The objective of this observational study is to evaluate the oncological safety of breast-conserving surgery combined with standard postoperative radiotherapy in adult female patients diagnosed with multifocal or multicentric breast cancer. The main questions it aims to answer are:

What is the local recurrence rate and the time distribution of local recurrence after breast-conserving therapy in patients with multifocal or multicentric breast cancer? What independent risk factors are associated with postoperative local recurrence among these patients? There is no separate comparison group in this single-arm prospective cohort study.

Participants will:

Receive standardized breast-conserving surgery plus routine postoperative radiotherapy and individualized adjuvant therapy in accordance with clinical guidelines; Complete regular clinical follow-up visits, telephone and WeChat follow-ups for at least 3 years after surgery to collect data on surgical complications, tumor recurrence, metastasis and survival status; Fill out standardized questionnaires including Breast-Q, SDS, SAS and EORTC QLQ-C30 to assess postoperative breast satisfaction, psychological status and quality of life.

This single-center, single-arm prospective observational cohort study enrolls female patients aged ≥18 years with unilateral multifocal or multicentric breast cancer treated at the Department of Breast Surgery, the First Affiliated Hospital of Nanjing Medical University. Subjects are screened in strict accordance with unified inclusion and exclusion criteria. All enrolled patients receive breast-conserving surgery followed by standardized postoperative radiotherapy and individualized adjuvant therapy without trial-specific interventions, and all treatment regimens are formulated in line with domestic guidelines for breast cancer diagnosis and treatment. All participants sign written informed consent prior to enrollment.

Uniform baseline data including demographic characteristics, menstrual and reproductive history, family history of malignancy, imaging findings, pathological profiles, molecular subtypes, and surgical information are collected. Postoperative 30-day surgical complications are documented to evaluate short-term surgical safety. Standardized long-term follow-up lasting no less than 3 years is conducted after surgery via outpatient visits, telephone calls and WeChat communication to continuously track oncological endpoints such as ipsilateral breast tumor local recurrence, regional lymph node recurrence, contralateral breast malignancy and distant metastasis, as well as data on disease-free survival and overall survival. Validated standardized questionnaires including Breast-Q, SDS, SAS and EORTC QLQ-C30 are administered at scheduled follow-up time points to assess patients' satisfaction with breast appearance, anxiety and depression status, and overall quality of life.

All data are double-entered, subjected to regular quality control and anonymized for privacy protection. Statistical analyses consist of descriptive statistics, univariate analyses, multivariate Logistic regression, Cox proportional hazards regression, and Kaplan-Meier survival analysis. Subgroup analyses stratified by molecular subtype, lesion distribution pattern and axillary lymph node status are performed, and a nomogram predicting the risk of postoperative local recurrence is constructed to facilitate individualized clinical risk assessment.

Patients with inflammatory breast cancer, extensive lesions precluding negative surgical margins, severe vital organ dysfunction, pregnancy or lactation incompatible with radiotherapy, or inability to complete long-term follow-up are excluded from the study. Lost-to-follow-up cases are treated as censored data in survival analyses, and baseline characteristics are compared between lost and fully followed patients to mitigate attrition bias. This study receives no external funding. All study procedures strictly comply with the Declaration of Helsinki and institutional ethical review requirements. The study aims to clarify the oncological safety of comprehensive breast-conserving therapy for multifocal or multicentric breast cancer, identify independent risk factors for postoperative local recurrence, and provide native evidence-based references for individualized surgical management of such patients in China.

Inclusion Criteria:

1. Female patients aged ≥18 years with an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1;
2. Pathologically confirmed invasive breast carcinoma or ductal carcinoma in situ (DCIS);
3. Radiologically confirmed unilateral multifocal or multicentric breast cancer;
4. No systemic distant metastasis detected on preoperative imaging;
5. Comprehensive clinical evaluation predicts all tumor lesions can be completely resected by breast-conserving surgery (BCS) to obtain negative surgical margins, and patients are suitable for standardized adjuvant whole-breast radiotherapy postoperatively;
6. Adequate function of vital organs (cardiac, hepatic, renal and hematopoietic systems) to tolerate surgery, subsequent adjuvant radiotherapy and chemotherapy;
7. Voluntarily agree to participate in the study after fully understanding the study purpose, procedures, potential risks and benefits, and provide written informed consent.

Exclusion Criteria:

1. Inflammatory breast cancer;
2. Extensive tumor burden that prevents complete resection via breast-conserving surgery or attainment of negative surgical margins;
3. Severe dysfunction or failure of vital organs (heart, liver, kidney, lung) making patients unable to tolerate surgery, radiotherapy or chemotherapy;
4. Pregnant or lactating women who are ineligible for postoperative radiotherapy;
5. Patients who are unwilling or unable to complete long-term postoperative follow-up and data collection.

This single-center prospective cohort enrolls women aged ≥18 years with an ECOG performance status of 0-1 and pathologically confirmed unilateral multifocal/multicentric breast cancer without distant metastasis. Eligible candidates are suitable for breast-conserving surgery with negative margins and postoperative adjuvant radiotherapy. Patients with inflammatory breast cancer, extensive unresectable lesions, severe vital organ dysfunction, pregnancy or lactation, or poor follow-up compliance will be excluded. All participants provide written informed consent and receive standard routine treatment, with long-term follow-up conducted to observe oncological outcomes, surgical complications and patient-reported quality of life.

The individual participant data involves sensitive personal clinical information of patients, and there is no established data sharing protocol approved by the ethics committee for external researchers at present. Therefore, IPD will not be shared after the study is finished.