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Replacement correction factors for cylindrical ion chambers in electron beams
Author(s) -
Wang L. L. W.,
Rogers D. W. O.
Publication year - 2009
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 - 10.1118/1.3213094
Subject(s) - monte carlo method , dosimetry , fluence , electron , physics , imaging phantom , calibration , atomic physics , computational physics , irradiation , optics , nuclear medicine , nuclear physics , mathematics , statistics , medicine , quantum mechanics
Purpose: In the TG‐21 dosimetry protocol, for cylindrical chambers in electron beams the replacement correction factor P repl(or the productp disp cavin the IAEA's notation), was conceptually separated into two components: the gradient correction( P gr)accounting for the effective point of measurement and the fluence correction( P fl)dealing with the change in the electron fluence spectrum. At the depth of maximum dose( d max) ,P gris taken as 1. There are experimental data available atd maxfor the values ofP fl(orP repl). In the TG‐51 dosimetry protocol, the calibration is at the reference depthd ref = 0.6 R 50 − 0.1( cm )whereP gris required for cylindrical chambers andP flis unknown and so the measured values atd maxare used with the corresponding mean electron energy atd ref. Monte Carlo simulations are employed in this study to investigate the replacement correction factors for cylindrical chambers in electron beams. Methods: Using previously established Monte Carlo calculation methods, the values of P replandP flare calculated with high statistical precision( < 0.1 % )for cylindrical cavities of a variety of diameters and lengths in a water phantom irradiated by various electron beams. The values ofP gras defined in the TG‐51 dosimetry protocol are also calculated. Results: The calculated values of the fluence correction factors P flare in good agreement with the measured values when the wall correction factors are taken into account for the plane‐parallel chambers used in the measurements. An empirical formula forP flfor cylindrical chambers atd refin electron beams is derived as a function of the chamber radius and the beam quality specifierR 50. Conclusions: The mean electron energy at depth is a good beam quality specifier for P fl. Thus TG‐51's adoption ofP flatd maxwith the same mean electron energy for use atd refis proven to be accurate. The values ofP grfor a Farmer‐type chamber as defined in the TG‐51 dosimetry protocol may be wrong by 0.3% for high‐energy electron beams and by more than 1% for low‐energy electron beams.