A case of autoimmune pulmonary alveolar proteinosis appearing as a localized ground-glass opacity.

Pulmonary alveolar proteinosis (PAP) is a rare diffuse lung disease caused by abnormal intra-alveolar surfactant accumulation; it commonly appears as a "crazy-paving" pattern on high-resolution computed tomography. Here, we report a rare case of autoimmune PAP appearing as localized ground-glass opacity. An 82-year-old woman underwent chest computed tomography (CT) at another facility for cough, and a 2-cm localized ground-glass opacity was detected at the bottom of the right upper lung lobe. When she presented for follow-up at our hospital 6 months later, she was asymptomatic. The CT examinations performed at that point and 2 months thereafter did not reveal any changes. However, a CT examination performed after 5 months revealed slight increases in size and concentration. Adenocarcinoma in situ or minimally invasive adenocarcinoma was suspected. Incomplete lobulation between the upper and middle lobes of the right lung was detected, and video-assisted thoracoscopic lobectomy of the upper lobe and partial resection of the middle lobe of the right lung were performed. Histological examination revealed alveoli and terminal bronchioles filled with eosinophilic proteinaceous material positive for periodic acid-Schiff stain. The histopathological diagnosis was PAP and positive serum anti-GM-CSF antibody findings confirmed autoimmune PAP.