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Ber‐EP4 Antibody as a Discriminant in the Differential Diagnosis of Malignant Mesothelioma Versus Adenocarcinoma

The pathologic diagnosis of malignant mesothelioma is often difficult, even with the benefit of special studies such as histochemistry, electron microscopy, and immunohistochemistry. Ber-EP4 is a newly characterized monoclonal antibody that reliably labels epithelial tissues but does not react with mesothelial cells. We evaluated Ber-EP4 on formalin-fixed, paraffin-embedded tissue sections from 115 malignant mesotheliomas and 83 adenocarcinomas. Although 72 cases (87%) of adenocarcinoma were positive for Ber-EP4, only one (1%) of the mesotheliomas was reactive. The only adenocarcinomas that did not stain were from the breast (eight of 25 cases nonreactive) and the kidney (all three cases nonreactive). The staining pattern in the positive adenocarcinomas was usually intense and membranous, but additional weak cytoplasmic staining was seen in many cases. The reactivity was diffuse in 59 cases and focal in 13 cases. The results of our study suggest that the Ber-EP4 antibody may have great use in the differential diagnosis of mesothelioma versus adenocarcinoma, particularly when only formalin-fixed tissue is available.