Introduction Women with advanced or recurrent endometrial carcinoma (EC) exhibiting HER2 overexpression or gene amplification are candidates for HER2-targeted therapy. Determining HER2 status involves immunohistochemistry (IHC) and, in equivocal cases, fluorescence in situ hybridization (FISH), which is costly, requires proficiency training and is not widely available. Many well-resourced centers have adopted next generation sequencing (NGS) platforms to detect pathogenic POLE mutations using gene panels that include the ERBB2 gene. Herein, we examine the correlation between HER2 status determined by IHC-FISH and NGS.

Methods Retrospective cases tested by IHC/FISH and NGS in a large academic center over 18 months. IHC (Ventana 4B5) and FISH (PathVysionHER2 DNA Probe kit) were evaluated on whole slide and scored according to ISGyP guideline recommendations. NGS used Oncomine Comprehensive Assay v3 on the S5 Prime NGS platform (Thermo Fisher) to detect ERBB2 deletions, single nucleotide variants (SNV) and amplification (figure 1).

Results HER2 status was determined by IHC/FISH and NGS in 45 cases: 19 serous, 18 high-grade (unspecified histologic type, HGU), 5 endometrioid 1 carcinosarcoma, 1 undifferentiated and 1 gastric-type. IHC-FISH HER2 was positive in 11/45 (24.4%) of cases, all serous or HGU. NGS identified 3 cases as amplified (concordant) and 1 non-pathogenic SNV. Although NGS had positive predictive value of 100% and negative predictive value of 81.0%, sensitivity was low (27.3%, 3/11). Table 1 describes the discordant cases.

Conclusion/Implications IHC/FISH are more sensitive than NGS for identification of HER2 positive cases. Intratumoral heterogeneity, tumour cellularity and HER2/CH17 ratio play significant role in reduced detection of HER2 amplification by NGS.

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Abstract EP204/#941 Figure 1

HER2 testing in endometrial carcinoma