Calculating Polarity Correction Factor

December 29, 2009 · Written by MDPhysics.com · mdphysicsblog@gmail.com

AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams contains many details to which we should pay close attention when performing necessary measurements for the calibration of high-energy beams. One such measurement is calculating the polarity correction factor (Ppol) for ionization chambers used in electron beam dosimetry. Since polarity effects vary with beam quality and other conditions (such as cable position), one must correct for these effects by making measurements each time clinical reference dosimetry is performed. To correct an ion chamber’s raw reading for polarity effects it is necessary to take readings with both polarities applied and tabulate Ppol by the following equation:

Ppol = Abs [(M+raw - M-raw)/2Mraw]
M+raw is the reading when positive charge is collected and M-raw is the reading when negative charge is collected

There may be some ambiguity when first encountering this equation as to which reading one should use for the Mraw value in the denominator. There are quite a few possibilities, but only one correct answer. So, which reading do you use for Mraw in the denominator?

1- The average of M+raw and M-raw
2- The larger value of M+raw and M-raw
3- The smaller value of M+raw and M-raw
4- Either M+raw or M-raw can be selected
5- None of the above

Click here for the answer

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