Asbestosis, pleural plaques and diffuse pleural thickening: three distinct benign responses to asbestos exposure.

The aim of this study was to investigate by computed tomography (CT) whether asbestosis, diffuse pleural thickening and/or pleural plaques are statistically associated. We also tried to find criteria to differentiate between diffuse and circumscribed pleural thickening. From 231 exposed workers, only those subjects whose radiograph showed neither bilateral calcified pleural plaques nor small pulmonary opacities higher than 1/1 grade according to the 1980 International Labour Office (ILO) Classification were considered. Scans were assessed for the presence of subpleural curvilinear lines, septal and intralobular lines, parenchymal bands, honeycombing, rounded atelectasis, pleural plaques and diffuse pleural thickening. CT scans revealed pleural and/or lung abnormalities in 99 workers. Pleural plaques were unilateral in one-third of cases with plaques. Diffuse pleural thickening, parenchymal bands and rounded atelectasis were unilateral in, respectively, 62 and 69 and 75% of cases with the abnormality. Septal and intralobular lines, and honeycombing were always bilateral. CT signs could be grouped into three patterns: 1) septal and intralobular lines, and honeycombing corresponding to pulmonary fibrosis; 2) pleural plaques corresponding to parietal pleural fibrosis; and 3) diffuse pleural thickening, rounded atelectasis and parenchymal bands corresponding to visceral pleural fibrosis. In these workers with a normal or near-normal radiograph, three groups of subjects with different responses were distinguished. Crow's feet and rounded atelectasis help to differentiate plaques from diffuse thickening.