Incidence patterns of spontaneous tumors in BN/Bi rats.

The occurrence of tumors in 236 female and 74 male BN/Bi rats that were allowed to live out their normal lifespans was described. For most neoplasms, the risk of a rat dying with a specific tumor increased with age. Some tumors, however, had a peak incidence; therefore, animals surviving these periods were at less risk than were their younger cohorts. The number dying with metastatic cancers was greatest in females over 30 months of age. Unexpectedly, males had a peak risk period at 25-30 months, so that males that died in older age groups had less risk of dying with a metastatic tumor. Lesions were often multiple, even in rats dying at a young age. Surprisingly, the percent dead with multiple tumors did not increase significantly with age. Some animals in every age group died without a tumor, and the percentage without neoplasms did not decrease, even in the older age groups.