OBJECTIVE To verify the reproducibility of patients irradiated after mastectomy on the immobilization system designed and manufactured for our hospital and to compare the Internal Protocol (IP) with the modified-No Action Level Protocol. BACKGROUND Application of forward IMRT techniques requires a good reproducibility of patient positioning. To minimize the set-up error, an effective immobilization system is important. METHODS The study was performed for two groups of 65 each. In the first group, portal images for anterior field were taken in 1-3 fractions and, subsequently, three times a week. In this group, the mNAL protocol was used. In the second group, the IP was used. The portal images from the anterior field and from the gantry 0 were taken during the 1-3 and 10 fractions. In both groups, image registration was performed off-line. For each group the systematic and random errors and PTV margin were calculated. RESULTS In the first group the value of the population systematic errors and random errors were 1.6 ± 1.6 mm for the left-right, and 1.5 ± 1.7 mm for the cranial-caudal directions, respectively, 1.7 ± 1.3 mm, and 1.9 ± 1.3 mm for the second group. The PTV margins for the left-right and cranial-caudal directions were 5.1 and 4.9 mm for the first group and 5.4 and 6.4 mm for the second group. CONCLUSIONS For patients immobilized with our support device treated according to the mNAL protocol or IP, a good set-up reproducibility was obtained. Implementation of IP limits the number of required images.