Investigating the characteristics of beams from a clinical linear accelerator for clinical dose calculations

For the accurate dose calculations in radiation therapy, the characteristics of beams from a clinical linear accelerator must be properly integrated in dose calculation algorithms. The purpose of this study was to investigate the characteristics of beams from a clinical linear accelerator and to use it for dose calculations in the clinical situation. To investigate the beam characteristics, Monte Carlo simulation was performed for the 6 MV photon beams from the CL23EX (Varian Medical Systems, Palo Alto, USA) accelerator by using EGS5. The validity of the simulation was confirmed by calculating the dose distributions in water based on the simulated phase space data and comparing them with the measured values in terms of the percentage depth dose (PDD) and the off axis ratio (OAR). In the evaluation of the PDD, calculated PDDs and the measured values were consistent within 1% after the buildup region. In the evaluation of the OAR, calculated OARs and the measured values were consistent within 3% in the radiation field. The results of the accelerator simulation were in good agreement with measurements. In addition, dose distributions of the clinical treatment plan were calculated based on the phase space data and compared with the one calculated by Varian Acuros XB algorithm. The local error of the dose at the isocenter was 0.11%. The gamma passing rate of the 3D gamma analysis in the patient body contour was 97.19% with 3%3mm criteria. These results showed that the dose distribution in the clinical situation could be calculated using MC without being inferior to commercial treatment planning system. (author)