The identification of features of malignancy in a desmoplastic mesothelioma requires adequate tissue and the amount of tissue in a closed pleural biopsy is often insuf- ficient. Large surgical biopsies are generally needed. High-grade sarcomas presenting in the pleura generally do not enter into the differential diagnosis of fibrous pleurisy versus desmoplastic mesothelioma. Features to separate the latter two are shown in Table 3. In the study by Mangano et al,7 the distinction of fibrous pleurisy from desmoplastic mesothelioma could be made by identifying one or more of the following features in a spindle cell proliferation of the pleura: invasive growth, bland necrosis, frankly sarcomatoid areas, and metastatic disease.
Stromal invasion is often more difficult to recognize in spindle cell proliferations of the pleura than in epithelioid proliferations. The invasive malignant cells are often deceptively bland, resembling fibroblasts, and pancytokeratin staining is invaluable in highlighting the presence of cytokeratin-positive malignant cells in regions where they should not normally be present: in the connective tissue,adipose tissue, or skeletal muscle deep to the parietal pleura or invading the visceral pleura and lung tissue (or other extrapleural structures present in the sample) (Figure 4, A and B). Although Mangano et al7 also found bland necrosis of paucicellular fibrous tissue to be a reliable criterion of malignancy, it may be subtle and one may be reluctant to base a diagnosis of malignancy solely on
its presence. Fortunately, most cases that show bland necrosis also show invasive growth. Similarly, the presence of ‘‘frankly sarcomatoid foci’’ is a distinctly subjective determination and one would be reluctant to base a diagnosis of malignancy on its presence alone because reactive processes may show marked cytologic atypia, albeit typically at the surface of the process.
Uniformity of growth and thickness of the process, surface atypia with deep maturation, and perpendicular thinwalled vessels are typical of reactive fibrous pleuritis (Figure 5, A and B), in contrast to the disorganized growth pattern and variable thickness of desmoplastic mesotheliomas. A helpful clue in desmoplastic mesotheliomas is the presence of expansile nodules of varying sizes with abrupt changes in cellularity between nodules and their surrounding tissue.