Sci‐YIS Fri ‐ 02: Calculated Pwall values in clinical photon beams

Dosimetry of high‐energy photon beams is based upon absorbed dose to water standards and requires the use of ionization chambers with several correction factors. This study investigates the wall correction factor, Pwall, in high‐energy photon beams for both cylindrical and parallel‐plate chambers using Monte Carlo calculations. For cylindrical chambers, dosimetry protocols use an empirical formula to determine Pwall despite high quality experimental evidence that there are problems with this formula. Wall corrections are not provided for parallel‐plate chambers in photon beams due to a lack of information available regarding the correction factors for these chambers. Monte Carlo calculations are carried out using the EGSnrc user‐code CSnrc to calculate the wall correction factor for a series of ion chambers using a correlated sampling variance reduction technique. Calculations of the wall correction are performed for a variety of chambers at the reference depth in photon beams, using realistic beam spectra from clinical accelerators, ranging in nominal energy from 60 Co to 24 MV. For cylindrical chambers, Pwall values differ by as much as 0.8% from the predicted values. This discrepancy is used to resolve previous experimental results that pointed to problems with the Pwall formalism. Pwall values are also shown for parallel‐plate chambers in high‐energy photon beams and have corrections up to 2%. These data should allow parallel‐plate chambers to be used in photon as well as in electron beams.