A range of HER2 directed therapies are approved to treat various malignancies, spanning a spectrum of HER2 protein expression. Several HER2 IHC scoring systems are in current use to determine eligibility for these therapies. For breast cancer this comprises the 2018 American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines with its 2025 modification to incorporate HER2 positive, low, ultralow and null groups. Australia has the added requirement of ISH amplification for access to subsidised trastuzumab. For gastric and gastro oesophageal junction cancer, the Bartley criteria are recommended. Australia has a more stringent definition of HER2 positivity in gastric cancer. In 2024, for the pan tumour indications of T-Dxd, the Bartley criteria were endorsed. In gynaecologic cancers, depending on whether Trastuzumab or T-Dxd is to be used, different HER2 scoring criteria apply. Of note, any or multiple sets of scoring criteria may be applied when interpreting the pattern of expression of HER2 on any given HER2 IHC test.
Leslie Gilham (Sun,) studied this question.