Key comparison BIPM.RI(I)-K9 of the absorbed dose to water standards of the LNE-LNHB, France, and the BIPM in medium-energy x-rays
A key comparison has been made between the absorbed dose to water standards of the LNE-LNHB, France, and the BIPM in the medium-energy x-ray range. The results show the standards to be in general agreement at the level of the expanded (k = 2) uncertainty of the comparison of 2 parts in 102. The results are presented in terms of degrees of equivalence for entry in the BIPM key comparison database. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCRI, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).
Monte Carlo and 60Co‐based kilovoltage x‐ray dosimetry methods
PURPOSE
This work seeks to investigate new methods to determine the absorbed dose to water from kilovoltage x rays. Current methods are based on measurements in air and rely on correction factors in order to account for differences between the photon spectrum in air and at depth in phantom, between the photon spectra of the calibration beam and the beam of interest, or in the radiation absorption properties of air and water. This work aims to determine the absorbed dose to water in the NIST-matched x-ray beams at the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL). This will facilitate the use of detectors in terms of dose to water, which will allow for a simpler determination of dose to water in clinical kilovoltage x-ray beams.


MATERIALS AND METHODS
A model of the moderately filtered x-ray beams at the UWADCL was created using the BEAMnrc user code of the EGSnrc Monte Carlo code system. This model was validated against measurements and the dose to water per unit air kerma was calculated in a custom built water tank. Using this value and the highly precise measurement of the air kerma made by the UWADCL, the dose to water was determined in the water tank for the x-ray beams of interest. The dose to water was also determined using the formalism defined in the report of AAPM Task Group 61 and using a method that makes use of a 60 Co absorbed dose-to-water calibration coefficient and a beam quality correction factor to account for differences in beam quality between the 60 Co calibration and kilovoltage x-ray beam of interest. The dose to water values as determined by these different methods was then compared.


RESULTS
The BEAMnrc models used in this work produced simulations of transverse and depth dose profiles that agreed with measurements with a 2%/2 mm criteria gamma test. The dose to water as determined from the different methods used here agreed within 3.5% at the surface of the water tank and agreed within 1.8% at a depth of 2 cm in phantom. The dose-to-water values all agreed within the associated uncertainties of the methods used in this work. Both the Monte Carlo-based method and the 60 Co-based method had a lower uncertainty than the TG-61 methodology for all of the x-ray beams used in this work.


CONCLUSION
Two new dose determination methods were used to determine the dose to water in the NIST-matched x-ray beams at the UWADCL and they showed good agreement with previously established techniques. Due to the improved Monte Carlo calculation techniques used in this work, both of the methods have lower uncertainties compared to TG-61. The methods presented in this work compare favorably with calorimetry-based standards established at other institutions.
Garnitures mécaniques d'étanchéité
Keywords: etancheite ; mecanique Reference Record created on 2005-11-18, modified on 2016-08-08