2009
Replacement correction factors for cylindrical ion chambers in electron beams
Publication
Publication
Medical Physics , Volume 36  Issue 10 p. 4600 4608
Purpose: In the TG21 dosimetry protocol, for cylindrical chambers in electron beams the replacement correction factor Prepl (or the product pdis pcav in 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 (Pfl) dealing with the change in the electron fluence spectrum. At the depth of maximum dose (dmax), Pgr is taken as 1. There are experimental data available at dmax for the values of Pfl (or Prepl). In the TG51 dosimetry protocol, the calibration is at the reference depth dref =0.6 R50 0.1 (cm) where Pgr is required for cylindrical chambers and Pfl is unknown and so the measured values at dmax are used with the corresponding mean electron energy at dref. 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 Prepl and Pfl are 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 of Pgr as defined in the TG51 dosimetry protocol are also calculated. Results: The calculated values of the fluence correction factors Pfl are in good agreement with the measured values when the wall correction factors are taken into account for the planeparallel chambers used in the measurements. An empirical formula for Pfl for cylindrical chambers at dref in electron beams is derived as a function of the chamber radius and the beam quality specifier R50. Conclusions: The mean electron energy at depth is a good beam quality specifier for Pfl. Thus TG51's adoption of Pfl at dmax with the same mean electron energy for use at dref is proven to be accurate. The values of Pgr for a Farmertype chamber as defined in the TG51 dosimetry protocol may be wrong by 0.3% for highenergy electron beams and by more than 1% for lowenergy electron beams.
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doi.org/10.1118/1.3213094  
Medical Physics  
Organisation  Department of Physics 
Wang, L.L.W., & Rogers, D.W.O. (2009). Replacement correction factors for cylindrical ion chambers in electron beams. Medical Physics, 36(10), 4600–4608. doi:10.1118/1.3213094
