Background. Lung adenocarcinomas represent a morphologically heterogeneous tumor composed of an admixture of different histologic subtypes (lepidic, papillary, acinar, and solid subtype). The presence of a solid subtype component is reported to be associated with a poorer prognosis. The aim of this study was to evaluate the characteristic immunophenotype of the solid subtype component compared with the immunophenotypes of other components. Methods. We analyzed the clinicopathological characteristics of stage I adenocarcinoma patientswith predominant solid subtype disease. Furthermore, we immunostained adenocarcinomaswith predominant lepidic, papillary, acinar, and solid subtype components (n = 23 each) for 10 molecular markers of tumor invasiveness and scored the results. Results. Patients showing predominance of the solid subtype component (solid subtype adenocarcinoma) had a poorer prognosis than those showing predominance of the lepidic, papillary, or acinar component. Lymphovascular invasion was more often detected in solid subtype tumors than in others. The solid subtype component showed a significantly stronger staining intensity of laminin-5 expression than the lepidic, papillary, and acinar components (P<0.001, P<0.001, and P = 0.016, respectively). The fibronectin and vimentin expression levels were also significantly higher in the solid subtype component than in other components. This immunostaining character was validated by using mixed-subtype adenocarcinomas containing all four components in the same tumor. Conclusions. This study concluded that the solid subtype component in lung adenocarcinomas exhibit the invasive immunophenotype, including increased laminin-5 expression, compared with the other components, which may be associated with a poorer prognosis.
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