Monte Carlo modeling of an electronic brachytherapy source using MCNP5 and EGSnrc

Transcription

1 Monte Carlo modeling of an electronic brachytherapy source using MCNP5 and EGSnrc Stephen D. Davis and Larry A. DeWerd University of Wisconsin, Madison, WI 13 th UK Monte Carlo User Group meeting March 29, 2007

2 Xoft Axxent TM electronic brachytherapy source 2 mm 2 mm Anode 10 mm Cathode Photos courtesy of Xoft, Inc. 2/26

3 Xoft Axxent TM electronic brachytherapy source X-Ray Probe Tip Detail Photos courtesy of Xoft, Inc. 3/26

4 Motivation Brachytherapy uses air-kerma strength, S K, as the measurement of source strength Air kerma rates can be measured using free-air ionization chambers, but S K is defined in vacuo Correction is required for attenuation in air from the source to the point of measurement (100 cm in our case) Air cross sections change rapidly at photon energies < 20 kev Photon spectrum needs to be accurately known for a low uncertainty on the air attenuation correction 4/26

5 Photon spectra Measurements with high purity germanium spectrometer Requires correction for the energy response of the detector Monte Carlo simulations Assumes Monte Carlo code has the low energy physics included to model x-ray spectra accurately 5/26

6 Photon spectra at 178 cm in air 120 Counts (arb. units) NIST HPGe MCNP Photon energy (kev) 6/26

7 Photon spectra at 178 cm in air NIST HPGe MCNP5 Counts (arb. units) W-L β W-L γ 5 W-L α Photon energy (kev) 7/26

8 Photon spectra at 0 cm in air Counts (arb. units) NIST HPGe MCNP Photon energy (kev) 8/26

9 EGSnrc January 2005 release included model by Iwan Kawrakow for electron impact ionization for K- and L- shells with binding energies above 1 kev October 2005 release included C++ class library with general purpose geometry modeling and user code cavity.cpp February 2007 release allowed for scoring of photon spectra in a circular plane using cavity.cpp 9/26

10 EGSnrc vs. MCNP5 Two codes have differences in: Electron impact ionization Binding effects and Doppler broadening for Compton interactions Atomic relaxation Electron transport Variance reduction techniques Scoring options 10/26

11 Doppler broadening Broomstick problem from MCNP5 LANL document 100 kev photons incident on 10 cm graphite rod with 10-6 cm radius 11/26

12 No Doppler broadening Fluence per unit incident fluence EGSnrc MCNP Photon energy (kev) 12/26

13 With Doppler broadening Fluence per unit incident fluence EGSnrc MCNP Photon energy (kev) 13/26

14 Atomic relaxation 30 kev photons incident on 10-5 cm thick yttrium slab 14/26

15 Atomic relaxation (cont.) 10 2 Fluence per unit incident fluence EGSnrc MCNP Photon energy (kev) 15/26

16 Atomic relaxation (cont.) Fluence per unit incident fluence Y-K α EGSnrc MCNP5 (Mode p,e) Y-K β Photon energy (kev) 16/26

17 Atomic relaxation (cont.) Fluence per unit incident fluence Y-K α EGSnrc MCNP5 (Mode p) Y-K β Photon energy (kev) 17/26

18 cavity.cpp model of Axxent TM source 18/26

19 Dose rates at 1 cm in water MC code P (r 0,θ 0 ) [cgy h -1 µa -1 ] MCNP5 345 ± 0.1% EGSnrc 346 ± 2.0% EGSnrc/MCNP ± 2.0% 19/26

20 Air kerma rates at 100 cm in air MC code P (r 0,θ 0 ) [cgy h -1 µa -1 ] MCNP ± 0.1% EGSnrc ± 3.1% EGSnrc/MCNP ± 3.1% 20/26

21 Calculated photon spectra at 1 cm in vacuum 9x10-4 8x10-4 Fluence per unit incident fluence 7x10-4 6x10-4 5x10-4 4x10-4 3x10-4 2x10-4 1x10-4 EGSnrc MCNP5 3.2% higher air kerma for EGSnrc Photon energy (kev) 21/26

22 Calculated photon spectra at 1 cm in vacuum 1.2x10-4 Fluence per unit incident fluence 1.0x x x x x10-5 EGSnrc MCNP5 <L>-<M> L III -<M> L II -<M> L II -<N> L I -<M> + L III -<N> EII contributes 2.6% of air kerma L I -<N> Photon energy (kev) 22/26

23 Calculated photon spectra at 1 cm in vacuum 9x10-4 Fluence per unit incident fluence 8x10-4 7x10-4 6x10-4 5x10-4 4x10-4 3x10-4 2x10-4 1x10-4 Y-K α EGSnrc MCNP5 Differences in Y fluorescent peaks led to 2.3% difference in air kerma Y-K β Photon energy (kev) 23/26

24 Calculated photon spectra at 1 cm in vacuum 2x10-4 Fluence per unit incident fluence 1x10-4 5x10-5 Removing EII and Y fluorescence differences, EGSnrc produces 2.4% lower air kerma than MCNP5 EGSnrc MCNP Photon energy (kev) 24/26

25 Conclusions Both codes produce similar water dose rates, air kerma rates, and photon spectra, and the small differences can be explained by different treatment of the low energy physics Further work will be necessary to compare these results to measurements, and to establish the required air attenuation corrections with low uncertainties 25/26

26 Acknowledgements Xoft, Inc. Iwan Kawrakow NIST UWMRRC staff and students UW ADCL customers Grid Laboratory of Wisconsin (GLOW) 26/26