Hypofractionated vs. Conventionally Fractionated Postmastectomy Radiotherapy After Implant-Based Reconstruction

Inclusion Criteria: * Female patients aged ≥ 18 years. * Pathologically confirmed unilateral primary invasive breast cancer. * Underwent skin-sparing (SSM) or nipple-sparing mastectomy (NSM) with axillary staging (sentinel lymph node biopsy or axillary lymph node dissection). * Underwent 1-stage (direct-to-implant) or 2-stage (tissue expander) reconstruction, or implant combined with autologous tissue, with prepectoral or subpectoral placement. * Received standard neoadjuvant or adjuvant systemic therapy (chemotherapy, endocrine, anti-HER2, or targeted therapy) according to guidelines or clinical trial protocols. * Definite indication for post-mastectomy chest wall ± regional nodal irradiation according to guidelines (typically c/pT1-3 N0-3 M0). * Interval of at least 6 weeks between reconstruction surgery and the initiation of radiotherapy. * ECOG performance status 0-1. * Prior non-breast malignancy permitted if disease-free for \> 5 years with low recurrence risk; prior cervical or colon carcinoma in situ, melanoma in situ, basal cell carcinoma, or squamous cell skin carcinoma permitted regardless of disease-free interval. * Estimated life expectancy ≥ 10 years. * Written informed consent provided. Exclusion Criteria: * Pregnant or lactating patients. * Prior history of breast cancer. * Bilateral breast cancer. * Prior radiotherapy to the ipsilateral chest wall or regional lymph node areas. * T4 disease (including, but not limited to, inflammatory breast cancer, regardless of response to neoadjuvant systemic therapy). * Non-breast malignancy within the past 5 years (except cervical or colon carcinoma in situ, melanoma in situ, basal cell carcinoma, or squamous cell skin carcinoma). * Severe non-malignant comorbidities (e.g., cardiovascular disease, pulmonary disease, systemic lupus erythematosus, or scleroderma) precluding radiotherapy tolerance or follow-up compliance.