Examination how the dependence of the mega voltage photon parameters in radiotherapy of the tumors using BEAMnrc code

Simulation using Monte Carlo technique is a practical method for assessment of the characteristics of the clinical photon beam from the treatment head. The accuracy of the simulation results is based on the validation of the Monte Carlo models used in the calculation. This study validated the simulation with BEAMnrc code for 6MV energy.
Subjects and The accelerators of Varian Clinic 2100C/D for photon beam 6MV were simulated using the BEAMnrc code system. A cylindrical mono energetic with 5.7, 5.9, 6MV energies and 1, 2, 3mm radius were applied. Two phase space files were scored below the exit window and above the phantom then analyzed the beam characteristics using BEAMdp and calculated the percentage depth dose and profile at 10cm using DOSXYZ code. In this work, measurements were performed using 0.13cc ion chamber inside a water phantom with dimension 50×50×50cm3. At the end of simulation, the results of the measured data were compared using gamma index with criteria 2%/2mm.
By increasing the electron energy, percentage depth dose in buildup region was reduced and became closer to measurements. With increasing the diameter of the electron source, the photons were diverged and the edges of the profiles showed a sharp drop compared to the dose measurements.
Finally good agreement (gamma index within2%/2mm) between the measurements and simulations was achieved for field sizes of 4×4 ,10×10 and 30×30cm2 with radial of incident electrons 0.2cm at source surface distances of 100cm.