The relationship between fibrosis and cancer in experimental animals exposed to asbestos and other fibers.

The association between occupational asbestos exposure and the development of both pulmonary fibrosis or asbestosis and pulmonary carcinomas is well documented. It has been suggested that the two pathological conditions are associated with asbestos-related carcinomas developing from areas of asbestosis and not occurring when exposure has been too low to produce this type of pulmonary scarring. Experimental inhalation studies so far published have not been designed to examine this association specifically, but many publications have reported that asbestos samples producing high levels of fibrosis is experimental animals are also very carcinogenic. Samples of asbestos or man-made fibers that produce little fibrosis also produce few tumors. These works are reviewed. In order to examine the association between fibrosis and tumor production in more detail, groups of animals with and without pulmonary tumors and with individual fibrosis measurements were assembled from a number of inhalation studies undertaken over a period of years at this Institute. It was found that animals with pulmonary tumors had almost double the amount of pulmonary fibrosis as animals of similar age that did not. In a few of the animals where tumors were found at an early stage of development, their origin from fibrotic areas could be confirmed, although in most cases where tumor deposits were widespread this was not possible. Experimental confirmation of the site of origin of most pulmonary tumors in asbestos-treated rats would require new studies with rats examined specifically at an age when early tumors would be expected.