mc simulation

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.

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.

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.

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.

The aim of the present study is to simulate 6 MV and 18 MV photon beam energies of a Siemens Primus Plus medical linear accelerator (Linac) and to verify the simulation by comparing the results with the measured data.
The main components of the head of Siemens Primus Plus linac were simulated using MCNPX Monte Carlo (MC) code. To verify the results, experimental data of percentage depth dose (PDD) and beam dose profile for 5 × 5 cm2, 10 × 10 cm2 and 20 × 20 cm2 field sizes were measured and compared with simulation results. Moreover, gamma function was used to compare the measurement and simulation data.
The results show a good agreement, within 1%, was observed between the data calculated by the simulations and those obtained by measurement for 6 MV photon beam, while it was within 2% for 18 MV photon beam, except in the build-up region for both beams. Gamma index values were less than unity in most data points for all the mentioned energies and fields. To calculate the dose in the phantom, cells were selected in different modes, one of the modes due to the lack of dose gradient and overlapping, produced better results than others produce.
There was good settlement between measured and MC simulation values in this research. The simulation programs can be used for photon modes of Siemens Primus Plus linac in conditions in which it is not possible to perform experimental measurements.

This study presents patient specific and organ dose estimation in computed tomography (CT) imaging of thorax directly from patient CT image using Monte Carlo simulation. Patient's CT image is considered as the patient specific phantom and the best representative of patient physical index in order to calculate specific organ dose.
EGSnrc /BEAMnrc Monte Carlo (MC) System was used for CT scanner simulation and DOSXYZnrc was used in order to produce patient specific phantom and irradiation of photons to phantom in step and shoot mode (axial mode). In order to calculate patient thorax organ dose, patient CT image of thorax as voxelized phantom was divided to a 64x64x20 matrix and 6.25 x 6.25 x 6.25 mm3 voxel size and this phantom was imported to DOSXYZnrc code. MC results in unit of Gy/particle were converted to absorbed dose in unit of mGy by a conversion factor (CF). We calculated patient thorax organ dose in MC simulation from all irradiated slices, in 120 kV and 80 kV photon energies.
Effective dose was obtained from organ dose and organ weighting factor. Esophagus and spinal cord received the lowest, and bone received the highest dose. In our study, effective dose in CT of thorax was 7.4 mSV and 1.8 mSv in 120 and 80 kV, respectively.
The results of this study might be used to provide the actual patient organ dose in CT imaging and calculation of real effective dose based on organ dose.

Making use of the orthovoltage machines in Radiotherapy, is one of the routine methods for the treatment of the superficial lesions. In this study, an important determinant of X-ray quality, the HVL (Half Value Layer), has been evaluated. The HVLs of a orthovoltage X-ray machine in 120 and 180 kVp are measured, using an empirical method, in which the HVLs are derived from the absorption curves. The measured HVLs are compared with calculated (Monte Carlo simulation) HVLs. Using the BEAMDP code of simulation, the output spectra are obtained and employed for the measurement of the HVLs. Comparing the calculated and measured HVL values, the results show that the highest and lowest differences between the two are 4.96% and 2.27%, respectively, which are, in fairly good agreement with those obtained in the former studies. This study shows that the EGSnrc simulation code is capable of being used for the extraction of the quality indices for the superficial X-ray radiotherapy machines. It seems that, the mentioned code, with the mentioned experimental method, can be employed as a routine clinical test tool for every superficial radiotherapy department.