Cytokeratin 5/6 expression, prognosis, and association with estrogen receptor α in high-grade serous ovarian carcinoma

High-grade serous ovarian carcinoma remains one of the most lethal malignancies in women. For histopathologic differentiation from mesothelioma cytokeratin, 5/6 immunohistochemistry is widely used. Another preferred marker for differential diagnosis to mesothelioma is estrogen receptor α (ER-α). In this study, we determined the rate of cytokeratin 5/6-positive cells in primary high-grade serous carcinoma. A cohort of 215 patients with high-grade serous ovarian carcinoma was evaluated immunohistochemically for the protein expression of cytokeratin 5/6. Most tumors demonstrated at least partly positive for cytokeratin 5/6 (n = 148; 68.3%), showing different staining patterns from scattered stained cells to a diffuse staining, at times with a distinctive tumor-stroma border motif. Sixty-seven (31%) were entirely negative. No correlation of cytokeratin immunoreactivity score (IRS) with conventional staging parameters could be demonstrated. From the different IRS values for cytokeratin 5/6, IRS = 12 (n = 6; 2.9%) seemed to indicate a worse prognosis, albeit not statistically significant. An association with ER-α expression could not be detected but the combination of cytokeratin 5/6 IRS = 12 and ER-α negativity resulted in a significant negative prognostic marker (overall survival: P = .003 and progression-free survival: P < .0001). We substantiate cytokeratin 5/6 protein expression as a frequent feature of high-grade serous ovarian carcinoma with various staining patterns, an important fact for the routine differential diagnosis with mesothelioma. Furthermore, cytokeratin 5/6 in combination with ER-α proved to be a negative prognostic marker, wherefore we suggest further investigation of its biological significance and possible manifestation of a basal differentiation.