Problem statement
Breast cancer human epidermal growth factor receptor–2 (HER2) can be tested by various assays including immunohistochemistry (IHC) and in-situ hybridisation (ISH). The 2018 American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) HER2 testing guidelines suggest initial HER2 testing using IHC and further testing IHC equivocal cases with ISH. However, many institutions perform both IHC and ISH on the same specimen. Repeat testing may be unnecessary and costly while potentially adding to delays in decision making and treatment. This study aims to demonstrate the concordance between HER2 IHC and ISH results tested on the same specimen.
Method
Patients diagnosed with breast cancer through BreastScreen NSW Sydney West program between January 2017 and December 2020 were identified and their HER2 IHC and HER2 ISH results on core needle biopsy (CNB) and surgical excisions (SE) were retrospectively collected. Specimens with both IHC and ISH results were then analysed for agreement and concordance using unweighted kappa values. Patients treated with neoadjuvant systemic therapy were not excluded. Equivocal IHC (2+) cases were excluded from concordance analysis.
Results
Overall, there were 240 invasive breast cancer specimens (CNB and SE) with both IHC and ISH recorded. Of these, two thirds (60.8%) were IHC equivocal (71 CNB and 75 SE). Concordance between HER2 IHC and ISH was 100% (95%CI:96.2–100%), kappa value 1.00 (P<0.0001). 94.5% of IHC equivocal cases were ISH negative. Concordance between IHC and ISH on CNB specimens (n=32) was 100% (95%CI:89.1–100%), kappa value 1.00 (P<0.0001). 91.5% of the CNB IHC equivocal cases were negative on ISH. Of the SE specimens (n=62) concordance was 100% (95%CI:93.9–100.0%), kappa value of 1.00 (P<0.0001). 97.3% of the SE IHC equivocal cases were negative on ISH.
Conclusion
There was perfect positive concordance and agreement between non-equivocal IHC and ISH results. Of the equivocal IHC cases the vast majority were negative on ISH. This reinforces that IHC alone can be utilised reliably for HER2 status consistent with the ASCO/CAP guidelines.

**Problem statement**
Breast cancer human epidermal growth factor receptor–2 (HER2) can be tested by various assays including immunohistochemistry (IHC) and in-situ hybridisation (ISH). The 2018 American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) HER2 testing guidelines suggest initial HER2 testing using IHC and further testing IHC equivocal cases with ISH. However, many institutions perform both IHC and ISH on the same specimen. Repeat testing may be unnecessary and costly while potentially adding to delays in decision making and treatment. This study aims to demonstrate the concordance between HER2 IHC and ISH results tested on the same specimen.
**Method**
Patients diagnosed with breast cancer through BreastScreen NSW Sydney West program between January 2017 and December 2020 were identified and their HER2 IHC and HER2 ISH results on core needle biopsy (CNB) and surgical excisions (SE) were retrospectively collected. Specimens with both IHC and ISH results were then analysed for agreement and concordance using unweighted kappa values. Patients treated with neoadjuvant systemic therapy were not excluded. Equivocal IHC (2+) cases were excluded from concordance analysis.
**Results**
Overall, there were 240 invasive breast cancer specimens (CNB and SE) with both IHC and ISH recorded. Of these, two thirds (60.8%) were IHC equivocal (71 CNB and 75 SE). Concordance between HER2 IHC and ISH was 100% (95%CI:96.2–100%), kappa value 1.00 (P<0.0001). 94.5% of IHC equivocal cases were ISH negative. Concordance between IHC and ISH on CNB specimens (n=32) was 100% (95%CI:89.1–100%), kappa value 1.00 (P<0.0001). 91.5% of the CNB IHC equivocal cases were negative on ISH. Of the SE specimens (n=62) concordance was 100% (95%CI:93.9–100.0%), kappa value of 1.00 (P<0.0001). 97.3% of the SE IHC equivocal cases were negative on ISH.
**Conclusion**
There was perfect positive concordance and agreement between non-equivocal IHC and ISH results. Of the equivocal IHC cases the vast majority were negative on ISH. This reinforces that IHC alone can be utilised reliably for HER2 status consistent with the ASCO/CAP guidelines.

COMPARISON OF BREAST CANCER HER2 STATUS TESTING WITH IMMUNOHISTOCHEMISTRY AND IN-SITU HYBRIDISATION.

We are pleased to welcome you to the Australasian International Breast Congress (AIBC) held jointly with the [Australasian Society for Breast Disease (ASBD)](https://www.asbd.org.au/),[6th World Congress on Controversies in Breast Cancer (CoBrCa)](http://cobrca.org/) and [Breast Surgeons of Australia and New Zealand (BreastSurgANZ) in Brisbane, Australia](https://www.breastsurganz.org/).

The Joint Congress will continue the tradition of directly addressing key issues facing clinicians in their daily practice, including medical oncology, surgery, radiation oncology, breast imaging, pathology, reconstruction, allied health and survivorship issues.

We are delighted to have assembled a stellar international and national faculty and thank all those who have agreed to participate and those who have submitted abstracts. We trust that the program is an informative and enjoyable experience.

Thank you for joining us.