Air kerma rate measurements from a miniature x-ray source using free-air ionization chambers

Transcription

1 Air kerma rate measurements from a miniature x-ray source using free- ionization chambers Stephen D. Davis 1, John A. Micka 1, Larry A. DeWerd 1, and Thomas W. Rusch 2 1 University of Wisconsin, Madison, WI 2 Xoft, Inc., Fremont, CA Young Investigators Symposium 48 th Annual Meeting of the American Association of Physicists in Medicine July 30, 2006

2 Xoft Axxent TM miniature x-ray source 2 mm 2 mm Anode 10 mm Cathode Photos courtesy of Xoft, Inc. 2/16

3 Xoft Axxent TM miniature x-ray source X-Ray Probe Tip Detail Photos courtesy of Xoft, Inc. 3/16

4 TG-43U1 protocol D& ( r ) ( r, θ ) ( r, θ ) G P θ = SK Λ g, G P 0 0, P () r F( r θ ) Miniature x-ray sources will be characterized using a modification of the TG-43U1 protocol NIST-traceable calibration will be through the kerma strength, equivalent to traditional brachytherapy sources 4/16

5 Wide-Angle Free-Air Chamber (WAFAC) Diagram from Seltzer et al. (2003) 5/16

6 Attix free- chamber (FAC) Beam defining aperture Signal Fixed center position Guard tube Sensitive collecting volume Collecting electrode Variable position plate Exit window Pushrod X-ray beam Entrance window High voltage bias Stepping motor controlled slide assembly #1 UWMRRC Stepping motor controlled slide assembly #2 Diagram from Coletti et al. (1995) 6/16

7 Air kerma measurements with Attix FAC K & di dl ρ K& = di dl 1 ρ A Air kerma rate at the free chamber aperture Change in current per change in plate separation Density of the ambient 0 W e A 0 W e g ki 1 1 g i k i Defining aperture area Energy required to liberate 1 C of charge in dry (33.97 J/C) Fraction of energy lost to radiative events Correction factors 7/16

8 Ritz free- chamber (FAC) Diagram from NIST Special Publication (2001) 8/16

9 Air kerma measurements with Ritz FAC K & I L ρ K& = I L 1 ρ A Air kerma rate at the free chamber aperture Measured current Ionization chamber collecting length Density of the ambient 0 W e A 0 W e g ki 1 1 g i k i Defining aperture area Energy required to liberate 1 C of charge in dry (33.97 J/C) Fraction of energy lost to radiative events Correction factors 9/16

10 Free- chamber correction factors Air attenuation from the beam-defining aperture to the center of the collecting volume Ionization created by scattered photons Energetic electrons reaching the chamber walls or collecting electrode Recombination of ions in the volume For S K determinations, the kerma measurements must be corrected to in vacuo 10/16

11 HDR 1000 Plus well chamber Sources will be measured with the well chamber prior to treatment to measure the -kerma strength (S K ) Special aluminum insert designed by Standard Imaging 11/16

12 Well chamber calibration K & NK I ( d ) N K = K& ( d ) I Well chamber calibration coefficient (in Gy/C) Air kerma rate in at 100 cm (in Gy/s) Well chamber current (in C/s) 12/16

13 HDR 1000 Plus well chamber calibration coefficients from FAC measurements (50 kv) Free- chamber N K ( 10 2 Gy/C) UW Attix (5 sources) ± 2.2% NIST Attix (3 sources) ± 5.6% NIST Ritz (4 sources) ± 5.3% Average across all 12 measurements ± 5.3% 13/16

14 Possible explanations for variability Azimuthal asymmetry in output from x-ray sources Sources were not rotated during FAC measurements Source alignment at NIST Very small differences in spectra in Air attenuation measurements were fly consistent Different measurement geometry in well chamber Very sensitive to any differences in the 3-D distribution of source output since it basically measures a large solid angle 14/16

15 Conclusions Air kerma rates at 100 cm in have been measured using free- chambers at UW and NIST but further work will be necessary to develop methods suitable for traceability to national measurement standards Conversion to -kerma strength in vacuo will require measurements and Monte Carlo simulations to determine accurate photon spectra 15/16

16 Acknowledgements Xoft, Inc. UWMRRC staff and students UW ADCL customers 16/16