Expression of muscle actins in diffuse mesotheliomas.

Ultrastructural examinations have shown myofibroblastoid differentiation in sarcomatoid/desmoplastic mesotheliomas, but immunohistochemical expression of muscle actins seldom has been documented. We examined 10 sarcomatoid, 12 epithelial, and five biphasic mesotheliomas immunohistochemically for the expression of muscle-specific actin (MSA) and smooth muscle actin (SMA) and compared it with that in 12 specimens of lung cancer. All of the sarcomatoid mesotheliomas were found to be positive for both MSA and SMA. The epithelial cells in nine epithelial and two biphasic mesotheliomas were positive for MSA, but SMA was only positive in one epithelial mesothelioma. Conversely, the lung cancers were negative for both MSA and SMA in the epithelial cells, except for one specimen that was weakly positive for MSA. The stromal cells in both the epithelial mesotheliomas and lung cancers were negative for cytokeratin but were positive for MSA and SMA, whereas the sarcomatoid and biphasic mesothelioma spindle cells were positive for all three antibodies. We concluded that sarcomatoid mesothelioma was positive for MSA and SMA, which is in support of its myofibroblastic differentiation, and that positivity for MSA in some epithelial mesotheliomas might be of diagnostic value in differentiation from lung cancers.