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The Four Corners Sign: A Specific Imaging Feature in Differentiating Systemic Sclerosis-related Interstitial Lung Disease From Idiopathic Pulmonary Fibrosis

Walkoff, Lara, MD*; White, Darin, B., MD*; Chung, Jonathan, H., MD; Asante, Dennis, MS; Cox, Christian, W., MD*

doi: 10.1097/RTI.0000000000000319
Original Articles

Purpose: Differentiating between systemic sclerosis-related interstitial lung disease (SSc-ILD) and idiopathic pulmonary fibrosis (IPF) is important because of the differences in workup, prognosis, and treatment. However, there is much overlap in the appearance of these 2 entities on high-resolution computed tomography. We propose that inflammation and/or fibrosis focally or disproportionately involving the bilateral anterolateral upper lobes and posterosuperior lower lobes [“Four Corners” Sign (FCS)] is specific for SSc-ILD.

Materials and Methods: Randomized high-resolution computed tomography studies from 74 IPF and 73 SSc-ILD cases were evaluated by 2 thoracic radiologists blinded to all patient data. For each case the reviewers noted whether the FCS was present and assigned a confidence level on the basis of a 7-point Likert scale. The same process was then performed on a randomized external validation group of 42 SSc-ILD and 42 IPF cases.

Results: For Likert scores of 6 or 7 (“mostly agree” or “entirely agree” that the FCS is present, respectively) the sensitivity in SSc was 16.4% (95% confidence interval, 9.7%, 26.6%), specificity 100.0% (95% confidence interval, 95.1%, 100.0%). There was a significant association between a confidently present FCS and SSc compared with a confidently present FCS and IPF (P=0.0003). Analysis on an external validation group of 42 SSc and 42 IPF cases conferred similarly high specificity for SSc in cases characterized as FCS with high confidence.

Conclusion: The FCS, a pattern of focal or disproportionate inflammation and/or fibrosis involving the bilateral anterolateral upper lobes and posterosuperior lower lobes, is specific for SSc-ILD when readers are confident of its presence.

*Department of Radiology, Mayo Clinic

Robert D and Patricia E. Kern Center of Science of Health Care Delivery, Health Sciences Research, Mayo Clinic, Rochester, MN

Department of Radiology, University of Chicago Medicine, Chicago, IL

The authors declare no conflicts of interest.

Correspondence to: Christian W. Cox, MD, Department of Radiology, Mayo Clinic, 200 First Street SW, Rochester, MN 55905 (e-mail:

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