1 INTRODUCTION Several studies have reported a worldwide increase in thyroid cancer incidence and a decreasing trend in mortality. 1 , 2 , 3 , 4 , 5 In addition, in Japan, the age‐adjusted thyroid cancer incidence rate increased: from 2.1 and 9.3 per 100 000 person‐years among men and women, respectively, in 1990 to 4.1 and 12.3 per 100 000 person‐years in 2012.
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