Development and Validation of a HER2-Low Focused Immunohistochemical Scoring System With High-Interobserver Concordance: The Australian HER2-Low Breast Cancer Concordance Study.

The DESTINY Breast-04 trial revealed survival advantages of trastuzumab deruxtecan for women with metastatic HER2-low breast cancer (1+ or 2+ immunohistochemistry [IHC], without amplification). Although this trial applied the 2018 Americal Society of Clinial Oncology (ASCO)/College of American Pathologists (CAP) HER2 IHC scoring criteria, the subjectivity and imprecision in IHC scoring have raised concerns that patients' treatment may be misaligned. Our group of 9 experienced breast pathologists collated a deidentified set of 60 breast cancer core biopsies from 3 laboratories, evaluated with the Ventana 4B5 HER2 assay and mostly scored locally as HER2 0 or 1+. Based on ASCO/CAP 2018 criteria and our extensive experience of reporting HER2 IHC, we specified scoring conventions for cancers with low levels of HER2 protein expression, articulating specific scoring pitfalls. Each pathologist then reviewed digitized whole slide images of the IHC slides and scored the HER2 expression for each case. At a subsequent consensus workshop, we reviewed the cases jointly to establish consensus scores for each case and determine the percentage of HER2 expressing tumor cells. Consensus was reached on all cases, with 40 classified as 1+ and 3 as 2+ (not amplified), totaling 43 (71.7%) HER2-low cancers. The remaining cases were HER2 0. In 93.3% of cases (56/60), the consensus score matched with the majority opinion of pathologists' independent scores. Seven (41.2%) of the 17 cases reported locally as HER2 0 were classified as HER2 low. Conversely, among 32 cases with local scores of 1+, 7 (21.8%) were reclassified as ultralow or null. Individual pathologists' accuracy in matching the consensus scores ranged from 73.3% to 91.67% (mean, 80.74%). Among HER2-low cancers those in which <20% of the tumor cells expressed HER2 had the lowest concordance levels. Observers Cohen's κ coefficients for concordance were excellent for 4, good in 1, and moderate in the 4 observers. This reference set of cases with expert consensus HER2 scores will be invaluable for peer training and development of our national external quality assurance program for HER2-low cancers. For assessing breast cancers at the low end of HER2 protein expression, our targeted scoring criteria and explicit instruction on pitfalls improved pathologists' accuracy and concordance.