CK5/6 in effusions: no difference between mesothelioma and pulmonary and nonpulmonary adenocarcinoma.

OBJECTIVE: To test the performance of CK5/6 for the differentiation between mesothelioma, adenocarcinoma and benign mesothelia/proliferations in effusion cytology. STUDY DESIGN: CKS/6 immunocytochemistry was applied to ethanol-fixed cytospin preparations from 74 benign and malignant effusions. RESULTS: Reactivity was seen in 7 of 8 mesotheliomas and in 9 of 11 benign mesothelial proliferations but also in 11 of l7 pulmonary adenocarcinomas and in 12 of 31 adenocarcinomas of nonpulmonary origin. Reactivity was also found in 3 of 5 non-small cell lung carcinomas and 1 of 1 squamous carcinoma. CONCLUSION: CK5/6 reactivity was found in a considerable proportion of metastatic adenocarcinomas of pulmonary and nonpulmonary origin. The high reactivity rate in pulmonary adenocarcinomas disagrees with the results obtained with histologic sections from solid tumor tissue, and CK5/6 seems to be of very limited value as an additional marker in effusion cytology.