Efficacy and Safety of Trastuzumab-rezetecan in HER2-Expressing Breast Cancer

Inclusion Criteria: 1. Age ≥ 18 years, with pathologically confirmed HER2-positive or HER2-low-expressing breast cancer: HER2-positive is defined as IHC 3+ or IHC 2+ with a positive ISH result; HER2-low-expressing is defined as IHC 1+ or IHC 2+ with a negative ISH result for HER2 amplification; hormone receptor status must be clearly documented. 2. Planned to receive treatment with Trastuzumab-rezetecan for Injection in the neoadjuvant, adjuvant, or advanced/metastatic setting; patients with advanced/metastatic disease must have progressed after at least one prior line of systemic therapy. 3. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0-2. 4. Women of childbearing potential must have a negative serum pregnancy test within 28 days prior to enrollment, and both male and female patients must be willing to use a highly effective, medically approved method of contraception during the study and for 1 year after the last dose of the study drug. 5. Non-pregnant and non-lactating female patients. 6. Not concurrently participating in any other ongoing clinical studies. The patient or their legal representative has provided informed consent by signing the informed consent form and is willing and able to comply with scheduled visits, the study treatment plan, laboratory tests, and other trial procedures. Exclusion Criteria: 1. Any evidence of severe or uncontrolled systemic disease, including uncontrolled hypertension, active bleeding disorders, active infections (such as hepatitis B, hepatitis C, and human immunodeficiency virus), or severely impaired bone marrow reserve or organ function (including hepatic and renal impairment), which, in the investigator's opinion, would significantly alter the risk-benefit balance. 2. At the initiation of treatment with Trastuzumab-rezetecan for Injection, the patient has not recovered from any prior treatment-related toxicity of CTCAE grade \>3. 3. Known history of hypersensitivity to Trastuzumab-rezetecan for Injection, its excipients, or drugs with similar chemical structures or classes as Trastuzumab-rezetecan. 4. Patients deemed unsuitable for enrollment by the investigator.