The goal of this phase III randomized controlled trial (PRADAIIBE) is to assess if preoperative radiation therapy (Preop-RT) combined with immediate breast reconstruction (IBR) can safely improve both aesthetic and quality of life outcomes in breast cancer patients, compared to the standard of care (SoC) therapy consisting of post-mastectomy radiation therapy (PMRT) and delayed/immediate breast reconstruction, in a population of breast cancer patients with an indication of mastectomy and PMRT. The following hypotheses and outcomes will be assessed at the primary endpoint of 1 year of follow-up: * Efficacy: Does Preop-RT+IBR lead to a higher BREAST-Q satisfaction with breasts score (primary endpoint), EQ-5D-5L VAS score , EQ-5D-5L Index score, AIS-Total Aesthetic Score, or a shorter treatment duration compared to SoC? * Safety: Does Preop-RT+IBR lead to an increase in adverse events (general or surgical), a lower rate of pathologic Complete Response (pCR), or worse survival outcomes compared to SoC? \[Note: this study was not powered as a non-inferiority trial, all outcomes will be pooled internationally with parallel studies\] Eligible and consenting participants will undergo screening and baseline assessments. They will then be randomised between experimental (Preop-RT+IBR) and control (SoC) groups, in a 1:1 stratified variable block size design. Follow-up will take place at 3 months, 1, 2, 5, and 10 years after the last study treatment. At baseline and during each follow-up visit each participant will complete the Breast Q 'satisfaction with breasts' and EQ-5D-5L scales, photographs will be taken. During follow-up pCR will be assessed if applicable, adverse events will be registered, and oncological follow-up will be recorded.