In the management of early breast cancer patients, postoperative radiotherapy following breast-conserving surgery has been established as the standard treatment. Over the past two decades, significant changes have occurred in radiotherapy for breast cancer, encompassing dose fractionation methods, radiotherapy techniques, and delineation of the radiation field. Hypofractionated radiotherapy has been reported in numerous randomized phase 3 studies to show equivalent tumor control rates, reduced acute side effects, and similar late side effects compared to conventional fractionation. Among hypofractionated radiotherapy approaches, particularly in early-breast cancer, accelerated partial breast irradiation (APBI) may be chosen over whole breast irradiation (WBI). This approach is based on data indicating that the majority of local recurrences in breast cancer occur around the site of the primary tumor. By targeting radiation to the involved breast region, it becomes possible to increase the dose per fraction while shortening the overall treatment duration due to the smaller radiation treatment volume. While careful patient selection is crucial, multiple clinical studies have demonstrated that APBI, particularly after breast-conserving surgery, yields satisfactory local control rates comparable to WBI. This is accompanied by reduced side effects, improved convenience, enhanced quality of life, and potential cost savings. Traditionally, APBI has been administered after surgery, but there is a recent trend to explore preoperative APBI. The rationale for performing preoperative APBI includes several considerations. Firstly, compared to postoperative APBI, the precise location of visually identifiable tumors allows for more accurate and targeted radiation therapy, minimizing the impact on adjacent normal tissues and achieving superior cosmetic results. Secondly, reducing the size of the preoperative lesion may enable more conservative surgery, decreasing the extent of the surgical procedure. Thirdly, it can serve as a preparatory treatment to assess tumor responsiveness. Fourthly, in some cases, it may be administered for definitive purposes, especially in elderly or frail women who may find it challenging to undergo standard treatment due to concurrent health issues. Several clinical studies have reported the effectiveness and validity of preoperative PBI in a few cases of early breast cancer. These studies have shown low rates of acute and chronic toxicity, with excellent cosmetic outcomes. While accumulating evidence supports preoperative PBI, there is currently a lack of comparative studies with postoperative PBI. Therefore, this study aims to compare and evaluate the overall clinical advantages, including safety and efficacy, of preoperative PBI and postoperative PBI in early breast cancer patients.