Quantitative assessment of fibroelastosis reveals distinct elastic and collagen fiber patterns in idiopathic and secondary pleuroparenchymal fibroelastosis.

Pleuroparenchymal fibroelastosis (PPFE) is a rare idiopathic interstitial lung disease; however, similar histopathological features are also observed in secondary PPFE (SPPFE) and in non-PPFE entities. This study aimed to distinguish idiopathic PPFE (IPPFE), SPPFE, and non-PPFE by quantitatively assessing elastic and collagen fibers using a deep learning-based approach. We analyzed 1728 Elastica van Gieson-stained slides obtained from 174 explanted lungs, including IPPFE (n = 14), SPPFE (n = 34), and non-PPFE (n = 126). Elastic and collagen fiber areas were quantified using DenseNet. Elastic fibers were classified into dense and faint types and assessed in upper and lower lobes. In the upper lobes, IPPFE showed a lower median collagen fiber area/fibrosis area ratio (0.480) than SPPFE (0.555, P = 0.0197) and non-PPFE (0.611, P < 0.0001), while SPPFE also showed a lower ratio than non-PPFE (P = 0.0063). IPPFE and SPPFE showed higher median dense elastic fiber area/fibrosis area ratios (0.117 and 0.097, respectively) than non-PPFE (0.041; P < 0.0001 for both). Likewise, IPPFE and SPPFE showed higher median faint elastic fiber area/fibrosis area ratios (0.385 and 0.342, respectively) than non-PPFE (0.295; P = 0.0001 and P < 0.0001, respectively). Similar trends were observed in the lower lobes, although differences in collagen fiber area/fibrosis area ratios were less pronounced. In conclusion, fibroelastosis in IPPFE is characterized by a uniform, dense elastic fiber framework with faint elastic fibers, whereas SPPFE shows heterogeneous fibroelastosis with mixed collagen fibers. These findings confirm that IPPFE is a distinct condition from SPPFE and non-PPFE.