Breast-conserving surgery requires radiation therapy. The reason is that even though the surgical margins are negative, it does not mean that the cancer is not present in the breast. Studies have shown that after breast-conserving surgery, the odds of having invasive and/or intraductal carcinoma-in-situ spot lesions >50px, 75px, and 100px from the primary tumor (which are still present in your breast) are 42%, 17%, and 10%, respectively. 1. However, the presence of these lesions does not necessarily mean that they will recur. By what means do doctors stop these potential lesions from recurring? The EBCTCG study showed that if the 10-year risk of recurrence after breast-conserving surgery without radiation was 35.0%, the risk of recurrence was reduced to 19.3% with radiation therapy. 2, the adverse consequences of radiotherapy, at present, due to the progress of technology, serious adverse reactions such as radiation pneumonia have been extremely low. Other radiation dermatitis, breast shape changes, etc. are no longer particularly important in terms of local control of tumor.