Morphologic, Immunophenotypic, and Molecular Features of Epithelial Ovarian Cancer

Epithelial ovarian cancer comprises a heterogeneous group of tumors. The four most common subtypes are serous, endometrioid, clear cell, and mucinous carcinoma. Less common are transitional cell tumors, including transitional cell carcinoma and malignant Brenner tumor. While in the past these subtypes were grouped together and designated as epithelial ovarian tumors, these tumor types are now known to be separate entities with distinct clinical and biologic behaviors. From a therapeutic standpoint, current regimens employ standard chemotherapy based on stage and grade rather than histotype. However, this landscape may change in the era of personalized therapy, given that most subtypes (with the exception of high-grade serous carcinoma) are relatively resistant to chemotherapy. It is now well-accepted that high-grade and low-grade serous carcinomas represent distinct entities rather than a spectrum of the same tumor type. While they are similar in that patients present with advanced-stage disease, their histologic and molecular features are entirely different. High-grade serous carcinoma is associated with TP53 mutations, whereas low-grade serous carcinomas are associated with BRAF and KRAS mutations. Endometrioid and clear cell carcinomas typically present as early-stage disease and are frequently associated with endometriosis. Mucinous carcinomas typically present as large unilateral masses and often show areas of mucinous cystadenoma and mucinous borderline tumor. It must be emphasized that primary mucinous carcinomas are uncommon tumors, and metastasis from other sites such as the appendix, colon, stomach, and pancreaticobiliary tract must always be considered in the differential diagnosis. Lastly, transitional cell tumors of the ovary, specifically malignant Brenner tumors, are quite uncommon. High-grade serous carcinoma often has a transitional cell pattern, and adequate sampling in most cases shows more typical areas of serous carcinoma. Immunohistochemical markers are routinely employed in the diagnosis of epithelial ovarian carcinomas. However, molecular testing of these tumors, unlike in endometrial carcinoma, is not routinely used in clinical practice.