Abstract
The thyroid glands of an 11-year-old castrated cow housed in a region contaminated by the Fukushima Daiichi Nuclear Power Plant accident were collected for histopathological examination. The estimated cumulative external exposure dose was 459 mSv. Macroscopically, the left lobe of the thyroid gland was firm and enlarged, and multilobular cystic structures could be observed on the cut surface. Histologically, the tumor cells were tall, cuboidal to columnar, and formed follicles of variable sizes and shapes. The fibrous capsule was unclear, and the neoplastic invasion of follicular tumor cells was obvious in the adjacent thyroid parenchyma. Immunohistochemically, the tumor cells were positive for thyroglobulin and thyroid transcription factor-1, and the MIB-1 index was 6.2%. Based on these findings, the neoplastic lesion was diagnosed as thyroid carcinoma. This is the first report of thyroid carcinoma in a cow exposed to radiation caused by the Fukushima Daiichi Nuclear Power Plant accident.