Primary pulmonary tumors

The development of naturally occuring lung tumors in rats is associated with aging. Rats younger than 12 months of age rarely have these neoplasms. The majority are founds in rats older than 24 months of age, but even then the prevalence is very low. Most of the available information consists of summarized data from 2-years carcinogenicity or life span studies. Little is known about the pathobiology of naturally occuring tumors, in part because of their infrequent occurence. There is a growing body of information derived from experimental studies of pulmonary carcinogenesis in the rat (see Mohr and Dungworth 1988; Mohr et al., 1 990), although here also information on cellular and molecular aspects of the process is still very sketchy. The important primary lung tumors in rats are bronchiolo-alveolar tumors and squamous cell tumors, both of which arise in peripheral regions of pulmonary parenchyma. These will be the main focus of this chapter. Naturally occuring tumors of the tracheobr onchial tree are so rare in rats that they will mentioned only briefly.