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The pulmonary apical cap (PAC) is a morphologically distinct type of unilateral or bilateral fibroelastotic scar involving the lung apices. Despite being relatively common and having been described more than a hundred years ago, it remains underappreciated as a unique diagnostic entity by clinicians, radiologists, and pathologists alike. Given the centrality of modern chest imaging in the workup of diseases of the lungs, it may be expected that the PAC will be biopsied with increasing frequency. As such, pathologists should be familiar with its presentation, appearance, and differential diagnosis. This article serves as a short overview of PAC and as a practical aid in its diagnosis for surgical pathologists.

We use a case-based format to review 4 relatively common, diagnostic challenges in surgical pathology of nonneoplastic lung diseases. All cases are linked to virtual slides so that the reader can participate in a manner that simulates the breakout session held at the 2008 New Frontiers in Pathology course at the University of Michigan, from which, this material was excerpted. Brief clinical histories and a summary of radiologic findings are followed by a description of the pertinent histologic findings and a concise topic review, intended to focus on practical diagnostic considerations. Our goal is that readers gain a greater understanding of those features most helpful in recognizing usual interstitial pneumonia, Langerhans cell histiocytosis, aspiration pneumonia, and Wegener granulomatosis.