Abstract
A computer user-code (TRANSMIT), based on the use of the EGSnrc Monte Carlo system, was developed to simulate the transmission of megavoltage x-rays through three different materials used for shielding purposes in radiotherapy. These materials are Lipowitz's metal, lead and Rad-block. The simulations were performed for 4, 6 and 10 MV x-rays using narrow beam geometry in air. The linear attenuation coefficients, in cm(-1), obtained from the simulated transmission curves for Lipowitz's metal were 0.494, 0.470 and 0.452 for 4, 6 and 10 MV, respectively. For lead, the linear attenuation coefficients were 0.532, 0.507 and 0.483 for 4, 6 and 10 MV, respectively. For Rad-block, the linear attenuation coefficients were 0.561, 0.537 and 0.518 for 4, 6 and 10 MV, respectively. Comparison of the simulation results with experimental results reported in the literature gave a percent deviation <5% which indicates the validity of the simulation results. In addition, broad beam geometry calculations were performed for a variety of field sizes (10 x 10 cm(2), 20 x 20 cm(2) and 40 x 40 cm(2)) using the three different attenuators. The results showed that broad beam attenuation coefficients decreased with increased field size and photon energy. For Lipowitz's metal, the difference in the linear attenuation coefficient data between the broad beam attenuation coefficient and the narrow beam value lies between 3.6% and 20% depending on x-ray energy and field size. Similarly, the difference between the broad beam attenuation coefficient and the narrow beam value for lead lies between 1.5% and 10.4%. For the Rad-block material, the difference lies between 0.53% and 6.8%.