Naturally arising tumors of the inbred WAB/Not rat strain. I. Classification, age and sex distribution, and transplantation behavior.

During a 3-year period, 97 naturally arising tumors were collected from an inbred colony of WAB/Not rats. These tumors were largely confined to the breeding groups of approximately 5,300 female and 2,700 male rats retained up to 2 years of age. The tumors included 39 nephroblastomas and 41 mammary tumors (24 of which were histologically benign); the remaining tumors were mainly connective tissue sarcomas and skin tumors with 1 lymphoma and 2 gastrointestinal lesions. With the exception of the incidence of mammary tumors, males and females had similar tumor incidence, although nephroblastomas in female rats had a marked preference for growth in the left kidney. Most tumors occurred in the first year of life, especially the histologically malignant mammary tumors and the nephroblastomas. Of the tumors investigated, 31 were transplantable. Poor correlation between histologic malignancy and transplantation was seen with the mammary tumors. Only 50% of the nephroblastomas were transplantable. Although most transplanted tumors retained histologic characteristics of the primary tumor, 1 mammary tumor, 1 gastrointestinal tumor, and 1 nephroblastoma deviated from this general finding. In the case of this nephroblastoma, separate transplant lines from opposite poles of the primary tumor also showed differences in histology.