Findings
    Initial chest radiograph shows bilateral pulmonary parenchymal infiltrates, primarily in both lower medial lung fields. Follow-up chest radiograph after two weeks shows more aggravation of bilateral pulmonary parenchymal infiltrates. HRCT scans after bronchoscopic biopsy show bilateral geographic distribution of ground-glass opacity with smooth thickening of inter- and intralobular septa. There is no cavitation, pleural effusion, or lymphadenopathy.

Discussion
    Pulmonary alveolar proteinosis(PAP) is a rare disease that consists of the alveolar filling with eosinophilic PAS-positive proteinaceous material, which is mostly phospholipid and protein constituents of both surfactant and degenerated cell membranes. PAP is a non-specific tissue response to a varity of injuries to alveolar macrophages, typeⅡ pneumocytes, or both. It is associated with altered immunity and exposure to dust and/or fumes. The pathogenesis of this disease is poorly understood, and the majority of cases are considered to be idiopathic. But proposed pathogenetic mechanism include unbalanced turnover and defective production or clearance of surfactant. The alveolar septa are usually normal or only slightly infiltrated, and interstitial fibrosis is unusual.  Insidious onset of nonproductive or mild productive cough and mild dyspnea are the most common symptoms, but hemoptysis, intermittent lower grade fever, pleuritic chest pain, weight loss, clubbing and cyanosis have been associated in a few patients. Pulmonary function test results typically show restrictive dysfunction with decreased diffusing capacity. Opportunistic superinfection, particularly with Nocardia, and pulmonary fibrosis are possible complications.
    The usual radiographic manifestations are bilateral, symmetric perihilar air-space disease in a "bat-wing distribution, simulating severe acute pulmonary edema but with normal heart size and rare air-bronchograms. The typical radiograph shows a bilateral, diffuse, perihilar or central, ill defined nodular or confluent pattern, which is usually worse in the base, but patchy disease and peripheral predominance can occur. CT can be useful in differential diagnosis and detection of focal pneumonia. The patterns of PAP on CT are not pathognomic, but common findings include bilateral ground-glass opacity, smooth interlobular septal thickening in abnormal areas, consolidation, and patchy or geographic distribution.
The diagnosis of PAP is made by biopsy, but detection of surfactant apoprotein A in serum could allow less invasive diagnosis.
    Bronchoalveolar lavage to remove the alveolar material is an effective therapy for patients with symptomatic PAP. The radiographic appearance after lavage have a characteristic pattern showing gradual but incomplete clearing of the washed lung within hours to weeks of successful treatment. Resolution is often spotty and asymmetric, and one area may improve as another worsens. Although in some patients the disease will resolve spontaneously or after a single lavage, most patients require chronic treatment for months or years.

References
1. Godwin JE, Muller NL, Takasugi JE. Pulmonary alveolar proteinosis: CT findings. Radiology 1988; 169:609-613
2. M. Elon Gale, Joel B. Karlinsky, Arthur G. Robins. Bronchopulmonary lavage in pulmonary alveolar lavage in pulmonary alveolar proteinosis. AJR 146:981-985, May 1986
3. Zimmer WE, Chew FS. Pulmonary alveolar proteinosis. AJR 1993;161:26