Data for the dosimetry of low- and medium-energy kV x rays

Abstract

Following the publication of the ICRU Report 90 (2016) on key data for measurement standards in radiation dosimetry, where ionometric air-kerma standards for kilovoltage (kV) x-ray beams are estimated to change by up to about 0.5%, an update of the backscatter factors and water/air ratios of mass energy-absorption coefficients in kV dosimetry protocols was deemed necessary for consistency through the entire dosimetry chain. In addition, numerical methods and Monte Carlo (MC) systems that did not exist at the time when air-kerma protocols were developed, are currently available.Calculations of the chamber-independent quantities required for the dosimetry of low- and medium-energy kV x rays were carried out using a consistent set of key data throughout the complete process. The quantities were based on MC calculations of a database for a dense grid of monoenergetic photons for different beam diameters and source-to-surface distances, followed by an averaging procedure to compute water/air energy-absorption coefficient ratios and backscatter factors for 342 experimental and calculated kV spectra. It was found that for a given HVL and field size the variation of backscatter factors for different kVs can be up to about 5%, a trend confirmed with independent calculations that shows the limitation of using only the HVL for the beam quality specification of kV x rays. Extensive tables as a function of beam quality in terms of kV and HVL were developed for configurations that might be encountered in clinical practice; the data are also available in the form of a GUI web app at http://52.233.195.208.Results were compared with data used at PTB for deriving low-energy ion chamber calibration coefficients, finding agreement within about 0.5%, and with independent full MC kerma calculations that agreed within better than about 1%. Compared with the data in the AAPM TG-61 protocol (Ma et al 2001 Med. Phys. 28 868–93) there was in general good agreement for the ratios of mass energy-absorption coefficients, although differences of up to 1.5% resulted when both kV and HVL were taken into account; more significant discrepancies, within about 2%–6%, were obtained for backscatter factors, the present values being generally higher.