The aim of post-mastectomy radiotherapy is to prevent local recurrence. The question of which breast cancer patients need radiotherapy after mastectomy has not been fully resolved. This is a very important issue, especially as there is now evidence that radiotherapy improves prognosis. The aim of radiotherapy is to prevent local recurrence The aim of radiotherapy is to prevent local recurrence (recurrence in the breast, chest wall or lymph nodes). This is an important goal because there can be a survival benefit from successful prevention of local recurrence. Even in very early-stage breast cancers, if a recurrence occurs after a mastectomy, about a quarter of these will eventually lead to death. Radiation therapy also appears to help delay the time to recurrence (5 years or more after the initial breast cancer diagnosis). Radiation therapy is recommended for breast cancer patients with high risk factors for recurrence according to published guidelines. High risk factors are T3 – T4 (tumor size greater than 5.0 cm or tumor invasion of the chest wall or skin) and/or N2-N3 (at least 4 axillary lymph node metastases, lymph node fusion, or internal breast lymph node metastases). Radiotherapy may be beneficial: for T1-T2 (invasive breast cancer less than 5 cm in diameter) and N1 (1 to 3 lymph node-positive) tumors. Radiotherapy may be beneficial if the factors summarized below are present. ER negative, PR negative, or both ER and PR negative. ki-67 >20% (Ki-67 is a proliferation index) Lymphovascular infiltration (cancer cells entering blood vessels or lymphatic channels). At least one extra-lymph node metastasis (cancer cells not only in the lymph node but also spreading outside the lymph node) Presence of at least one positive surgical margin. Young age (under 40 years old).