Abstract. Summary: the objective of this preliminary evaluation is to determine the set-up reproducibility of a commercially available pelvis and abdomen immobilization system and compatible pelvis thermoplastic masks. Material and method: to carry out this evaluation we used the pelvis and abdomen immobilization system, PEL-FIX / eaSyGroin (AnatGe), compatible pelvis thermoplastic masks, eXaCast (AnatGe) and an Alderson Radiation Therapy (ART) human-like phantom. The phantom was placed on the immobilization system and immobilized with a thermoplastic mask previously softened during 1 to 2 minutes on a water bath. Once the mask was placed on the phantom and hardened, a CT scan was acquired and the mask is removed. The images were sent to Pinnacle TPS (Philips Radiation Oncology Systems, Fitchburg, WI). Two days later, the immobilization process of the phantom was repeated, the verification CT images were acquired and sent to the TPS. This process was repeated during 10 days and we acquired 5 groups verification CT images, that were analyzed in the fusion application of the TPS (Fig.1). Results: The data set obtained from 5 CTs groups of phantom's repositioning were analyzed to get the following average of set-up errors: 0,21 mm (longitudinal), 0,18 mm (lateral) and 0,12 mm (Vertical). Conclusion: This immobilization system is a workable solution. This system offers great adaptability to the morphology of the patient, which leads to improve precision and reproducibility of abdominal and pelvis area in radiotherapy treatments.