Histopathologic features of usual interstitial pneumonia and related patterns: what is important for radiologists?

In interstitial lung diseases, the diagnosis of idiopathic pulmonary fibrosis is important where pathology and radiology show usual interstitial pneumonia (UIP) pattern. Recently, revised guidelines of idiopathic pulmonary fibrosis were published in which the diagnostic algorithm requires a stricter definition for both pathology and radiology. In spite of the progress, there are issues that still need to be considered especially when facing cases where there may be a lower confidence of UIP diagnosis. As a matter of course, various etiologic backgrounds represent UIP pattern. However, their distinction is important irrespective of the etiology, because the histologic UIP pattern indicates a significantly worse prognosis than other chronic interstitial lung diseases. In this review, we describe the histologic features of UIP, effects of revised guidelines, interobserver agreement, etiologic variations of UIP pattern, and finally we include a few of our hypothetical thoughts on the "UIP bucket."