LAUSR

Supplemental Digital Content is available in the text. Primary mucinous carcinoma of the ovary is uncommon, and while numerous studies have focused on improving our ability to distinguish these tumors from gastrointestinal metastases, recent data suggest that up to one fifth are still misdiagnosed with a previously underrecognized culprit: endometrioid carcinoma. Using an index case of an ovarian endometrioid carcinoma with mucinous differentiation masquerading as a mucinous carcinoma, we sought to identify the most efficient biomarker combination that could distinguish these 2 histotypes. Eight immunohistochemical markers were assessed on tissue microarrays from 183 endometrioid carcinomas, 77 mucinous carcinomas, and 72 mucinous borderline tumors. Recursive partitioning revealed a simple 2-marker panel consisting of PR and vimentin. The combination of PR absence and vimentin absence could predict mucinous tumors with a sensitivity of 95.1%, a specificity of 96.7%, and an overall accuracy of 96.0%. Additional marker combinations did not improve accuracy. The 5-yr ovarian cancer-specific survival for mucinous carcinoma was significantly worse than endometrioid carcinoma (70% vs. 86%, respectively, P=0.02). Our proposed 2-marker algorithm allows diagnostic distinction between mucinous and endometrioid ovarian carcinomas when morphology is not straightforward. Given key differences in the underlying biology and clinical behavior of these 2 histotypes, improved diagnostic precision is essential for guiding appropriate management and treatment.