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

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

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

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

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

HVL 120 kvp, 4.77 mm Al Varian OBI HVL at 120 kvp Exposure (pc) 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 y = 1479.7e -0.148x R² = 0.9952 0 1 2 3 4 5 mm Al

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

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

Equivalent Spectrum 0.035 0.03 Varian OBI Eqivalent Full Bowtie Aluminum beam hardner Original TASMIP Optimized Spectrum 0.025 0.02 0.015 0.01 0.005 0 0 20 40 60 80 100 120 140 mean average energy = 51.9502 kev HVL1 = 4.731

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

Bowtie Profile Measurements

Bowtie Profile Ion Chamber

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

Bowtie Profile w/ Spline Interpolated Ion Chamber Data

Ion Chamber vs Interpolated Ion Chamber

Gafchromic XR opaque, active layer, laminated layer 0.1 20cGy Photopolymerization One photochemical reaction can cause thousands of molecular monomer reactions to form a polymer At least ten hours to fully polymerize

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

Bowtie Profile w/ Film

Film vs Ion Chamber

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.

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

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).

Equivalent FBT Film Beam Profile

Equivalent FBT Spline Interpolated Beam Profile

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

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