Treatment with anti‐HER2 therapy could be beneficial for patients with HER2‐positive endometrial and ovarian clear cell carcinoma (CCC). We studied HER2 overexpression by immunohistochemistry (IHC) using three different antibodies, including concordance with amplification by in‐situ hybridisation (ISH).

IHC and ISH were performed on tissue microarrays of 101 tumours: 58 endometrial pure CCC, 19 endometrial mixed carcinomas with a CCC component and 24 ovarian pure CCC. IHC was performed using SP3, 4B5 and HercepTest antibodies, and was scored by two independent observers. ISH was performed using dual‐colour silver ISH. Using IHC, agreement was poor between SP3/4B5 (61.4%), poor between SP3/HercepTest (68.3%) and reasonable between 4B5/HercepTest (75.2%). Interobserver agreement was substantial to almost perfect for all antibodies (SP3: linear weighted κ = 0.89, 4B5: κ = 0.90, HercepTest: κ = 0.76). HER2‐positivity by ISH was 17.8% (endometrial pure CCC: 24.1%, endometrial mixed: 0%, ovarian pure CCC: 16.7%). IHC/ISH concordance was poor, with a high false‐negative rate of all three IHC antibodies: sensitivity (38.9–50.0%) and positive predictive value (PPV) (37.5–58.3%) were poor; specificity (81.9–94.0%) and negative predictive value (NPV) (87.1–88.3%) were reasonable. When excluding 2+ cases, sensitivity declined (26.7–43.8%) but PPV (80.0–87.5%) and specificity (98.6–98.7%) improved.

In ovarian and endometrial CCC, there is considerable difference in HER2 overexpression by different IHC antibodies and marked discordance with ISH. As such, no single antibody can be considered conclusive for determining HER2 status in CCC. Based on these results, the lack of predictive value of different HER2 testing methods, as used in other studies, could be explained.