"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 exposed chronically to a number of particulates. A variety of diagnostic terms have been applied to these pulmonary lesions but no consensus exists as to their most proper morphological classification. In an attempt to obtain a consensus for cystic keratinizing pulmonary lesions produced 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 consensus on a suitable descriptive diagnostic term. All participants agreed 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 lesions was "proliferative keratin cyst" (PKC). In addition, the panel agreed on the following descriptive text: "The lesions are cysts lined by a well-differentiated stratified squamous epithelium with a central keratin mass. Growth appears to have occurred by keratin accumulation and by peripheral extension of the metaplastic change into the adjacent alveolar spaces. The lesions are sharply demarcated except in those areas in which there has been extension of metaplasia into adjacent alveoli. The squamous epithelium has few mitotic figures and dysplasia is absent."