CBCT Equivalent Source Generation Using HVL and Beam Profile Measurements. Johnny Little PSM - Medical Physics Graduate Student University of Arizona

Transcription

1 CBCT Equivalent Source Generation Using HVL and Beam Profile Measurements. Johnny Little PSM - Medical Physics Graduate Student University of Arizona

2 Introduction CBCT has become a routine procedure for image guided radiation therapy in many clinics AAPM TG 75 suggests that: The introduction of more intensive imaging procedures for IGRT now obligates the clinician to evaluate therapeutic and imaging doses in a more balanced manner The purpose of this study is to develop a method of modeling the Varian OBI CBCT source for use in accurate Monte Carlo dosimetry simulations

3 Overview of method Goal: model the energy spectrum and bowtie filtration using empirical and semi-empirical methods. Half Value Layer (HVL) will be used to characterize the energy spectrum. 2D dose profile measurements with a farmer chamber and Gafchromic film used to characterize the bowtie filter. Measurements will be used to generate an equivalent source Equivalent energy spectrum and equivalent bowtie filter 120 kvp was used for this work

4 Varian Linac OBI kv Detector kv Source Figure 1. Varian Clinac OBI

5 HVL 120 kvp, 4.77 mm Al Varian OBI HVL at 120 kvp Exposure (pc) y = e x R² = mm Al

6 Equivalent Spectrum generation algorithm Kerma at a point directly downstream of a bowtie filter made of Al and initial spectrum I 0,E and a defined central thickness of t 0. kvp K 0 = I 0,E E exp µ E,Al t 0 E=0 µ tr ρ E,air Kerma at a point downstream of a bowtie filter made of some thickness, t Al, of aluminum is HVL obtained when kvp K Al = I 0,E E exp µ E,Al t Al E=0 µ tr ρ E,air K Al = 1 I 0,EE exp µ E,AlHVL = kvp K 0 2 E=0 I 0,E E exp µ E,Al t 0 µ tr ρ E,air µ tr ρ E,air

7 Generating equivalent spectra soft tungsten spectrum is iteratively hardened by an increasingly thick slab of hardening material The HVL of each hardened spectrum is calculated Equivalent spectrum is defined as the hardened spectrum with a calculated HVL equal to the measured HVL

8 Equivalent Spectrum Varian OBI Eqivalent Full Bowtie Aluminum beam hardner Original TASMIP Optimized Spectrum mean average energy = kev HVL1 = 4.731

9 Ionization Chamber measurements Scandatronix/Wellhofer CC13 ionization chamber in air Sun Nuclear 1-D Scanner Exposure profile was sampled every 1 cm across the bowtie filter Source to measurement plane distance of 38 cm 120kVp, 100 mas, 20 x 20 cm field Each measurement was normalized to the exposure at center

10 Bowtie Profile Measurements

11 Bowtie Profile Ion Chamber

12 Cubic Spline Interpolation Interpolation between data points with piecewise cubic polynomials Cubic spline has the following form over interval [ i, i+1 ]: x t = a x t 3 + b x t 2 + c x t + d x y t = a y t 3 + b y t 2 + c y t + d y Coefficients different for each interval Finds values at intermediate points of a F(x,y) that underlies data

13 Bowtie Profile w/ Spline Interpolated Ion Chamber Data

14 Ion Chamber vs Interpolated Ion Chamber

15 Gafchromic XR opaque, active layer, laminated layer cGy Photopolymerization One photochemical reaction can cause thousands of molecular monomer reactions to form a polymer At least ten hours to fully polymerize

16 Film Scans Films cut to fit dimensions of scan bed to standardize orientation Measurements taken with 20 x 20cm field Red channel extracted OD = ln PV bkg PV xposed dt Dose = ϕ ρdx Since OD ϕ, OD Dose 2-D wiener used to reduce outliers

17 Bowtie Profile w/ Film

18 Film vs Ion Chamber

19 Equivalent bowtie filter generation Want dimensions of an equivalent bowtie that attenuates the equivalent spectrum in the same manner that the actual bowtie filter attenuates the actual cone beam spectrum Information needed: (a) the equivalent spectrum, (b) the beam profile measurements.

20 Equivalent bowtie filter generation algorithm Kerma at a point directly downstream of a bowtie filter made of Al and equivalent spectrum I Eq,E and a defined central thickness of t 0. kvp K 0 = I Eq,E E exp µ E,Al t 0 E=0 Kerma at a point downstream of a bowtie filter made of some thickness, t Al, of aluminum is Normalized Kerma kvp K Al = I Eq,E E exp µ E,Al t Al E=0 µ tr ρ µ tr ρ E,air E,air K Al K 0 = kvp E=0 I Eq,E E exp µ E,Al t Al I Eq,E E exp µ E,Al t 0 µ tr ρ µ tr ρ E,air E,air

21 Equivalent bowtie filter generation algorithm (1) Using bowtie profile, the measured exposures are normalized. (2) The equivalent spectrum will be transmitted through the defined central ray thickness, the Kerma, K 0, will be calculated. (3) The equivalent spectrum will then be transmitted through a very thin, uniform sheet of aluminum and the subsequent Kerma, K Al, in air will be calculated. (4) The ratio of the K Al from (3) to the K 0 from (2) will be calculated. (5) Steps (3)-(4) will be repeated iteratively, increasing the Al until the difference between the normalized bowtie profile and normalized equivalent bowtie are minimized. This minimization results in a Al thickness that is unique to the beam profile measurement data location from (1).

22 Equivalent FBT Film Beam Profile

23 Equivalent FBT Spline Interpolated Beam Profile

24 Conclusion A method has been described that produces an energy spectrum and bowtie filter model Will be used for Monte Carlo dosimetry simulations Method uses actual dose measurements and interpolated dose measurents on CBCT of interest Future work will be performed to evaluate accuracy of Monte Carlo simulations that use equivalent source models

25 References A. C. Turner and D. Zhang, A method to generate equivalent energy spectra and filtration models based on measurement for multidetector CT Monte Carlo dosimetry simulations, Med. Phys. 36, L. C. Ku, IGRT with the Varian On-Board Imager P. Alaei, Review of the Doses from Cone Beam CT and Their Inclusion in the Treatment Planning J. M. Boone, Equivalent spectra as a measure of beam quality, Med.Phys. 136, J. M. Boone and J. A. Seibert, An accurate method for computer generating tungsten anode x-ray spectra from 30 to 140 kv, Med. Phys. 2411, J. H. Siewerdsen, A. M. Waese, D. J. Moseley, S. Richard, and D. A. Jaffray, Spektr: A computational tool for x-ray spectral analysis and imaging system optimization, Med. Phys. 3111,