Incidence of spontaneous tumors in a group of retired breeder female brown Norway rats.

A group of 80 female retired breeder inbred Brown Norway (BN) rats with recorded breeding histories was followed for 150 weeks with regard to spontaneous occurrence of tumors. The median survival time of the whole group was 134 weeks. Ninety tumors (42 benign tumors, 23 sarcomas, and 25 carcinomas) were found in 64 animals. Compared with previous findings on virgin female BN rats, the retired breeders used in the present study had a strikingly higher incidence (34%) of tumors of the genital system but a lower incidence (4%) of breast tumors. The breeding histories revealed no relationship between the recorded number of litters, number of newborns, age of the mother at the time of her first litter, preweanling mortality, and the occurrence of cancer or survival in general. Although all animals were kept under identical environmental conditions, animals born in the period July-December appeared to have a lower incidence of utero-vaginal tumors than animals born in the first half of the year. The growth rates of 32 tumors inoculated into syngeneic rats were monitored for 26 weeks. They did not correlate with the histopathologic characteristics of the specific tumor. Malignant or benign tumors showed the same growth variability.