SU‐EE‐A2‐05: Central Electrode Correction Factor, Pcel, for Steel and Wire Electrodes

Purpose: Beam quality conversion factors, k Q , are required for use with the TG‐51 clinical dosimetry protocol. TG‐51 values of k Q are calculated using the ratio of several correction factors including the central electrode correction factor, P cel , which is known for ion chambers with aluminum or graphite electrodes. This study calculates P cel values in photon beams for more recent chambers that use electrodes composed of high‐Z materials. Method and Materials: Monte Carlo simulations using the EGSnrc user‐code egs_chamber are used to calculate the absorbed dose to the gas in an ion chamber with and without the central electrode. The ratio of the two doses gives P cel . Manufacturers' blueprints and user's manuals are used to model the ion chambers. The source is modeled as a collimated point source from tabulated spectra for Cobalt‐60 and linear accelerator photon beams. Spectra used include some linear accelerators without flattening filters to investigate the value of P cel in ‘soft’ beams. Results: For small volume ion chambers with electrodes composed of high‐Z materials the central electrode effect is unlike that of aluminum electrodes in standard Farmer‐like chambers. This results in k Q factors which are up to 1.5% larger for clinical beams than those used in current dosimetry protocols. For standard Farmer‐like chambers, P cel does not depend on the degree of filtration in the beam but for chambers with high‐Z electrodes P cel is different by up to 1.5% in lightly‐filtered beams compared to values in typical clinical beams. Conclusion: Calculations of the central electrode correction factor in photon beams have shown that ion chambers with electrodes made of high‐Z materials have significantly larger electrode effects (1 to 2.5%) than those with aluminum electrodes (<0.8%). It would be best to avoid such high‐Z electrode materials.