Location of Fibroblastic Foci: Does the Lesion You Observe Really Suggest Usual Interstitial Pneumonia?

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/cm² were counted, and the percentage of each location's FF was calculated. IPF showed more total FF and peripheral FF than NSIP. FHP had more cFF than CVD (P = .026) and NSIP (P = .018). The dFF was higher in IPF than that in CVD (P = .018) and NSIP (P = .039). The aFF/total FF ratio was higher in CVD than that in FHP (P = .021) and IPF (P < .001). A high cFF/total FF ratio was correlated with FHP versus IPF (P = .032). In conclusion, FF with existing peripheral and distorted/dense fibrosis were more closely related to IPF, whereas cFF were highly correlated with FHP. Moreover, a high aFF/total FF ratio was suggestive of CVD.