For the patient, it is critically important to know if you have a malignant (cancerous) mesothelioma or a benign (noncancerous) tumor. True malignant mesothelioma is an aggressive malignancy with a dismal prognosis. There are, however, a number of benign mesothelial proliferations that must be distinguished from malignant (cancerous) mesothelioma. Examples of benign mesothelioma are discussed below.
Multicystic mesothelioma, also called multiocular peritoneal inclusion cyst, is a benign lesion, or area of abnormal tissue, that is characteristically formed by multiple cysts arranged in grape-like clusters. Adenomatoid mesotheliomas are benign lesions of the genital system. Mesothelioma of the atrioventricular node is neither a mesothelioma nor a tumor. This lesion represents congenital heterotopia of the endodermal sinus in the atrioventricular node. Well-differentiated papillary mesothelioma is found more often in the abdominal cavity of young women. Histologically, it is formed by multiple papillary structures covered by cytologically benign mesothelial cells. The lesion is benign, but there have been occasional cases in which several years after diagnosis the patient developed a true mesothelioma. Localized mesothelioma, better referred to as localized fibrous tumor of the pleura (FTP), is similar to other fibrous tumors found elsewhere in the body. The tumor cells have a benign appearance and are usually characteristically negative for cytokeratin (a marker of mesothelial cells) and positive for CD34, which suggests that these cells are not of mesothelial origin. The most important predictive factor in the prognosis of FTP is whether the tumor can be completely resected. Pedunculated tumors have a much better prognosis than tumors that grow over a broad pleural area. A pedunculated tumor grows on a stalk and so can be easily and cleanly snipped off from the surrounding healthy tissue.
Tumor array studies comparing variant histologies with classical epithelial malignant mesothelioma may in the future provide us with tools to identify these rare "benign malignant mesotheliomas". Thus far, the feasibility of using gene expression arrays to distinguish mesothelioma markers that were significant in the expression arrays are already used as part of the standard immunohisotochemical panel.