Thyroid proliferative findings in the Crl:WI (Han) rat in 2-year carcinogenicity studies – An update

Joint BSTP/ESTP Toxicologic Pathology Congress 2025 -- The thyroid gland is an important and frequent target in carcinogenicity studies due to the inducibility of follicular tumours secondary to liver changes. C-cell tumours are among the most frequent spontaneous neoplasms in Han Wistar rats. They do not share the above-described pathogenesis as follicular tumours. Historical control data (HCD) are helpful for differentiating spontaneous and test item-related findings, such as marginally greater incidences and/or severity of proliferative changes in treated animals compared to controls, and unexpected increases or decreases of tumour incidences in study control animals. HCD can reveal trends in tumour biology and behavior that may evolve over time in rodent models. The aim of this study is to provide updated HCD on proliferative findings of the thyroid in Han Wistar rats as fluctuations in the incidence of these findings in recent carcinogenicity studies have been pointed out by our team of pathologists.