821 Use of immunohistochemistry for correct ovarian cancer diagnosis and histotyping in clinical practice in The Netherlands

Introduction/Background Immunohistochemistry (IHC) is essential for correctly diagnosing and histotyping epithelial ovarian cancer. Key markers for origin determination include CDX2, cytokeratin 7, cytokeratin 20, GATA3, and PAX8, while oestrogen receptor (ER), Napsin A, p16, p53, progesterone receptor (PR), and Wilms' Tumour 1 (WT1) are essential for histotyping. We examined the use of these markers in diagnosing and histotyping epithelial ovarian cancer in the Netherlands. Methodology In this retrospective cohort study, clinical and pathological data from all patients diagnosed with ovarian cancer in 2018 in the Netherlands were retrieved from the Netherlands Cancer Registry and the Dutch nationwide pathology databank, respectively. Information about immunohistochemistry was extracted from the microscopy section of the pathology reports. Results IHC was conducted in 94.3% (n=1,020) of the 1,082 cases. In clear cell (CCC) and mucinous (MC) carcinoma, IHC was omitted most frequently (17.9% and 10.1%, respectively). Approximately 25% of CCC, MC, endometrioid (EC), and low-grade serous (LGSC) carcinomas lacked origin-determining IHC. The key markers p53 and WT1 were concurrently assessed in 81.6% of high-grade serous carcinoma (HGSC) and 72.1% of LGSC cases. The key markers WT1, and ER or PR, were used in 70% of EC and 34.3% of MC cases. In CCC, Napsin A was analysed in 39.2% of cases, but when absent, WT1 and ER or PR were assessed in 35.4%. Conclusion The high number of tumours in which key markers were omitted, and the presence of cases in which no IHC was conducted, underscore the urgency for standardised histotype-specific IHC sets. We propose assessment of at least WT1 and p53 for HGSC and LGSC, WT1 and PR for EC and MC, and Napsin A for CCC, aiming for diagnostic precision and consistency in epithelial ovarian cancer histotyping. Disclosures The authors have no disclosures to declare that are relevant to the content of this study.