ABSTRACT
This chapter aims to evaluate the occurrence of multiple tumors in rats and their occurrence in rats of different ages. The age-associated risk for cancer to metastasize seemed to be greater in older than younger rats. Evaluation of the risk to develop multiple tumors per rat with increasing age was done using linear regression analysis. The total tumors were tabulated irrespective of tumor type or their apparent benign or malignant morphology. Some were considered to be malignant as judged by local invasion or direct extension from the primary site to surrounding tissues. Others were classified as malignant based on their cellular morphology. Some primary tumors also had distant metastases. The other rat groups had fewer tumors with distant metastases, with numbers ranging from only 3 to 7. An assessment of the age-associated incidence is more difficult.
