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Comparing calibration methods of electron beams using plane‐parallel chambers with absorbed‐dose to water based protocols
Author(s) -
Stewart K. J.,
Seuntjens J. P.
Publication year - 2002
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 - 10.1118/1.1449876
Subject(s) - absorbed dose , ionization chamber , calibration , laser beam quality , monte carlo method , dosimetry , electron , beam (structure) , physics , ionization , cathode ray , plane (geometry) , computational physics , materials science , optics , nuclear physics , atomic physics , nuclear medicine , radiation , mathematics , ion , medicine , geometry , statistics , laser , quantum mechanics , laser beams
Recent absorbed‐dose‐based protocols allow for two methods of calibrating electron beams using plane‐parallel chambers, one using the N D , wCofor a plane‐parallel chamber, and the other relying on cross‐calibration of the plane‐parallel chamber in a high‐energy electron beam against a cylindrical chamber which has an N D , wCofactor. The second method is recommended as it avoids problems associated with the P wallcorrection factors at60 Co for plane‐parallel chambers which are used in the determination of the beam quality conversion factors. In this article we investigate the consistency of these two methods for the PTW Roos, Scanditronics NACP02, and PTW Markus chambers. We processed our data using both the AAPM TG‐51 and the IAEA TRS‐398 protocols. Wall correction factors in60 Co beams and absorbed‐dose beam quality conversion factors for 20 MeV electrons were derived for these chambers by cross‐calibration against a cylindrical ionization chamber. Systematic differences of up to 1.6% were found between our values of P walland those from the Monte Carlo calculations underlying AAPM TG‐51, and up to 0.6% when comparing with the IAEA TRS‐398 protocol. The differences in P walltranslate directly into differences in the beam quality conversion factors in the respective protocols. The relatively large spread in the experimental data of P wall , and consequently the absorbed‐dose beam quality conversion factor, confirms the importance of the cross‐calibration technique when using plane‐parallel chambers for calibrating clinical electron beams. We confirmed that for well‐guarded plane‐parallel chambers, the fluence perturbation correction factor at d maxis not significantly different from the value at d ref . For the PTW Markus chamber the variation in the latter factor is consistent with published fits relating it to average energy at depth.