Background: To reduce the dose to normal tissues surrounding the treated breast, a uniform magnetic field was used within a humanoid phantom in breast radiotherapy. Materials and Methods: Monte Carlo simulations were performed with GEANT4, irradiating humanoid phantoms in a magnetic field. To reconstruct phantoms, computed tomography (CT) data slices of four patients were used for the Monte Carlo simulations. All of them had left breast cancer either or not mastectomy. In the simulations, the planning and methods of chest wall irradiation were similar to the actual clinical planning. Results: Utilizing magnetic field will help to produce uniform dose distribution to the breast with a sharp dose-volume histogram (DVH) curve for the planning target volume (PTV), however, for the ipsilateral lung and chest wall skin the mean dose was reduced by a mean of 16% and 12% at 1.5 T, and 9% and 7% at 3 T, respectively. The magnetic field was shown to restrict the lateral spread of secondary electrons to the contralateral organs, resulting in significient dose reductions to the contralateral breast (CB) and contralateral chest wall skin (CCWS) by a mean (range) of 28% (21-37%) and 58% (44-75%) at 1.5 T, and 48% (32-81) and 66% (54-73%) at 3 T, respectively. Conclusion: The simulations established that the magnetic field can reduce the dose to the internal and contralateral tissues and increase it to the PTV with sharper edge DVH curve.