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

Abstract

Purpose: Beam quality conversion factors, kQ, are required for use with the TG‐51 clinical dosimetry protocol. TG‐51 values of kQ are calculated using the ratio of several correction factors including the central electrode correction factor, Pcel, which is known for ion chambers with aluminum or graphite electrodes. This study calculates Pcel 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 Pcel. 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 Pcel 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 kQ factors which are up to 1.5% larger for clinical beams than those used in current dosimetry protocols. For standard Farmer‐like chambers, Pcel does not depend on the degree of filtration in the beam but for chambers with high‐Z electrodes Pcel 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.