CK7, CK20, CDX2 and MUC2 immunohistochemical staining used to distinguish metastatic colorectal carcinoma involving ovary from primary ovarian mucinous adenocarcinoma

Objective: Colorectal adenocarcinoma, the most common tumor that metastasizes to the ovary, is often difficult to distinguish from primary ovarian mucinous adenocarcinoma (POMA). Obtaining the correct diagnosis is difficult but crucial to treatment and prognosis. Methods: We evaluated the immunohistochemical (IHC) expression of cytokeratin 7 (CK7), cytokeratin 20 (CK20), CDX2, CEA, MUC2, MUC5AC and α-methylacyl-CoA racemase (AMACR) in 22 POMAs and 41 metastatic colorectal adenocarcinomas (MCAOs) involving ovaries. Results: MCAOs, in contrast with POMAs, were almost always negative for MUC5 (97.6%), often negative for CK7 (82.9%), focal or diffuse positive for CDX2 (73.2%), diffuse positive for CK20 (65.9%), focal or diffuse positive for MUC2 (51.2%), diffuse positive for CEA (41.5%) and negative for AMACR (41.5%). We therefore considered CK7 (-), CK20 (diffuse +), CDX2 (+) and MUC2 (+) to be colonic markers and regarded cases with expression of more than two colonic markers as MCAO, those with no expression of colonic markers as POMA and those with expression of one colonic marker as indeterminate. Using CK7/CK20/CDX2/MUC2, 82.5% of the cases were correctly classified, 6.3% were misclassified and 6.3% were indeterminate. Conclusion: CK7, CK20, CDX2 and MUC2 IHC staining is a useful adjunctive diagnostic tool to differentiate MCAOs from POMAs, in addition to clinical history and gross and microscopic findings.