The purpose of this study was to quantify the differences in doses in the organs at risk and to compare the PTV coverage and dose homogeneity of patients with pendulous breasts between prone and supine position. In 10 patients a CT-based treatment plan was made in prone and supine position. Data about dose homogeneity and doses to organs at risk were collected. Dose homogeneity turned out to be better in prone position and doses in organs at risk were lower, but with the current technique, this occurred at the cost of a reduced PTV coverage.