Monte Carlo and ⁶⁰Co‐based kilovoltage x‐ray dosimetry methods

Abstract

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 60Co absorbed dose‐to‐water calibration coefficient and a beam quality correction factor to account for differences in beam quality between the 60Co 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 60Co‐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.

Characterizing a PTW microDiamond detector in kilovoltage radiation beams

“…A Comet (Flamatt, Switzerland) MXR-320/26 x-ray tube assembly, with a maximum tube voltage of 320kV, is utilized for generating kilovoltage x-ray beams. 11 The target is made of tungsten and has an angle of 20 o . The tube contains a 2-mm-thick berylium window with 1 mm of inherent beryllium filtration.…”

“…The tube contains a 2-mm-thick berylium window with 1 mm of inherent beryllium filtration. 11 The M and L series beams were designed to match the NIST beams based on tube potential, first half value layer (HVL1), second half value layer (HVL2), and homogeneity coefficient (HC) which was calculated from HVL1 and HVL2 measurements. Table I shows the relevant properties of the UWADCL M and L series beams utilized in this study with the known airkerma rates at 100 cm distance from the source.…”

“…The methods of Lawless et al were used to calculate absorbed dose to water rates for the M-series x-ray beams. 11 For all in-phantom irradiations, the detector was placed at 2 cm depth with a source to surface distance (SSD) of 100 cm. The known dose rates at the reference depth for each x-ray beam are tabulated in Table II.…”

“…The EGSnrc model employed in this study was previously validated by Lawless et al using EBT3 radiochromic film and A12 ion chamber measurements. 11 Variance reduction techniques (VRT) such as Russian roulette and photon cross-section enhancement, were utilized to increase the computational efficiency of the simulation. 13 EGSnrc code FLURZnrc was used to quantitate the magnitude of average photon beam energy spectra and average energy spectra of the secondary electrons generated at various depths of the water phantom for UW250-M, UW100-M, and UW50-M x-ray beams.…”

“…The FLURZnrc Monte Carlo model employed in this work was previously validated by Lawless et al using in-air energy spectra measurements and A12 ion chamber measurements in the water phantom. 11 The average energy, E ave , at each depth was calculated by:…”