Context: The separation of benign from malignant mesothelial proliferations is crucial to patient management but is often a difficult problem for the pathologist.
Objective: To review the pathologic features that allow separation of benign from malignant mesothelioma proliferations, with an emphasis on new findings.
Data sources: Literature review and experience of the authors.
Conclusions: Invasion is still the most reliable indicator of malignancy. The distribution and amount of proliferating mesothelial cells are important in separating benignity from malignancy, and keratin stains can be valuable because they highlight the distribution of mesothelial cells. Hematoxylin-eosin examination remains the gold standard, and the role of immunochemistry is extremely controversial; we believe that at present there is no reliable immunohistochemical marker of malignancy in this setting. Mesothelioma in situ is a diagnosis that currently cannot be accurately made by any type of histologic examination. Desmoplastic mesotheliomas are characterized by downward growth of keratin-positive spindled cells between S100-positive fat cells; some cases of organizing pleuritis can mimic involvement of fat, but these fat-like spaces are really S100-negative artifacts aligned parallel to the pleural surface. Fluorescence in situ hybridization on tissue sections to look for homozygous p16 gene deletions is occasionally useful, but many mesotheliomas do not show homozygous p16 deletions. Equivocal biopsy specimens should be diagnosed as atypical mesothelial hyperplasia and another biopsy requested if the clinicians believe the process is malignant.