This is an observational clinical study of MRD as a predictor of local treatment in patients with oligometastatic breast cancer. The aim of the study is to assess the correlation between MRD levels and prognosis in patients with oligometastatic breast cancer who have undergone local treatment in order to identify the population that would benefit from local treatment and to develop an individualised treatment plan. Subjects will agree to participate in this study and will be asked to collect as much medical history and demographic information as possible prior to enrolment to confirm that they meet the inclusion/exclusion criteria, and that they will undergo imaging and MRD testing prior to their first study treatment. Furthermore, subjects will undergo tumour imaging and MRD testing every 3 months (±7 days) after MRD treatment for the duration of the study, with additional investigations or more frequent tumour imaging evaluations as clinically indicated during the study.

This is an observational study to investigate the correlation between MRD levels and prognosis in patients with locally treated oligometastases of breast cancer, who have no more than 5 systemic metastases of breast cancer, which may not be confined to the same organ (with the exception of brain metastases), and who have stable systemic control of the disease, and who are to undergo localised treatment. Subjects will read and fully consider the "Informed Consent Form" for this study, sign it in writing if they decide to participate in the study, and complete the Screening Visit no later than 28 days prior to the start of study treatment. For ethical reasons, the screening visit may not be repeated if it was performed prior to the signing of the Informed Consent Form. Subjects were asked to complete imaging and MRD testing prior to their first study treatment, and to undergo on-tumour imaging and MRD testing every 3 months (±7 days) for the duration of the study, and additional investigations or more frequent on-tumour imaging evaluations could be performed during the study based on clinical indications. During this period, subjects continued study treatment until disease progression occurred, toxicity was intolerable, the subject voluntarily withdrew informed consent, or the investigator determined that the subject needed to withdraw from the study. Subjects with disease progression or early withdrawal were also required to complete an exit visit and then enter safety follow-up.

Inclusion Criteria:

1. Histologically or cytologically confirmed breast cancer.
2. Previously confirmed metastatic breast cancer by imaging.
3. Eastern Cooperative Oncology Group (ECOG) physical status score of 0-2. (4) Age ≥ 18 years at the time of signing the informed consent form. (5) Subjects willing and able to comply with the study protocol throughout the study period.

(6) Female subjects of childbearing potential must agree to use a highly effective method of contraception during the study.

(7) Voluntarily agree to and sign a written informed consent form and be willing and able to comply with all aspects of the trial protocol, which can be withdrawn by the patient at any time without prejudice.

Exclusion Criteria:

1. Patients with active/refractory infections that require ongoing anti-infective therapy.
2. Severe cardiovascular compromise (history of congestive heart failure greater than New York Heart Association (NYHA) class II), unstable angina or myocardial infarction within the past 6 months, or severe arrhythmia.
3. Subjects with allogeneic organ transplants requiring immunosuppressive therapy. (4) Subjects known to be human immunodeficiency virus (HIV) positive.

5) Subjects with a prior malignancy other than breast cancer, cervical cancer in situ, and non-melanoma skin cancer, unless the prior malignancy has been previously diagnosed and definitively treated for at least 5 years without evidence of subsequent recurrence.

6) Any medical condition that, in the opinion of the Investigator, makes the subject unsuitable for entry into the study.

The overall study proposes to enrol patients with no more than five lesions of systemic metastases from breast cancer, which may not be confined to the same organ (with the exception of brain metastases), who have stable control of systemic disease, and who are proposed to be treated locally.