Fibroblastic foci (FF) are considered important findings of usual interstitial pneumonia (UIP)
however, they are not only specific to UIP but also observed in various fibrotic interstitial lung diseases (ILDs). Previous studies have reported the significance of FF comparing UIP with nonspecific interstitial pneumonia (NSIP) or secondary interstitial pneumonia, such as collagen vascular disease-related ILD (CVD-ILD) or fibrotic hypersensitivity pneumonitis (FHP). However, only few studies have mentioned their location, and no reports have shown significant results regarding their location. This study aimed to compare the spatial distribution of FF across various forms of ILDs, based on anatomical location. Among patients who underwent lung transplantation at Kyoto University Hospital between April 1, 2008, and March 31, 2023, those diagnosed with idiopathic pulmonary fibrosis (IPF) (n = 24), idiopathic NSIP (n = 11), CVD-ILD (n = 36), and FHP (n = 12) were included, and 744 slides were obtained. FF were classified into 4 categories: peripheral, such as subpleural/paraseptal
intralobular, along the alveolar wall (aFF)
centrilobular (cFF)
and distorted or dense fibrotic lesions. The number of total and each location's FF/cm