Napsin A and WT 1 are useful immunohistochemical markers for differentiating clear cell carcinoma ovary from high‐grade serous carcinoma

Clear cell carcinoma (CCC) of the ovary is an uncommon, but an aggressive epithelial ovarian cancer (EOC), which has overlapping histopathologic features with other ovarian tumours. Lately, Napsin A has been identified as its useful diagnostic immunohistochemical (IHC) marker. Fifty‐eight prospectively diagnosed ovarian CCCs, 53 high‐grade serous carcinomas (HGSCs), 16 endometrioid adenocarcinomas (EMACs), six mixed carcinomas, containing components of CCC and EMAC, seven metastatic mucinous adenocarcinomas and six ovarian yolk sac tumours (YSTs) were tested for Napsin A immunostaining. Fifty ovarian CCCs, 50 HGSCs, seven ovarian EMACs and five mixed carcinomas were tested for WT1 immunostaining. Napsin A was positively expressed in all 58 (100%) CCCs; was focally positive in 1 of 6 YSTs; in 1/16 EMACs and in six cases of mixed carcinomas, while it was negative in all 53 HGSCs and in seven metastatic mucinous adenocarcinomas. Other IHC markers expressed in cases of CCC ovary were CK7 (31/31) (100%), WT1 (0/50), p53 (20/26, ‘wild type’), ER (4/31, focal) (12.9%), PAX8 (14/14) (100%), glypican‐3 (4/10, focal) (44.4%), p16INK4 (5/5, focal) and CK20 (0/5). Various IHC markers expressed in HGSCs were WT1 (48/50) (96%), p53 (31/31, mostly ‘mutation type’), CK7 (9/9) (100%) ER (13/16, variable) (81.2%) and PAX8 (14/14) (100%). IHC markers expressed in EMACs were ER (15/16) (93.7%), CK7 (2/2) (100%) and WT1 (0/7). IHC markers expressed in mixed carcinomas were CK7 (2/2) (100%), WT1 (0/2), focal Napsin A (6/6) and focal ER (5/6). The sensitivity and specificity of Napsin A for the diagnosis of CCC ovary was 100% and 90.9%, respectively. The sensitivity and specificity of WT1 for diagnosis of HGSC ovary was found to be 96% and 100%, respectively. Napsin A and WT1 are highly sensitive and specific IHC markers for diagnosing ovarian CCCs and HGSCs, respectively, and in differentiating these tumours from their mimics. Napsin A is useful in identification of component of CCC in certain EMACs.