Prevalence of ectopic intrathyroidal thymus in Japan: The Fukushima Health Management Survey - Fukushima, T; Suzuki, S; Ohira, T; Shimura, H; Midorikawa, S; Ohtsuru, A; Sakai, A; Abe, M; Yamashita, S; Suzuki, S; Thyroid Examination Unit of the Radiation Medical Center for the Fukushima Health Management Survey (Thyroid, 2015)