Guidelines for combining neoplasms for evaluation of rodent carcinogenesis studies

In a continuing review of long-term toxicology and carcinogenesis studies in rats and mice, the National Toxicology Program (NTP) is confronted with many problems concerning the interpretation of tumor data. A frequently raised question is: "Should certain neoplasms be combined for overall assessment of rodent carcinogenesis data?" NTP policy is that certain neoplasms may be combined for statistical assessment of tumor data and that hyperplastic responses may be used as supportive evidence. The primary reason for combining neoplastic lesions is to gain more insight into the evidence of the carcinogenicity of a given chemical in that species of animal. This report gives the rationale, criteria, and guidelines used by the NTP for combining neoplasms for the evaluation of long-term rodent toxicology and carcinogenesis studies. The guidelines are based mainly on lesions occurring in the F344/N inbred rat and (C57BL/6 X C3H)F1 mouse and may or may not be appropriate for other strains or species. The concepts of combining neoplasms and sites should be viewed in terms of the study as a whole, since tumor formation is only one of many responses caused by chemicals in mammals. The resulting information becomes part of the "weight of the evidence" for estimating the potential hazard of a given chemical.