Lack of cage effects on liver tumor incidence in B6C3F1 mice

Twenty-three chemicals evaluated as hepatocarcinogens in B6C3F1 mice by the National Toxicology Program (NTP) and nine showing equivocal liver tumor effects were examined to determine if the occurrence of liver neoplasms was associated with caging, i.e., if there were significant differences in tumor incidence among cages. A total of 79 dosed groups showing increased liver tumor incidence were evaluated. The analysis showed that the number of instances in which liver tumors showed significant cage effects agreed closely with chance expectation. Experimental design protocols now utilized by the NTP (including random assignment of columns of cages to dosed and control groups, periodic rotation of cage location, and individual caging of mice) further reduce the likelihood that factors associated with the housing of the animals could influence tumor incidence.