Malignant disease of the pleura: a histopathological study with special emphasis on diagnostic criteria and differentiation from reactive mesothelium.

In order to define the histopathological criteria for the diagnosis of malignant neoplasm involving pleura, we studied 11 pleurectomy specimens containing undoubted malignant cells. The most important feature for a diagnosis of malignancy was the pattern of neoplastic infiltration of the fibrous sub-pleural tissue. These criteria were subsequently applied in a re-analysis of 76 pleural needle biopsies with a diagnosis of cancer. The patterns most frequently seen were a multi-pronged growth of at least four clusters of atypical cells in the fibrous sub-pleural tissue, a solid pattern with a large collection of atypical cells and a confluent one where large numbers of small clusters of atypical cells appeared almost fused in the stroma. Less frequently, malignant cells covered the surface of fibromuscular tissue, or were found in an intravascular or intramuscular location, but often associated with any of the three main patterns. Five needle biopsies were considered suspicious and 11, considered benign, contained a prominent mesothelial reaction. The benign control cases showed mesothelial cells mainly in a peripheral growth pattern or as clusters of cells interspersed with fibrin, without any mitotic activity; granulation tissue and lympho-histiocytic infiltrates were more frequently seen in the control group.