SU‐CC‐J‐6C‐09: Calculated Pwall Values in Clinical Electron Beams
Purpose: Dosimetry of high‐energy electrons beams is based upon absorbed dose to water standards and requires the use of ionization chambers with several correction factors. There is little information regarding the details of many of these correction factors for electron beam dosimetry. This study investigates the wall correction factor, Pwall, in high‐energy electron beams for both cylindrical and parallel‐plate chambers using Monte Carlo calculations. Dosimetry protocols use a wall correction factor of unity in high energy electron beams, despite some evidence that there may be an effect greater than 1%. Method and Materials: Monte Carlo calculations are carried out using the EGSnrc system. In particular, the user‐code CSnrc is used to calculate the wall correction factor for a series of ion chambers using a correlated sampling variance reduction technique. The wall correction is computed as the ratio of doses to the air cavity for a chamber having a wall made entirely of water to that having a realistic chamber geometry. Calculations of the wall correction are performed for a variety of chambers at the reference depth in electron beams, using realistic electron beam spectra from clinical accelerators, ranging in nominal energy from 5 MeV to 25 MeV. Results: For parallel‐plate chambers, the wall correction is between 1.5% and 1.8% at the lower energies and varies from 0.5% to 1% at the highest energies. For cylindrical chambers, the wall corrections are up to 0.7% for the energy range investigated. Conclusion: EGSnrc calculations of the wall correction factors for ion chambers in high energy electron beams show that this effect is, in many cases, greater than 1%. This differs significantly from dosimetry protocols, which assume a correction of unity in these beams. Chamber‐specific values of the wall correction for parallel‐plate chambers are parametrized as a function of the beam quality.