Pulmonary alveolar proteinosis with multiple localized ground-glass nodules mimicking lung cancer: A case report

Background Pulmonary alveolar proteinosis (PAP) is a rare disorder characterized by the accumulation of surfactant within distal airspaces. Chest computed tomography (CT) typically shows a crazy-paving pattern, with interlobular septal thickening overlaps with ground-glass opacities(GGOs). Localized ground-glass nodules (GGNs) are uncommon and may mimic early-stage lung cancer. Case Presentation A 69-year-old man with a history of rectal cancer surgery five years earlier subsequently underwent multiple pulmonary resections for metastatic rectal cancer. During postoperative surveillance, follow-up chest CT revealed localized GGNs in the right S6 and left S3. The left S3 lesion demonstrated interval growth, raising suspicion for primary lung cancer. Video-assisted thoracic surgery (VATS) S3 segmentectomy was performed. Histopathological examination showed no malignancy; instead, eosinophilic granular material within the alveolar spaces stained positive on periodic acid–Schiff (PAS), suggesting PAP. Serum anti–granulocyte–macrophage colony-stimulating factor (GM-CSF) antibody levels were elevated, confirming a diagnosis of autoimmune PAP. The patient remained asymptomatic, and conservative management was selected. Conclusion In typical PAP, opacities extend centrifugally from the perihilar regions toward the pleura and rarely present as isolated peripheral lesions. The presence of localized GGNs, as in this case, is therefore unusual. Differentiating PAP from lung cancer based solely on CT findings can be challenging, particularly when interval growth is observed. This case highlights the importance of considering PAP in the differential diagnosis of enlarging GGNs, even when radiologic findings are atypical.