Comparing the 2017 ASCO/CAP guideline for gastroesophageal adenocarcinoma surgical specimen to the 2018 ASCO/CAP guideline for breast cancer in assessing the HER2 status in primary mucinous ovarian carcinoma.

The successful experiences of HER2 inhibitors in patients with HER2 ( +) breast cancer (BC) and advanced gastroesophageal adenocarcinoma (GEA) have encouraged us to continuously explore the HER2 status and its potential as a therapeutic target in primary mucinous ovarian carcinoma (mOC). Using 49 primary mOC samples, we compared the assay characteristics of HER2 status between both 2017 American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) for GEA and 2018 ASCO/CAP for BC guideline recommendations. We demonstrated moderate to strong agreement between their HER2 IHC results (Weighted Kappa = 0.78) and perfect agreement between their HER2 FISH results (Kappa = 1.00). The overall concordance of non-equivocal HER2 IHC and HER2 FISH results was 97.56% (kappa = 0.93) by 2017 ASCO/CAP for GEA criteria and 100% (kappa = 1.00) by 2018 ASCO/CAP for BC criteria. The number (n = 8; 16.32%) of HER2 IHC equivocal (score + 2) by 2017 ASCO/CAP for GEA criteria was twofold higher than that (n = 4; 8.16%) by 2018 ASCO/CAP for BC criteria. Additionally, we identified one false-positive (FP) case (n = 1; 2.04%) that was HER2 IHC positive (score + 3), but HER2 FISH non-amplified result by the 2017 ASCO/CAP for GEA criteria. In conclusion, owing to the absence of FP/ FN and fewer equivocal cases of HER2 IHC, we recommend that the 2018 ASCO/CAP for BC are more appropriate than 2017 ASCO/CAP for GEA criteria in appraising the HER2 status in mOC and justifying the inclusion of eligible subjects for basket clinical trials of the newly developmental anti-HER2 treatments.