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The central electrode correction factor for high‐Z electrodes in small ionization chambers
Author(s) -
Muir B. R.,
Rogers D. W. O.
Publication year - 2011
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3532818
Subject(s) - ionization chamber , electrode , laser beam quality , imaging phantom , ionization , beam (structure) , photon , atomic physics , materials science , monte carlo method , physics , optics , ion , laser , statistics , mathematics , quantum mechanics , laser beams
Purpose: Recent Monte Carlo calculations of beam quality conversion factors for ion chambers that use high‐Z electrodes [B. R. Muir and D. W. O. Rogers, Med. Phys., – (2010)] have shown large deviations ofk Qvalues from values calculated using the same techniques as the TG‐51 and TRS‐398 protocols. This report investigates the central electrode correction factor,P c e l, for these chambers. Methods: Ionization chambers are modeled and P c e lis calculated using the EGSnrc user code egs_chamber for three cases: in photon and electron beams under reference conditions; as a function of distance from an iridium‐192 point source in a water phantom; and as a function of depth in a water phantom on which a 200 kVp x‐ray source or 6 MV beam is incident. Results: In photon beams, differences of up to 3% between P c e lcalculations for a chamber with a high‐Z electrode and those used by TG‐51 for a 1 mm diameter aluminum electrode are observed. The central electrode correction factor for a given value of the beam quality specifier is different depending on the amount of filtration of the photon beam. However, in an unfiltered 6 MV beam,P c e l, varies by only 0.3% for a chamber with a high‐Z electrode as the depth is varied from 1 to 20 cm in water. The difference betweenP c e lcalculations for chambers with high‐Z electrodes and TG‐51 values for a chamber with an aluminum electrode is up to 0.45% in electron beams. The central electrode correction, which is roughly proportional to the chambers absorbed dose sensitivity, is found to be large and variable as a function of distance for chambers with high‐Z and aluminum electrodes in low‐energy photon fields. Conclusions: In this work, ionization chambers that employ high‐Z electrodes have been shown to be problematic in various situations. For beam quality conversion factors, the ratio of P c e lin a beam quality Q to that in a Co‐60 beam is required; for some chambers,k Qis significantly different from current dosimetry protocol values because of central electrode effects. It would be best for manufacturers to avoid producing ion chambers that use high‐Z electrodes.