Purpose: Despite the well-known fact that intensity-modulation could significantly improve the dose distributions in breast irradiation, its clinical implementation has been hindered by deficiencies in the current inverse planning systems and by the lack of a comprehensive procedure. The aim of this work was to develop a general scheme of intensity-modulated breast treatment using MSRT and to demonstrate its superiority over IMRT as well as standard tangential-field (TF) technique using 20 clinical cases of various breast sizes.Methods: The patient setup and target definition were the same as that used in the TF treatment. Two planning methods were studied: (1)manual forward planning, and (2)computer MSRT optimization. A 3D planning system with a trial-and-error was used in the manual MSRT planning to stack MLC segments on top of the standard TFs to get a sensible MSRT plan. The underlying reason for the approach to be a viable choice is that the initial TFs have already brought the system to the vicinity of optimal solution. As a result, it is often sufficient to add 1∼3 segments to greatly improve the dose distribution. MSRT optimization was also developed using a gradient method. The DVH-based score function depended on both segment weights and shapes. MLC constraints were included to prevent unphysical MLC configurations. The target volume, defined at the patient setup based on the palpable breast tissue, was used by the algorithm for calculation.The segmented fields were concatenated to form a step-and-shoot delivery. An algebraic method was devised to determine the segmented MU to optimally compensate the MLC transmission. The fluence map and MU were independently checked. 20 patients were planned and the results were compared with the standard TF plans, as well as the TF and multiple-field IMRT plans.Results: The MSRT significantly improved target dose uniformity. Figure 1 shows a manual plan. It was also possible to reduce the lung/heart dose with a slight deterioration of target dose. Optimization yielded consistent beam apertures and weights and became advantageous for complicated cases. Our results revealed that MSRT could easily reduce the dose uniformity from 105∼120% (prescription was at 90%) to 100∼112%. The results were comparable or even more favorable than the conventional TF IMRT plans. It was noticed that IMRT with 3∼7 beams was inferior in that more normal tissues were irradiated.Conclusion: IMRT deviates from the conventional approach and requires additional steps in the treatment process. MSRT bridges the gap between conventional and IMRT treatments. For breast cancer, MSRT is a natural extension of standard procedure and improves the treatment without paying the excessive overhead associated with current IMRT. Purpose: Despite the well-known fact that intensity-modulation could significantly improve the dose distributions in breast irradiation, its clinical implementation has been hindered by deficiencies in the current inverse planning systems and by the lack of a comprehensive procedure. The aim of this work was to develop a general scheme of intensity-modulated breast treatment using MSRT and to demonstrate its superiority over IMRT as well as standard tangential-field (TF) technique using 20 clinical cases of various breast sizes. Methods: The patient setup and target definition were the same as that used in the TF treatment. Two planning methods were studied: (1)manual forward planning, and (2)computer MSRT optimization. A 3D planning system with a trial-and-error was used in the manual MSRT planning to stack MLC segments on top of the standard TFs to get a sensible MSRT plan. The underlying reason for the approach to be a viable choice is that the initial TFs have already brought the system to the vicinity of optimal solution. As a result, it is often sufficient to add 1∼3 segments to greatly improve the dose distribution. MSRT optimization was also developed using a gradient method. The DVH-based score function depended on both segment weights and shapes. MLC constraints were included to prevent unphysical MLC configurations. The target volume, defined at the patient setup based on the palpable breast tissue, was used by the algorithm for calculation. The segmented fields were concatenated to form a step-and-shoot delivery. An algebraic method was devised to determine the segmented MU to optimally compensate the MLC transmission. The fluence map and MU were independently checked. 20 patients were planned and the results were compared with the standard TF plans, as well as the TF and multiple-field IMRT plans. Results: The MSRT significantly improved target dose uniformity. Figure 1 shows a manual plan. It was also possible to reduce the lung/heart dose with a slight deterioration of target dose. Optimization yielded consistent beam apertures and weights and became advantageous for complicated cases. Our results revealed that MSRT could easily reduce the dose uniformity from 105∼120% (prescription was at 90%) to 100∼112%. The results were comparable or even more favorable than the conventional TF IMRT plans. It was noticed that IMRT with 3∼7 beams was inferior in that more normal tissues were irradiated. Conclusion: IMRT deviates from the conventional approach and requires additional steps in the treatment process. MSRT bridges the gap between conventional and IMRT treatments. For breast cancer, MSRT is a natural extension of standard procedure and improves the treatment without paying the excessive overhead associated with current IMRT.