This study is a multicenter, single-arm, phase II clinical trial, aiming to evaluate the safety and efficacy of tumor bed dose escalation radiotherapy for patients with primary breast lesions achieving pCR (ypT0) after neoadjuvant chemotherapy and breast-conserving surgery.

New neoadjuvant chemotherapy (NAC) is mainly used for patients with locally advanced or those who wish to preserve their breast. It not only helps to downstage the tumor but also increases the success rate and possibility of breast-conserving treatment for patients. Moreover, NAC provides doctors with the opportunity to assess the chemotherapy response, which can be used to predict the survival of patients. Studies have shown that patients who achieve pathological complete response (PCR) after NAC and surgery have a lower local-regional recurrence rate, higher disease-free survival rate and overall survival rate. Currently, there are no treatment guidelines for this group of patients. Avoiding unnecessary tumor bed dose escalation can reduce radiotherapy toxicity and help promote the effective utilization of medical resources. This study is the first prospective single-arm study to explore the prognostic significance of NAC and breast-conserving surgery without tumor bed dose escalation for patients with pCR breast cancer. The aim is to identify the appropriate population for exemption from tumor bed dose escalation and to develop more precise adjuvant radiotherapy plans.

Inclusion Criteria:

(1) Voluntary participation, good compliance, able to cooperate with the trial observation, and having signed a written informed consent form; (2) Confirmed as invasive cancer through pathological tissue examination, and received baseline breast MR or PET/MR examination before neoadjuvant therapy; (3) Received the standard neoadjuvant treatment plan recommended by the guidelines based on molecular typing; (4) Single lesion, initially diagnosed as cT1-3N0-3a; (5) Underwent breast-conserving surgery, with negative surgical margins and a margin distance of ≥1mm, and achieved pCR of the primary breast lesion after NAC (ypT0); (6) Age 30-75 years, ECOG score 0-1; (7) Met the basic requirements for adjuvant radiotherapy, including basically normal peripheral blood count, no significant abnormalities in heart, liver, and kidney functions, and basically normal electrocardiogram; (8) Had no previous chest radiotherapy.

Exclusion Criteria:

(1) Pregnant women, lactating women, or women with reproductive capacity but without contraceptive measures in place; (2) Known to carry mutated susceptibility genes for breast cancer (including BRCA-1 and BRCA-2); (3) Diagnosed with cT4, N3b-c, bilateral breast cancer, with multiple primary lesions, multicentricity or non-bulky enhancement at the primary site; (4) Not receiving the standard neoadjuvant treatment regimen recommended by guidelines, or the primary breast lesion did not achieve pCR (including ypTis) after NAC; (5) Suffering from other malignant tumors or participating in other clinical trials simultaneously; (6) Unable to start radiotherapy within 8 weeks after breast cancer surgery; (7) Diagnosed with distant metastasis through initial imaging or pathology; (8) Previous history of breast cancer, history of radiotherapy in the neck, chest or ipsilateral axilla; (9) Severe non-tumor-related medical comorbidities affecting the implementation of radiotherapy; (10) Suffering from uncontrollable neurological, mental disorders or mental impairments, with poor compliance and inability to cooperate and describe treatment responses.

This study only included female breast cancer patients.