Immunohistochemical differentiation of sarcomatoid mesotheliomas from other spindle cell neoplasms

A sizable proportion of pleural malignant mesotheliomas are sarcomatoid (22%) or biphasic (24%) and may need to be distinguished from other spindle cell neoplasms that occur as primary or metastatic tumors of the chest wall or lungs. To determine the utility of immunohistochemistry in the differentiation of sarcomatoid malignant mesotheliomas from other spindle cell neoplasms, the authors studied the immunostaining patterns of nine antibodies in four sarcomatoid mesotheliomas, one desmoplastic mesothelioma, two epithelial mesotheliomas, four spindle cell squamous carcinomas, and eight sarcomas of various differentiation. Three of the four sarcomatoid mesotheliomas and the desmoplastic mesothelioma had positive results for cytokeratin, which distinguishes them from sarcomas but not from spindle cell squamous carcinomas. The immunostaining pattern of 14 examples of benign spindle cell mesothelial proliferation was similar to that of the sarcomatoid mesotheliomas, which supports the theory that these tumors are mesothelial in origin. Although other antibodies occasionally may be helpful, depending on the differentiation of the sarcoma, cytokeratin immunostaining appears to be the best method to discriminate sarcomatoid mesotheliomas from sarcomas.