CBCT Equivalent Source Generation Using HVL and Beam

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. • 2 D 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 k. Vp was used for this work

Varian Linac OBI

HVL 120 k. Vp, 4. 77 mm Al Varian OBI HVL at 120 k. Vp 1500 1400 R 2 = 0. 9952 1300 1200 1100 1000 Exposure (p. C) • 900 800 700 600 500 400 300 200 100 0 0 1 2 mm Al 3 4 5

Equivalent Spectrum generation algorithm •

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

Ionization Chamber measurements • Scandatronix/Wellhofer CC 13 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 – 120 k. Vp, 100 m. As, 20 x 20 cm field • Each measurement was normalized to the exposure at center 9

Bowtie Profile Measurements

Bowtie Profile Ion Chamber

Cubic Spline Interpolation •

Bowtie Profile w/ Spline Interpolated Ion Chamber Data

Ion Chamber vs Interpolated Ion Chamber

Photopolymerization Gafchromic XR opaque, active layer, laminated layer 0. 1 – 20 c. Gy One photochemical reaction cause thousands of molecular monomer reactions to form a polymer At least ten hours to fully polymerize

Film Scans

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 •

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

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, 2154 -2164 2009. 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– 868 1986. J. M. Boone and J. A. Seibert, “An accurate method for computer generating tungsten anode x-ray spectra from 30 to 140 k. V, ” Med. Phys. 2411, 1661– 1670 1997. 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– 3067 2004.