Anatomic Pathology: Pulmonary Pathology

• Malignant mesothelioma is an aggressive malignancy that arises from mesothelial-lined serosal spaces. This includes pleura, peritoneum, and, less commonly, tunica vaginalis.

• This malignancy is associated with asbestos exposure and occurs decades after exposure. Lung fibrosis can also occur from asbestos exposure, but lung fibrosis need not be present for mesothelioma to occur.

• Histologically, malignant mesothelioma can be epitheloid, biphasic or sarcomatoid. Epitheloid patterns of growth are quite varied, although many cases have papillary, tubule-papillary or solid growth.

• Immunohistochemistry panels are required for accurate diagnosis. These include a mixture of markers positive in mesothelial cells/mesothelioma and those positive in epithelial cells/carcinoma.

• Invasion, including invasion of adipose tissue, is an important criterion in determining malignancy. Reactive mesothelial cells can be quite atypical, making the distinction of reactive mesothelial proliferations from malignant ones difficult.

• Loss of a p16 tumor suppressor gene appears to be an important event in the development of malignant mesothelioma. In pleural mesothelioma, loss of p16 expression is often due to homozygous deletion of the p16 gene on chromosome 9.