Experimental determination of ionization chamber overall correction factor in medium-energy X-ray beams

Abstract

Introduction According to the IAEA TRS 398, in kilovoltage X-ray beams the calibration factor in terms of absorbed dose to water ( N D , w , Q ) for an ionization chamber can be derived from its calibration factor in terms of air kerma ( N K , Q ) if the chamber overall correction factor, p Q , is known. However, literature p Q values are lacking, especially when referring to the reference depth of 2 g/cm 2 . Purpose To determine the p Q factor at 2 g/cm 2 depth using a new absorbed-dose-to-water ( D w ) primary standard based on an in-water-phantom graphite calorimeter recently established for medium-energy X-ray beams. Materials and methods N D , w , Q factors are directly determined against the D w primary standard. Using the same beam qualities, the N K , Q factors are also determined. Values of p Q are obtained by comparing the N D , w , Q factors with the analogous calibration factors derived by N K , Q using the IAEA TRS 398 formalism. Ratios of mean mass-energy absorption coefficients, water to air, are taken from literature. Results For a Farmer type ionization chamber, the p Q factor was respectively 1.002 and 1.012 for the 180 kV and the 250 kV medium-energy quality of the CCRI series, with a relative combined standard uncertainty of 2%. This figure is going to be reduced to around 1% with the ongoing improvements on the new D w primary standard. Conclusion The new D w primary standard based on graphite calorimetry, although affected by a larger uncertainty compared to primary standards based on water calorimetry, allows independent measurements of p Q factors for ionization chambers currently used in radiotherapy dosimetry.