Background: Conventional radiation dosimetry methods in computed tomography (CT) are not able to measure the dose distribution along the patient’s longitudinal axis. To calculate the dose index on a CT scan, the dose distribution from the center of the radiation field must be calculated. In this study, the most appropriate integral interval for calculating the CT dose index in the axial mode was determined using the Monte Carlo (MC) method based on X-ray photon energy and slice thickness. Materials and Methods: The computed tomography dose index (CTDI) phantom was simulated in the EGSnrc/BEAMnrcMC system and was irradiated with several X-ray energies and several slice thicknesses and dose profiles in phantom were investigated. The area under the dose profile and the scatter to primary radiation dose ratio (SPR) were calculated. Results: The range of scattered beams from the center of the radiation field reaches 450 mm in 140 kV and a 40 mm slice thickness. The SPR value for all levels of X-ray photon energy (between 80 and 140 kV) significantly decreases as slice thickness increases. CT scan imaging technical factors greater than 310 mm from the center of the slice thickness have no effect on the behavior of the scattered radiation. Conclusion: The primary beams are more affected by the energy of the photons, and the scatter beams are more strongly affected by the slice thickness. For 64-slice scanners, the polymethyl methacrylate (PMMA) phantom length should be between 700 mm and 900 mm to yield accurate CTDI estimations.