Updated beam parameters for Monte Carlo simulation of five Varian megavoltage photon beams (4, 6, 10, 15 and 18 MV)

Significant discrepancies between Monte Carlo dose calculations and measurements for the Varian 18 MV photon beam with a large field size (40 × 40 cm²) were reported by different investigators. Chibani and Ma used the GEPTS Monte Carlo code to investigate these discrepancies. They suggested that geometrical data used to model the linac head, and more precisely the primary collimator, are inaccurate. Their findings were later confirmed by the manufacturer: the primary collimator entrance and exit radii are 2 mm larger than expected. More recently, the manufacturer made available new detailed drawings for (i) the tungsten and lead shielding components located between the monitor chamber and the Y jaws, (ii) the mirror frame and the (iii) tungsten base plate located downstream of the X jaws. The purpose of this study is to investigate the impact of more precise linac head geometry on dose distributions and also to extend the results we obtained for the 18 MV photon beam (Med. Phys. Vol. 34 2007 p.1206) to other energies: 4, 6, 10, and 15 MV. Contrary to most publications, the beam parameters (energy spectrum and lateral intensity distribution of primary electrons) are checked for a wide range of field sizes (2x2 cm² to 40x40 cm²), not only for 10x10 cm2. Our preliminary results show that beam models can be tuned to mach measured depth dose and dose profile distributions within 1% difference for any depth larger than 1 cm. Monte Carlo simulation of the ionization chamber response shows that difference between calculated and measured percent depth ionizations can be reduced to less than 3% even at zero depth where half of the ionization chamber is outside the phantom.