QuantifyHER

This trial will follow approximately 200 patients with HER2-low (IHC<2+) MBC who are starting T-DXd. The study will test a newly developed test to measure HER2 protein on tumor cells using an antibody that 'lights up' when attached to HER2. The 'light' is called fluorescence. Cells with HER2 light up when viewed under a microscope with a special light. The fluorescence strength matches with the amount of HER2 protein on the breast cancer cell. We propose to prove this test works in patients with MBC that is HER2 low (IHC<2+) and who are beginning treatment with T-DXd. Each patient in this study will have their tumor tested using this new fluorescence test (Quantitative Immunofluorescence, or QIF). Patients will be followed for response to T-DXd. We will compare the QIF score with IHC score as they relate to patients who benefit from T-DXd. We propose that QIF will more reliably tell