PROLIFERATIVE KERATIN CYST, A LESION IN THE LUNGS OF RATS FOLLOWING CHRONIC EXPOSURE TO PARA-ARAMID FIBRILS

Cystic and keratinizing squamous lesions have been observed in rats ex posed chronically to a number of particulates. A variety of diagnostic terms have been applied to these pulmonary lesions but no consensus e xists as to their most proper morphological classification. In an atte mpt to obtain a consensus for cystic keratinizing pulmonary lesions pr oduced in rats by Kevlar para-aramid fibrils and TiO2 powder, a panel of medical and veterinary pathologists was invited to participate in a workshop addressing the morphology of the lesions and to reach a cons ensus on a suitable descriptive diagnostic term. All participants agre ed that the cystic keratinizing lesions were not malignant neoplasms. The majority was of the opinion that the lesions were not neoplasms. A minority (3/13) considered the lesions to be benign tumors. The panel considered that the most appropriate morphologic diagnosis for the le sions was ''proliferative keratin cyst'' (PKC). In addition, the panel agreed on the following descriptive text: ''The lesions are cysts lin ed by a well-differentiated stratified squamous epithelium with a cent ral keratin mass. Growth appears to have occurred by keratin accumulat ion and by peripheral extension of the metaplastic change into the adj acent alveolar spaces. The lesions are sharply demarcated except in th ose areas in which there has been extension of metaplasia into adjacen t alveoli. The squamous epithelium has few mitotic figures and dysplas ia is absent.'' (C) 1994 Society of Toxicology.