Absorbed‐dose beam quality conversion factors for cylindrical chambers in high energy photon beams

Abstract

Recent working groups of the AAPM [Almond et al., Med. Phys. 26, 1847 (1999)] and the IAEA (Andreo et al., Draft V.7 of "An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water," IAEA, 2000) have described guidelines to base reference dosimetry of high energy photon beams on absorbed dose to water standards. In these protocols use is made of the absorbed-dose beam quality conversion factor, kQ which scales an absorbed-dose calibration factor at the reference quality 60Co to a quality Q, and which is calculated based on state-of-the-art ion chamber theory and data. In this paper we present the measurement and analysis of beam quality conversion factors kQ for cylindrical chambers in high-energy photon beams. At least three chambers of six different types were calibrated against the Canadian primary standard for absorbed dose based on a sealed water calorimeter at 60Co [TPR10(20)=0.572, %dd(10)x=58.4], 10 MV [TPR10(20)=0.682, %dd(10)x=69.6), 20 MV (TPR10(20)=0.758, %dd(10)x= 80.5] and 30 MV [TPR10(20) = 0.794, %dd(10)x= 88.4]. The uncertainty on the calorimetric determination of kQ for a single chamber is typically 0.36% and the overall 1sigma uncertainty on a set of chambers of the same type is typically 0.45%. The maximum deviation between a measured kQ and the TG-51 protocol value is 0.8%. The overall rms deviation between measurement and the TG-51 values, based on 20 chambers at the three energies, is 0.41%. When the effect of a 1 mm PMMA waterproofing sleeve is taken into account in the calculations, the maximum deviation is 1.1% and the overall rms deviation between measurement and calculation 0.48%. When the beam is specified using TPR10(20), and measurements are compared with kQ values calculated using the version of TG-21 with corrected formalism and data, differences are up to 1.6% when no sleeve corrections are taken into account. For the NE2571 and the NE2611A chamber types, for which the most literature data are available, using %dd(10)x, all published data show a spread of 0.4% and 0.6%, respectively, over the entire measurement range, compared to spreads of up to 1.1% for both chambers when the kQ values are expressed as a function of TPR10(20). For the PR06-C chamber no clear preference of beam quality specifier could be identified. When comparing the differences of our kQ measurements and calculations with an analysis in terms of air-kerma protocols with the same underlying calculations but expressed in terms of a compound conversion factor CQ, we observe that a system making use of absorbed-dose calibrations and calculated kQ values, is more accurate than a system based on air-kerma calibrations in combination with calculated CQ (rms deviation of 0.48% versus 0.67%, respectively).

Study of the uncertainty in the determination of the absorbed dose to water during external beam radiotherapy calibration

“…As has been reported, the polarity effect for photon beams and cylindrical chambers does not significantly depend on energy, (18) and so the same value can be assumed both for ${}^{60}\mathrm{normalC}\mathrm{o}$ beam quality and our photon beam qualities $(<0.1\%)$. For the Markus chamber, values below 0.2% have been reported (19) .…”

Measured and Monte Carlo calculated kQ factors: Accuracy and comparison

“…1,4,27,28 This provides an excellent method of examining the reliability of Monte Carlo calculated k Q factors. Our previous study 2 allows an investigation of the sleeve effect in two ways.…”

“…This work determines the reliability of the k Q factors obtained in both ways and provides a detailed comparison of measured and calculated k Q factors. Measured k Q factors have been determined in previous publications using water calorimetry [3][4][5][6][7][8] or Fricke dosimetry. [9][10][11][12] However, aside from the work of McEwen, 1 experimental determination of k Q has been limited to only a few chamber types.…”

Uncertainty Assessment: Relative versus Absolute Point Dose Measurement for Patient Specific Quality Assurance in EBRT

“…19) The variation in the charge reading depends on the following factors, reproducibility, display resolution, electrometer linearity, correct nulling of the electrometer and long term stability of the chamber. The uncertainty in the charge reading for both chambers (reference and field chamber) is estimated to 0.3%.…”

“…16) The uncer tainty associated with the polarity factor was estimated by accounting the machine reproducibility with the external monitor chamber.…”