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Monte Carlo calculations of electron beam quality conversion factors for several ion chamber types
Author(s) -
Muir B. R.,
Rogers D. W. O.
Publication year - 2014
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 - 10.1118/1.4893915
Subject(s) - ionization chamber , absorbed dose , dosimetry , monte carlo method , laser beam quality , beam (structure) , imaging phantom , percentage depth dose curve , ion , materials science , electron , atomic physics , physics , optics , computational physics , ionization , nuclear medicine , nuclear physics , radiation , mathematics , medicine , laser , statistics , quantum mechanics , laser beams
Purpose: To provide a comprehensive investigation of electron beam reference dosimetry using Monte Carlo simulations of the response of 10 plane‐parallel and 18 cylindrical ion chamber types. Specific emphasis is placed on the determination of the optimal shift of the chambers’ effective point of measurement (EPOM) and beam quality conversion factors. Methods: The EGSnrc system is used for calculations of the absorbed dose to gas in ion chamber models and the absorbed dose to water as a function of depth in a water phantom on which cobalt‐60 and several electron beam source models are incident. The optimal EPOM shifts of the ion chambers are determined by comparing calculations of R 50 converted from I 50 (calculated using ion chamber simulations in phantom) to R 50 calculated using simulations of the absorbed dose to water vs depth in water. Beam quality conversion factors are determined as the calculated ratio of the absorbed dose to water to the absorbed dose to air in the ion chamber at the reference depth in a cobalt‐60 beam to that in electron beams. Results: For most plane‐parallel chambers, the optimal EPOM shift is inside of the active cavity but different from the shift determined with water‐equivalent scaling of the front window of the chamber. These optimal shifts for plane‐parallel chambers also reduce the scatter of beam quality conversion factors, k Q , as a function of R 50 . The optimal shift of cylindrical chambers is found to be less than the 0.5 r cav recommended by current dosimetry protocols. In most cases, the values of the optimal shift are close to 0.3 r cav . Values of k ecal are calculated and compared to those from the TG‐51 protocol and differences are explained using accurate individual correction factors for a subset of ion chambers investigated. High‐precision fits to beam quality conversion factors normalized to unity in a beam with R 50 = 7.5 cm ( k Q ′ ) are provided. These factors avoid the use of gradient correction factors as used in the TG‐51 protocol although a chamber dependent optimal shift in the EPOM is required when using plane‐parallel chambers while no shift is needed with cylindrical chambers. The sensitivity of these results to parameters used to model the ion chambers is discussed and the uncertainty related to the practical use of these results is evaluated. Conclusions: These results will prove useful as electron beam reference dosimetry protocols are being updated. The analysis of this work indicates that cylindrical ion chambers may be appropriate for use in low‐energy electron beams but measurements are required to characterize their use in these beams.