:f1271
2. Tentative Summary Results of the Fukushima Health Management Survey (Thyroid Ultrasound Examination Programme) 2014. (http://www.pref.fukushima.lg.jp/sec/21045b/kenkocyosa-kentoiinkai-15.html) [in Japanese]
3. Shibuya K, Gilmour S, Oshima A. Time to reconsider thyroid cancer screening in Fukushima. Lancet 2014;383:1883-4
4. Kamo K, Katanoda K, Matsuda T, Marugame T, Ajiki W, Sobue T. Lifetime and age-conditional probabilities of developing or dying of cancer in Japan. Japanese journal of clinical oncology 2008;38:571-6
5. Wun LM, Merrill RM, Feuer EJ. Estimating lifetime and age-conditional probabilities of developing cancer. Lifetime data analysis 1998;4:169-86
6. Ahn HS, Kim HJ, Welch HG. Korea's thyroid-cancer "epidemic"--screening and overdiagnosis. The New England journal of medicine 2014;371:1765-7
Competing interests: S. Tsugane is a member of the review committee of the health examination program in Fukushima prefecture, Japan.