Pathology Implementation Advice Panel: Guidance for Reporting Diagnostic Classifications for Breast Cancer

The expression of estrogen receptor (ER) and progesterone receptor (PR) levels, as determined by immunohistochemistry (IHC), and fluorescence in situ hybridization (FISH)-negative (no amplified) for human epidermal growth factor receptor 2 (HER2) are integral to the diagnosis of triple-negative breast cancer. Uncertainty regarding jurisdictional differences in how ER and PR scores are reported has been identified. Additionally, the pan-Canadian Oncology Drug Review Expert Review Committee’s recommendation of trastuzumab deruxtecan for HER2-low metastatic breast cancer endorsed the pathologist recommendation that archival (i.e., prior to 2022) HER2 IHC 0 samples be re-read, as this could identify patients who would be eligible for trastuzumab deruxtecan.

Canada's Drug Agency convened a pan-Canadian implementation advice panel of pathologists. This panel provided advice for the following implementation question:

• How should the new diagnostic classifications, HR-low (i.e., ER/PR scores of 1% to 10% as the low level) and HER2-low, be consistently tested and reported?

Harmonization between this pathology panel and the Provisional Funding Algorithm Panel was crucial and deliberate to effectively discuss treatment strategies for breast cancer.