The study plans to include 100 patients and divide them into cohort A (HER2 low expression, 70 cases) and cohort B (HER2 IHC 0, 30 cases) based on the tissue HER2 status. This study adopts a non-random design, and patients will be directly included in the corresponding cohort according to their baseline tissue HER2 status. All enrolled patients will receive standard T-DXd treatment after signing the informed consent form until disease progression, occurrence of intolerable toxic reactions, or study termination. The primary endpoint of the study is to explore the correlation between the HER2 expression status of CTCs/CTECs and ORR. The secondary endpoints include the correlation between the baseline HER2 expression status of CTCs/CTECs and the DCR, PFS, and OS of the entire population; the correlation between the baseline serum HER2 ECD level and the ORR, DCR, PFS, and OS of the entire population; and the ORR and PFS of each cohort. Patients will be followed up from the first administration and samples of peripheral blood will be collected for biomarker dynamic monitoring before treatment, at 2 cycles of treatment, and at the time of disease progression (the number of tests may be increased or decreased according to the patient's condition) until disease progression, death, loss to follow-up, or study closure.
