Expression of cytokeratin 5/6 in epithelial neoplasms: an immunohistochemical study of 509 cases.

Cytokeratin 5/6 (CK 5/6) immunoreactivity has been observed in the vast majority of cases of malignant mesothelioma but only rarely in pulmonary adenocarcinomas. Thus, CK 5/6 has been used to distinguish malignant mesothelioma from pulmonary adenocarcinoma. However, the utility of CK 5/6 in distinguishing pleural malignant mesothelioma from pleural metastases from nonpulmonary adenocarcinoma, as well as peritoneal malignant mesothelioma from peritoneal metastatic adenocarcinoma, has not yet been adequately addressed because the tissue expression of CK 5/6 in nonpulmonary neoplasms has not been well defined. We have studied the CK 5/6 expression in 509 cases of various epithelial tumors by immunohistochemistry. We found that the vast majority of cases of squamous cell carcinoma, basal cell carcinoma, thymoma, salivary gland tumor, and biphasic malignant mesothelioma were positive for CK 5/6. In addition, CK 5/6 immunoreactivity was detected in 15 of 24 cases (62%) of transitional cell carcinoma, in 5 of 10 cases (50%) of endometrial adenocarcinoma, in about one third of cases of pancreatic adenocarcinoma (38%) and breast adenocarcinoma (31%), and in one quarter of cases of ovarian adenocarcinomas (25%). Our study confirms the diagnostic utility of CK 5/6 immunohistochemistry in distinguishing biphasic mesothelioma from pulmonary adenocarcinoma but raises caution about its use for the differential diagnosis of pleural or peritoneal malignant mesothelioma versus pleural or peritoneal metastatic nonpulmonary adenocarcinoma, because many types of nonpulmonary adenocarcinomas may be positive for CK 5/6.