Morphologic and Molecular Characteristics of Mixed Epithelial Ovarian Cancers

Epithelial ovarian cancer consists of 5 major histotypes: high-grade serous carcinoma (HGSC), endometrioid carcinoma (EC), clear cell carcinoma (CCC), mucinous carcinoma (MC) and low-grade serous (LGSC). Each can have a broad spectrum of morphological appearances, and one histotype can closely mimic histopathological features more typical of another. Historically, there has been a relatively high frequency of mixed, defined by 2 or more distinct histotypes present based on routine histopathological assessment, histotype carcinoma diagnoses (3–11%), however recent immunohistochemical studies identifying histotype specific markers and allowing more refined histotype diagnoses suggests a much lower incidence. We reviewed hematoxylin and eosin stained slides from 871 cases of epithelial ovarian cancer and found the frequency of mixed carcinomas to be 1.7% when modern diagnostic criteria are applied. Through international collaboration, we established a cohort totaling 22 mixed epithelial ovarian cancers, consisting of 9 EC/CCC, 4 EC/LGSC, 3 HGSC/CCC, 2 CCC/MC and 4 other combinations. We interrogated the molecular differences between the different components of each case using immunohistochemistry, gene expression and hotspot sequencing analyses. Immunohistochemical data alone suggested 9 of the 22 cases were not mixed tumors as they presented a uniform immuno-phenotype throughout, and these cases most probably represent morphological mimicry and variation within tumors of a single histotype. Synthesis of molecular data further reduces the incidence of mixed carcinomas. Based on these results, true mixed carcinomas with both morphological and molecular support for the presence of more than one histotype within a given tumor represent less than 1% of epithelial ovarian cancers.