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 CCC s, 53 high‐grade serous carcinomas ( HGSC s), 16 endometrioid adenocarcinomas ( EMAC s), six mixed carcinomas, containing components of CCC and EMAC , seven metastatic mucinous adenocarcinomas and six ovarian yolk sac tumours ( YST s) were tested for Napsin A immunostaining. Fifty ovarian CCC s, 50 HGSC s, seven ovarian EMAC s and five mixed carcinomas were tested for WT 1 immunostaining. Napsin A was positively expressed in all 58 (100%) CCC s; was focally positive in 1 of 6 YST s; in 1/16 EMAC s and in six cases of mixed carcinomas, while it was negative in all 53 HGSC s and in seven metastatic mucinous adenocarcinomas. Other IHC markers expressed in cases of CCC ovary were CK 7 (31/31) (100%), WT 1 (0/50), p53 (20/26, ‘wild type’), ER (4/31, focal) (12.9%), PAX 8 (14/14) (100%), glypican‐3 (4/10, focal) (44.4%), p16 INK 4 (5/5, focal) and CK 20 (0/5). Various IHC markers expressed in HGSC s were WT 1 (48/50) (96%), p53 (31/31, mostly ‘mutation type’), CK 7 (9/9) (100%) ER (13/16, variable) (81.2%) and PAX 8 (14/14) (100%). IHC markers expressed in EMAC s were ER (15/16) (93.7%), CK 7 (2/2) (100%) and WT 1 (0/7). IHC markers expressed in mixed carcinomas were CK 7 (2/2) (100%), WT 1 (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 WT 1 for diagnosis of HGSC ovary was found to be 96% and 100%, respectively. Napsin A and WT 1 are highly sensitive and specific IHC markers for diagnosing ovarian CCC s and HGSC s, respectively, and in differentiating these tumours from their mimics. Napsin A is useful in identification of component of CCC in certain EMACs.