1-Week Versus 3-Week PBI

Inclusion Criteria: * Female patients with an estimated life expectancy greater than 5 years. Meets criteria for either primary invasive breast cancer or pure ductal carcinoma in situ (DCIS). * For invasive breast cancer: Age 40-75 years; Underwent breast-conserving surgery with sentinel lymph node biopsy or axillary lymph node dissection; pT ≤ 3 cm (unicentric/unifocal or total multifocal size ≤ 3 cm); favorable histology (invasive ductal, mucinous, tubular, or solid papillary carcinoma, DCIS component permitted); ER+ and/or PR+; pN0(i- or i+); M0; surgical margins ≥ 2 mm. * For pure DCIS: Age 18-75 years; Underwent breast-conserving surgery with or without sentinel lymph node biopsy; low-to-intermediate nuclear grade; tumor size ≤ 2.5 cm; surgical margins ≥ 3 mm. * Presence of surgical clips or distinct postoperative seroma/tissue changes in the tumor bed allowing precise target delineation. * ECOG performance status 0 or 1. * Eligible for systemic therapy, without prior neoadjuvant systemic therapy. * Able to tolerate radiotherapy procedures and position immobilization. * No contraindications to radiotherapy. * Written informed consent provided. Exclusion Criteria: * Unfavorable histological subtypes (e.g., invasive lobular carcinoma, invasive micropapillary carcinoma). * Positive axillary, internal mammary, or supraclavicular lymph nodes. * Presence of distant metastasis. * Paget's disease of the nipple. * History of prior ipsilateral or contralateral invasive breast cancer or DCIS. * Bilateral breast cancer. * Known germline BRCA1 or BRCA2 mutation. * Prior radiotherapy to the ipsilateral breast, chest wall, or regional lymph nodes. * Active or prior second malignancy (except adequately treated non-melanoma skin cancer). * Severe non-malignant comorbidities (e.g., cardiovascular disease, pulmonary disease, systemic lupus erythematosus, or scleroderma) precluding definitive radiotherapy. * Pregnant or lactating patients. * Inability or unwillingness to comply with scheduled follow-up visits.