2010
Replacement correction factors for planeparallel ion chambers in electron beams
Publication
Publication
Medical Physics , Volume 37  Issue 2 p. 461 465
Purpose: Planeparallel chambers are recommended by dosimetry protocols for measurements in (especially lowenergy) electron beams. In dosimetry protocols, the replacement correction factor Prepl is assumed unity for "wellguarded" planeparallel chambers in electron beams when the front face of the cavity is the effective point of measurement. There is experimental evidence that ion chambers which are not wellguarded (e.g., Markus) have nonunity Prepl values. Monte Carlo simulations are employed in this study to investigate the replacement correction factors for planeparallel chambers in electron beams. Methods: Using previously established Monte Carlo calculation methods, the values of Prepl are calculated with high statistical precision for the cavities of a variety of planeparallel chambers in a water phantom irradiated by various electron beams. The dependences of the values of Prepl on the beam quality, phantom depth, as well as the guard ring width are studied. Results: In the dose falloff region for lowenergy beams, the Prepl values are very sensitive to depth. It is found that this is mainly due to the gradient effect, which originates from the fact that the effective point of measurement for many planeparallel chambers should not be at the front face of the cavity but rather shifted toward the center of the cavity by a fraction of a millimeter. Using the front face of the cavity as the effective point of measurement, the calculated values of Prepl at dref are not unity for some wellguarded planeparallel chambers. The calculated Prepl values for the Roos chamber are close to 1 for all electron beams. The calculation results for the Markus chamber are in good agreement with the measured values. Conclusions: The appropriate selection of the effective point of measurement for planeparallel chambers in electron beams is an important issue. If the effective point of measurement is correctly accounted for, the Prepl values would be almost independent of depth. Both the guard ring width and the ratio of the collecting volume diameter to the cavity thickness can influence the values of Prepl. For a diameter to thickness ratio of 5 (e.g., NACP02 chamber), the guard width has to be 6 mm for the chamber to be considered as wellguarded, i.e., have a Prepl value of 1.00.
Additional Metadata  

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doi.org/10.1118/1.3276735  
Medical Physics  
Organisation  Department of Physics 
Wang, L.L.W. (Lilie L. W.), & Rogers, D.W.O. (2010). Replacement correction factors for planeparallel ion chambers in electron beams. Medical Physics, 37(2), 461–465. doi:10.1118/1.3276735
