Experimental squamous cell lung tumors in Sprague-Dawley and murine pneumonitis-free rats

Squamous cell bronchogenic carcinoma was produced in 34% of a group of specific pathogen-free rats in a mean time of 139 days by the use of point-source irradiation. In a group of Sprague-Dawley rats, identical tumors developed in 54% of animals after an average exposure time of 147 days. This study was undertaken to clarify whether the bronchogenic squamous cell carcinomas produced by this method are due to the effects of the point source of irradiation alone or to the additive effects of irradiation and chronic murine pneumonitis. The results of this study in specific pathogen-free rats indicate that murine pneumonitis is not a prerequisite for the development of neoplasia by this method. The electron microscopic findings presented support the histologic diagnosis of a well-differentiated squamous cell bronchogenic carcinoma, similar in appearance to the tumor found in humans.