In the Supplemental Material in Supplemental discussion on dismissed or equivocal rodent mammary carcinogens, we describe examples where signicant FBA induction was dismissed, or where FBA incidence was combined with mammary adenomas and carcinomas during analysis to eliminate the statistical signicance of the latter tumor types.

Theme 2: nonmonotonic dose responses. Many toxicological assessments assume that chemicals exert their strongest eects at higher doses and are weaker at lower doses.131,153 However, numerous studies have shown EDCs eliciting nonmonotonic dose responses, including in the mammary gland,10,21,33,153156 because hormones (and therefore disruptions in hormonal signaling) produce dierent eects at dierent concentrations. For example, the US EPA dismissed the signicant induction of mammary tumors from the pesticides malathion157 and alachlor158 in the middleand low-dose groups, respectively, because tumor incidence in the high-dose group was not statistically signicantly dierent from controls (Excel Table S2 and Supplemental Material in Supplemental discussion on dismissed or equivocal rodent mammary carcinogens).

Theme 3: high-dose toxicity. High doses of chemicals in the bioassay can render food unpalatable or cause systemic toxicity, either of which can reduce the animals body weight. Because lower body weight reduces mammary tumor incidence,9,159,160 it can mask what would be a treatment-related increase in tumors. 017002-11 132(1) January 2024

In some studies, mammary tumor induction in high-dose groups becomes signicant if results are adjusted for body weight,160 but this adjustment is inconsistently applied.161165 We found several examples where statistically signicant increases in mammary tumors at lower doses were dismissed owing to the lack of further increases at higher doses (nonmonotonic doseresponse), and many of these were accompanied by body weight reductions (Excel Table S2 and Supplemental Material in Supplemental discussion on dismissed or equivocal rodent mammary carcinogens). We expect that standardized approaches for maintaining assay sensitivity in the presence of altered body weight would improve 2-y bioassay accuracy and consistency.