Study of the effective point of measurement for ion chambers in electron beams by Monte Carlo simulation.

In current dosimetry protocols for electron beams, for plane-parallel chambers, the effective point of measurement is at the front face of the cavity, and, for cylindrical chambers, it is at a point shifted 0.5r upstream from the cavity center. In this study, Monte Carlo simulations are employed to study the issue of effective point of measurement for both plane-parallel chambers and cylindrical thimble chambers in electron beams. It is found that there are two ways of determining the position of the effective point of measurement: One is to match the calculated depth-ionization curve obtained from a modeled chamber to a calculated depth-dose curve; the other is to match the electron fluence spectrum in the chamber cavity to that in the phantom. For plane-parallel chambers, the effective point of measurement determined by the first method is generally not at the front face of the chamber cavity, which is obtained by the second method, but shifted downstream toward the cavity center by an amount that could be larger than one-half a millimeter. This should not be ignored when measuring depth-dose curves in electron beams. For cylindrical chambers, these two methods also give different positions of the effective point of measurement: The first gives a shift of 0.5r, which is in agreement with measurements for high-energy beams and is the same as the value currently used in major dosimetry protocols; the latter gives a shift of 0.8r, which is closer to the value predicted by a theoretical calculation assuming no-scatter conditions. The results also show that the shift of 0.8r is more appropriate if the cylindrical chamber is to be considered as a Spencer-Attix cavity. In electron beams, since the water/air stopping-power ratio changes with depth in a water phantom, the difference of the two shifts (0.3r) will lead to an incorrect evaluation of the water/air stopping-power ratio at the point of measurement, thus resulting in a systematic error in determining the absorbed dose by cylindrical chambers. It is suggested that a shift of 0.8r be used for electron beam calibrations with cylindrical chambers and a shift of 0.4r-0.5r be used for depth-dose measurements.