Background: The aim of this study was to compare the differences of the dosimetric parameters between three-dimensional conformal radiotherapy (3D-CRT) and simultaneous-integrated boost intensity-modulated radiotherapy (SIB-IMRT) techniques in the prone and supine positions for breast irradiation. Materials and Methods: Ten patients underwent a computed tomography simulation in both the prone and supine positions. For each set-up position, the treatment plans were created with 3D-CRT and SIB-IMRT. The dosimetric parameters were obtained from dose-volume histograms. Results: High-dose regions in the whole breast were decreased in IMRT with a simultaneous integrated boost technique. The lung doses were significantly reduced for all patients, and the heart doses were lower in left-sided breast cancer patients in the prone position. The heart doses except mean dose were not significantly lower with SIB-IMRT in the prone position.  Conclusion: SIB-IMRT allowed a more conformal dose distribution regardless of position. The prone position is superior to the supine treatment regarding doses in the ipsilateral, contralateral lung, and heart. The contralateral breast doses were increased in the prone position. Prone IMRT can be chosen for simultaneous integrated boost treatment in women with pendulous breasts.