Rapid Communication MICROSCOPIC REVIEW OF para-ARAMID-INDUCED CYSTIC KERATINIZING SQUAMOUS LESIONS IN THE LUNGS OF RATS

Abstract

In a 2-yr inhalation study in rats with para-aramid (p-AR) respirable, fiber-shaped particulate (RFP), cystic lung lesions, originally diagnosed as cystic keratinizing squamous-cell 3 carcinoma, were produced in some rats exposed to either 100 or 400 RFP/cm. However, the consensus diagnosis of an international panel of pathologists convened in 1992 was that the p-AR RFP-associated lesions were best designated as proliferative keratin cysts. Similar lesions have been produced in rats following exposure to many different dusts. The diagnosis and human relevance of these lesions are controversial. In 1995 in Hannover, Germany, another international panel of pathologists proposed criteria and diagnostic terminology for the spectrum of squamous lesions produced in the rat lung following exposure to dusts. Using the criteria established by this panel, the p-AR RFP-induced squamous lesions were reevaluated by a panel of four pathologists. Unanimous agreement was reached by the pathologists for a diagnosis of pulmonary keratin cyst for 9 of 10 cystic keratinizing squamous lesions produced in female rats. The one remaining cystic squamous lesion was more difficult to classify. One pathologist considered the lesion to be a cystic keratinizing epithelioma, and three considered it to be a pulmonary keratin cyst. The squamous lung lesion that occurred in one male rat was unanimously diagnosed as a squamous cell carcinoma. The cystic keratinizing lung lesions produced following exposure to p-AR RFP and many other dusts appear to be unique to the rat, and further investigation is necessary to better understand their biological nature. However, these lesions are probably not relevant for human risk assessment of pulmonary cancer.