An Assessment of Dose Distribution for Asymmetric Fields by Monte Carlo Symulation

The Monte Carlo simulation techniques are the most accurate methods to predict the dose distribution in radiotherapy. The main drawback of these methods is their very long computing time for a reasonable accuracy although the time of calculations has been reduced by the use of parallel processing techniques. The MCNP-4C Monte Carlo code was used to simulate the 9 MV photon beam generated by a Neptun 10PC linear accelerator. To verify the model, the calculated dose data (depth dose, beam profile, output factors) were compared to the corresponding measured values for open fields and were found to be within the acceptable limits. The model has been applied to the simulation of asymmetric fields. The depth dose on central beam axis and collimator axis was calculated for three field sizes; 6×6, 10×10 and 15×15 cm2 with an offset of 1.5, 3.5 and 6 cm, respectively. The lateral dose profiles at two depths have also been calculated for the irradiation fields. All of the computed data were compared to the practically measured values. The measurements were carried out using a Scanditronix dose scanning system and a 0.12 cm3 RK ionization chamber. The comparison of the calculated depth dose on the central beam axis and collimator axis versus water phantom measurements in asymmetric fields showed a very good agreement (better than 2%). The result of Monte Carlo calculation of the dose profiles at 2 and 10 cm depth in water for various field sizes were found to be comparable to the corresponding measured data. Discussion and The obtained results demonstrate the accuracy of the geometry simulation and the photon source model for Neptun 10pc accelerator. This model can be used to predict the dose distribution in the other complex fields