OBJECTIVES. The goal of this research was to investigate whether there is a difference in patientʼs mobility with regard to the use of the bellyboard and to examine whether there is a difference in safety margins with regard to the use of this aid. Also, we wanted to determine if there is a correlation between the age and sex of the patient with the deviation on the bellyboard. STUDY DESIGN. Cross-sectional study. PARTICIPANTS AND METHODS. The study included subjects who were treated at the Institute of Oncology, Osijek Clinical Hospital Center. The required data is obtained from the computer program used at the Department. The data included the periods from 2004 to 2009 and from 2011 to 2017. Patients were positioned in the radiotherapy position to 2015 with older patient fixing aids, while a modern bellyboard aids is being used since 2015. RESULTS. The measured deviation in patients with mean value and standard deviation was 4.7 and 4.2 in the vertical axis, while in the lateral axis was 3.7 and 3.5. In the longitudinal axis the deviation was 3.5 and 3.5. The maximum vertical axis displacements were -26 and 23 mm, lateral -22 and 24 mm, and longitudinal -17 and 11 mm. Based on these values, the safety margins are 7,7, 6,9 and 8,1 mm. CONCLUSION. There is a difference in patientʼs mobility with regard to the use of the bellyboard. There was also a statistically significant difference in SM values with respect to the use of the aids and a statistically significant correlation between the age of the patient and the deviation on the bellyboard in all 3 axes (p <0.005). The influence of the sex of the patient with regard to the shift on the bellyboard can not be proven in this research.