Room: Exhibit Hall | Forum 3
Purpose: Report on recent updates and additions to the EGSnrc Monte Carlo simulation system improving calculation accuracy of dosimetric quantities for kV and MV photon beams.
Methods: Add option to use photo-electric cross-sections with a normalization correction which closely reproduces multi-configuration Dirac-Fock cross-sections. The application egs_fac with a BEAMnrc x-ray tube source is used to calculate half-value layers (HVL) and free-air chamber (FAC) attenuation corrections (Aatt). Mean ionization energies for water and graphite are updated in EGSnrc following the recommendations of ICRU-90. The crystalline density of graphite is used to determine the density correction while the bulk density is used in the actual simulation. Calculated beam quality conversion factor kQ for MV photon beams is compared with values based on ICRU-37 parameters and with measurements for graphite-walled and plastic-walled chambers and several MV beams. Dose to water and dose to the chamber cavity are calculated using the EGSnrc application egs_chamber.
Results: HVL values for lightly filtered x-ray beams differ from measurements by as much as 35 % at 80 kV when using un-renormalized photo-electric cross sections while use of the renormalized data reduces this difference to about 5 % for all energies. Similarly, the agreement of calculated FAC Aatt values with measured values improves from 0.5 % to 0.1 % for the 80 kV lightly filtered x-ray beam. A difference of less than 0.14 % in calculated kQ factors is observed which is contradictory to the predictions in ICRU-90 that suggest differences up to 0.5 % in high-energy photon beams. A slightly better agreement with experimental kQ values is obtained when using ICRU-90 recommendations.
Conclusion: Renormalized photo-electric cross sections improve the agreement between measured and calculated HVL and FAC attenuation corrections. ICRU-90 recommendations on key dosimetric data have little impact on MC-based kQ values of high-energy photon beams.