Refining the Radiologic Recognition of Pulmonary Alveolar Proteinosis: A Crazy-Paving Pattern Approach

Pulmonary alveolar proteinosis (PAP) is a rare airspace disease classically associated with the crazy-paving pattern on high-resolution computed tomography (HRCT). While highly suggestive, this imaging pattern is not pathognomonic and appears across a wide spectrum of pulmonary pathologies. In this review, we adopt a phenotype-first approach, using representative imaging cases to walk the reader through the differential diagnosis of crazy-paving, with attention to radiologic distribution, clinical context, and disease acuity. We emphasize distinguishing features between PAP and its mimics-including pulmonary edema, diffuse alveolar hemorrhage, organizing pneumonia, mucinous adenocarcinoma, exogenous lipoid pneumonia, acute fulminant PAP, and COVID-19 pneumonia-using side-by-side imaging and contextual pearls. Special attention is given to the radiologic clues favoring autoimmune versus secondary PAP, including geographic distribution of ground-glass opacities, subpleural sparing, and lower lobe predominance. The review concludes with a summary of diagnostic strategies, pathologic correlation, and treatment options, including insights from post-pandemic diagnostic pitfalls. This pattern-based framework is designed for the radiologist and serves as a practical guide for recognizing PAP within the broader spectrum of airspace diseases.