Intensity-modulated radiotherapy (IMRT) provides better sparing of normal tissue. We evaluated the optimum beam configuration for IMRT based on inverse treatment planning in adjuvant radiotherapy for breast cancer in a case of left-sided tumor. In addition to radiotherapy planning with the conventional technique of tangential wedged 6-MV photon beams and an oblique 15-MeV electron beam, we performed inversely planned IMRT with the step-and-shoot-technique. Dose calculation was carried out using the treatment planning system Virtuos with the inverse optimization module KonRad adapted to it. IMRT plans were generated for 2 to 16 beams. The results were compared with conventional techniques. For a maximum treatment time of 20 minutes, it is shown that IMRT with 12 modulated photon beams and 7 intensity steps is best suited for treatment in the presented case. Compared with a conventional technique with photons combined with electrons, dose conformality and homogeneity of the planning target volume was increased. The mean heart dose was reduced from 9.1 Gy to 6.1 Gy. The volume of heart irradiated with a dose higher than 30 Gy was reduced from 7.6% to 1.9%, and the volume of the left lung from 13.6% to 11.5% as well. Inverse optimization for IMRT with multiple beams is feasible in the adjuvant treatment of breast cancer. Because of the reduction of the high-dose area of a substantial cardiac volume, it is superior to conventional techniques in cases where the parasternal lymph nodes should be integrated into the target volume. Here, a clinical advantage might be detectable.