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A useful semiquantitative method of using computed tomographic (CT) images to evaluate therapeutic response in pulmonary alveolar proteinosis (PAP) has not been established, although the extent score or grading score of ground-glass opacities has been used.The purpose of this study was to establish a semiquantitative method for evaluating therapeutic response in PAP.CT scans were obtained within 1 month before and after therapy from 32 patients with PAP who participated in a multicenter phase II trial of granulocyte-macrophage colony-stimulating factor inhalation therapy. The scans were evaluated by two chest radiologists independently. Increased parenchymal opacity was evaluated on the basis of its intensity and extent (CT grade), and the severity scores were compared with CT scores based on the extent alone (CT extent), as well as on the basis of physiological and serological results.CT grade score and CT extent score had significant correlation with diffusing capacity of the lung for carbon monoxide percent predicted (%DlCO), PaO2, VC percent predicted (%VC), Krebs von den Lungen (KL)-6, and surfactant protein D. The change in CT grade score between pre- and post-treatment examinations (ΔCT grade) correlated better with difference of PaO2 between pre- and post-treatment examinations (ΔPaO2) than ΔCT extent (difference of CT extent score between pre- and post-treatment examinations). In univariate analysis, ΔCT grade, ΔCT extent, ΔKL-6, Δ%DlCO, Δ%VC, and change in surfactant protein D correlated significantly with ΔPaO2. In multivariate analysis, ΔCT grade and ΔKL-6 correlated more closely with ΔPaO2.Although a number of CT variables were collected, the currently proposed grading system that correlates well with PaO2 should be viewed as a retrospective scoring system that needs future validation with another PAP cohort.