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
Note from Adam: you probably need an introduction/motivation slide here where you quickly make the point that the use of CBCT for IGRT has become very common and should lay out the purpose of your talk. This should be a quickly presented slide.

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
Note from Adam: This is a suggested slide to explain the goal of the equivalent source.

4. Varian Linac OBI
Measurements for Source and Path Length Characterization

5. HVL
120 kVp, 4.77 mm Al

6. Equivalent Spectrum generation algorithm
Kerma at a point directly downstream of a bowtie filter made of Al and the initial spectrum I0,E and a defined central thickness of t 0 .
K 0 = E=0 kVp I 0,E E exp − μ E,Al t μ tr ρ E,air
Kerma at a point downstream of a bowtie filter made of some thickness, tAl , of aluminum is
K Al = E=0 kVp I 𝟎,E E exp − μ E,Al t Al μ tr ρ E,air
HVL obtained when
K Al K 0 = 1 2 = E=0 kVp I 0,E E exp − μ E,Al HVL μ tr ρ E,air I 0,E E exp − μ E,Al t 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

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. Photopolymerization Gafchromic XR
opaque, active layer, laminated layer
0.1 – 20cGy
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
Dose=ϕ∙ dT ρdx
Since OD∝ϕ, OD ∝ Dose
2-D wiener used to reduce outliers

17. Bowtie Profile w/ Film
The polarization effects and energy dependence effects can be see in the in the correction functions. At the gradient of the bowtie filter, were the energy change is the steepest there is an exaggerated response of the film. But the response does level out as bow tie filter levels off, i.e. energy gradient levels out

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.
NOTE FROM ADAM: This is a lot of words. Try to summarize this a little bit better and then you can explain the slide using words.

20. Equivalent bowtie filter generation algorithm
Kerma at a point directly downstream of a bowtie filter made of Al and the equivalent spectrum IEq,E and a defined central thickness of t 0 .
K 0 = E=0 kVp I Eq,E E exp − μ E,Al t μ tr ρ E,air
Kerma at a point downstream of a bowtie filter made of some thickness, tAl , of aluminum is
K Al = E=0 kVp I Eq,E E exp − μ E,Al t Al μ tr ρ E,air
Normalized Kerma
K Al K 0 = E=0 kVp I Eq,E E exp − μ E,Al t Al μ tr ρ E,air I Eq,E E exp − μ E,Al t μ tr ρ 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
Actual height dimension of bowtie is 2.7 cm

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, 861– 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, 1661– 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, 3057–