Toward standard HER2 testing of endometrial serous carcinoma: 4-year experience at a large academic center and recommendations for clinical practice

HER2 overexpression and/or amplification have been reported in endometrial serous carcinoma, suggesting that HER2 may be a promising therapeutic target. However, there is considerable variation in the reported rates of HER2 overexpression and amplification, likely at least in part resulting from variability in the testing methods, interpretation, and scoring criteria used. Unlike in breast and gastric cancer, currently there are no established guidelines for HER2 testing in endometrial carcinoma. A total of 108 endometrial carcinoma cases-85 pure serous carcinomas and 23 mixed endometrial carcinomas with serous component were identified over a 4-year period. All H&E and HER2 immunohistochemical slides were reviewed and HER2 FISH results (available on 52 cases) were retrieved from pathology reports. HER2 immunohistochemical scores were assigned according to the FDA criteria and the current breast ASCO/CAP scoring criteria. Clinical information was retrieved from the patients' medical records. Thirty-eight cases (35%) showed HER2 overexpression and/or gene amplification, 20 of which (53%) had significant heterogeneity of protein expression by immunohistochemistry. Lack of apical membrane staining resulting in a lateral/basolateral staining pattern was observed in the majority of HER2-positive tumors. Five of the HER2-positive cases (13%) demonstrated discrepant immunohistochemical scores when using the FDA versus ASCO/CAP scoring system. The overall concordance rate between HER2 immunohistochemistry and FISH was 75% (39/52) when using the FDA criteria, compared with 81% (42/52) by the ASCO/CAP scoring system. In conclusion, in this largest comprehensive study, 35% of endometrial serous carcinoma harbors HER2 protein overexpression and/or gene amplification, over half of which demonstrate significant heterogeneity of protein expression. The current breast ASCO/CAP scoring criteria provide the highest concordance between immunohistochemistry and FISH. Assessment of HER2 immunohistochemistry on multiple tumor sections or sections with large tumor areas is recommended, due to the significant heterogeneity of HER2 protein expression.