Carcinogenicity in the 21st Century: Challenges of New Methodologies, Strategies, and Their Impact on Pathologists—Session 1

The 2025 joint British and European Society of Toxicologic Pathology Congress commenced with a thought-provoking keynote lecture detailing the history, learnings, and limitations of the 2-year rodent bioassay. Animal carcinogenicity studies and the implementation of the Ames test for mutagenicity have enhanced our understanding of carcinogenesis, toxicokinetics, metabolism, and carcinogenic modes of action. These learnings have challenged the relevance of the 2-year bioassay for the determination of human risk and have raised questions about the need for its continued use. The first session was dedicated to carcinogenesis and Best Practice concepts with three presentations providing an introduction and general overview of rodent carcinogenicity studies: a toxicologist's perspective on the complexities of carcinogenicity studies pertaining to study design, data interpretation, and determination of human safety relevance; an experienced pathologist's overview regarding the intricacies and challenges of histopathological evaluation of carcinogenicity studies, the harmonization of nomenclature, and the data interpretation challenges; and a CRO (contract research organization) pathologist's perspective on the evolution of carcinogenicity studies with an emphasis on peer-review nuances. A case study presentation on hepatocellular foci in rodents advocated for the need to record all foci of cellular alterations with recommended implementation of size/lower threshold criteria for small lesions.