Abstract P6-04-17: High Intra- and Inter-block concordance of HER2 immunohistochemistry (IHC) scores across breast cancer samples and impact of decalcification procedures

Abstract Background & Objective Trastuzumab deruxtecan is effective in HER2-positive (IHC3+ or ISH-positive) and HER2-low (IHC1+; IHC2+/ISH-negative) breast cancer. We assessed consistency of HER2-positive and HER2-low categorization by IHC across and between breast cancer samples with a range of HER2 expression. The impact of decalcification procedures on HER2 IHC staining was also assessed to determine whether HER2 expression is likely to be compromised in decalcified samples from metastatic bone disease. Methods Three non-consecutive tumor sections (approximately 100 μm apart) from 50 commercially obtained breast cancer resections (2 blocks/resection) and 20 biopsies (sections ~40 μm apart) representing a range of HER2 IHC expression, and with known ISH status, were stained using the VENTANA anti-HER2/neu 4B5 IHC assay, scored per ASCO-CAP (2018) guidelines and intra-block/inter-block concordance reported for HER2-positive and HER2-low categorization. To investigate the impact of decalcification, four resection samples were formalin-fixed, divided into four pieces, of which three were pre-treated with either 10% formic acid, Decal STAT or Richard Allan Scientific< sup>TM Decalcifying Solution before processing and staining with the 4B5 IHC assay. Results A single pathologist reviewed all samples. Intra-block agreement for HER2-low (proportion of blocks in which 3 of 3 sections had identical status) was 89.5% for biopsies (17/19) and 80.6% for resection samples (n=79/98). Inter-block agreement (proportion of resections in which two independent blocks had the same majority status for HER2-low) was 91.7%. Intra and inter-block agreement for HER2-positive was >90%. Exposure to Decal STAT or Richard Allan Scientific< sup>TM Decalcifying Solution caused a significant reduction in HER2 IHC1+ and IHC2+ staining in breast cancer samples. 10% formic acid had no apparent impact on HER2 IHC staining in all four samples tested. Conclusion There was high HER2-positive and HER2-low agreement within breast cancer biopsies and resection blocks and highly consistent HER2-positive and HER2-low categorization between different blocks from tumor resections. Our results suggest HER2 staining heterogeneity does not impact determination of HER2-positive and HER2-low status in the majority of cases. Accurate assessment of HER2 IHC1+ and IHC2+ status was observed to be compromised by Decal STAT and Richard Allan decalcification solutions. Decalcification of bone metastasis samples for HER2-positive and HER2-low status determination may be possible using formic acid, but this would require further validation. Citation Format: Asimina Tsirka, Charo Garrido, Marietta Scott, Paul Scorer, Anne-Marie Boothman, Tsuyoshi Matsuo, Craig Barker. High Intra- and Inter-block concordance of HER2 immunohistochemistry (IHC) scores across breast cancer samples and impact of decalcification procedures [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P6-04-17.