Treatment Intermittent Periods Based on ctDNA-MRD（Circulating Tumor DNA-minimal Residual Disease） Status in Advanced HER2-Positive Breast Cancer

Breast cancer seriously threatens the health of women worldwide. The HER-2-positive subtype accounts for approximately 15% to 20%, featuring strong invasiveness and poor prognosis. In recent years, anti-HER-2 targeted therapy has developed rapidly. The application of various drugs has improved the remission rate of advanced patients, leading to an increase in the number of patients achieving complete remission (CR). However, CR patients still face the risk of recurrence. Moreover, treatment decisions are confronted with dilemmas such as the contradiction between the cumulative toxicity of continuous treatment and the increased risk of recurrence due to premature drug withdrawal, as well as the lack of step-down treatment guidance strategies in disease-free states. Circulating tumor DNA (ctDNA)-mediated molecular residual disease (MRD) detection shows potential in predicting prognosis and monitoring recurrence in various solid tumors, with certain progress also made in the field of breast cancer. However, there are few studies on its application in guiding treatment decisions for advanced HER-2-positive breast cancer patients after CR. This study adopts a prospective, observational cohort study design. It intends to screen advanced HER-2-positive breast cancer patients, include those who have achieved CR with negative ctDNA-MRD and negative CA153, and divide them into a drug intermission group and a continued salvage treatment group based on patients' autonomous choices. The study will conduct long-term follow-up on the two groups, dynamically monitor the ctDNA-MRD status, observe the disease changes when treatment is paused in the drug intermission group, as well as the treatment response and disease progression in the continued salvage treatment group. It will evaluate the impact of the drug intermission strategy on prognostic indicators such as progression-free survival (PFS) and overall survival (OS), clarify whether ctDNA-MRD can accurately predict recurrence