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The polarity effect of compact ionization chambers used for small field dosimetry
Author(s) -
Looe Hui Khee,
Büsing Isabel,
Tekin Tuba,
Brant Andre,
Delfs Björn,
Poppinga Daniela,
Poppe Björn
Publication year - 2018
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 - 10.1002/mp.13227
Subject(s) - monte carlo method , ionization chamber , dosimetry , physics , polarity (international relations) , polarity symbols , ionization , computational physics , optics , beam (structure) , atomic physics , ion , voltage , nuclear medicine , chemistry , medicine , biochemistry , statistics , mathematics , breakdown voltage , quantum mechanics , cell
Purpose The recent developments of compact air‐filled ionization chambers for use in small photon beams have raised the needs to address the associated polarity effect. The polarity effect of five compact ionization chambers has been quantified at small field sizes. The origins of the polarity effect are studied experimentally and through Monte‐Carlo simulations. For this purpose, the one‐dimensional lateral dose–response functions were determined using positive and negative chamber's polarity. Monte‐Carlo simulations were performed to study the underlying mechanism of the polarity effect by quantifying the charge imbalance in the collecting electrode and cable. Methods Five novel compact ionization chamber designs have been studied ( PTW ‐Freiburg: Semiflex 3D 31021, PinPoint 3D 31022 and PinPoint 31023; IBA Dosimetry: Razor chamber CC 01‐G and Razor Nano‐chamber CC 003). Output ratios were measured down to a nominal field side length of 3 mm using both polarities to evaluate the polarity effect at different field sizes. The small field output correction factors were derived using a scintillator detector as reference. To identify the origins of the polarity effect, slit beam measurements were performed to obtain their lateral dose–response functions. All measurements were performed using three chamber orientations: axial, radial crossplane, and radial inplane. The chambers were modeled according to the manufacturers' blueprints using the Monte‐Carlo package EGS nrc. The charge imbalance due to electrons entering and leaving the inner electrode and cable was studied using an adapted user‐code. Results The output ratios obtained using all five chambers show field size‐dependent polarity effects at small field sizes in the axial orientation, whereas no significant field size dependence of the polarity effect has been observed in the radial orientations. The chambers' lateral dose–response functions reveal that the radiation‐induced charge imbalance in the inner electrode and cable is the main cause of the observed polarity effect at small field sizes. The effect is weakest for the largest PTW 31021 chamber but intensifies for smaller chambers with decreasing sensitive air volume. Consistent results have been obtained between Monte‐Carlo simulations and measurement data. Conclusions Awareness needs to be raised so that the polarity effect of novel compact ionization chambers is appropriately accounted for in small field dosimetry. The results in this work are useful to identify the magnitude of the polarity effect correction and to assist in the decisions on choosing the appropriate chambers and setups during measurements. The origins of the observed polarity effect have been elucidated using the chambers' lateral dose–response functions. The adapted Monte‐Carlo user‐code has been used to compute the radiation‐induced charge imbalance in the chamber's components. It opens the possibility to study the chamber's design with the aim to minimize its polarity effect. Small field output correction factors computed according to TRS 483 have been reported for these investigated chambers.