Skip to Main Content
 

Global Search Box

 
 
 
 

Files

File List


FINAL-DL.pdf (5.49 MB)

ETD Abstract Container

Abstract Header

Acquisition and Analysis of Megavoltage Linac Beam Transmission Data for Direct Verification of Photon Spectra Models in a Treatment Planning System

Leheta, Dimitra
http://rave.ohiolink.edu/etdc/view?acc_num=mco1416566156

Abstract Details

2014, Master of Science (MS), University of Toledo, Biomedical Sciences (Medical Physics: Diagnostic Radiology).
The dose calculation in Philips Pinnacle Treatment Planning System (TPS) uses collapsed cone convolution algorithm, which relies on beam spectrum information for calculation of scatter contribution. Typically beam spectra are derived from a set of measurements collected during commissioning of a linear accelerator. The purpose of this project is to measure beam spectra independently using transmission methodology and compare with those modeled by the Pinnacle TPS. Three photon beam energies were measured and analyzed, 6MV, 6MV flattening-filter-free (FFF), and 10MV. Transmission measurements were conducted for two materials, having high and low atomic numbers (Z), using a standard Farmer ionization chamber fitted with high-Z and low-Z buildup caps. The combination of two materials and two caps with different Z served to enhance discrimination between absorption of low and high-energy portions of the spectrum, thus improving the accuracy of the results. We chose commonly found attenuating materials, lead and polycarbonate, and standard build-up caps, made of polymethyl methacrylate (PMMA) and brass, with a goal of adaptation of an “ideal” setup to that achievable in a typical clinical setting. The data was analyzed using a regularization technique implemented through spreadsheet-based calculations, enabling a potential user to restore spectra from transmission measurements without elaborate programming efforts. We successfully unfolded the spectra using transmission measurements and found the resultant spectra to deviate from those derived in the TPS beam models. The effect of such deviations on treatment planning was evaluated for 6MV-FFF beam through side-by-side comparison of representative plans, calculated with both the commissioned model spectrum and that obtained from the measurements. The differences between the model and unfolded spectra were reviewed through isodose distributions, and quantified in terms of maximum dose values for critical structures. While in most cases switching the model spectrum with the measured one did not result in drastic differences in the calculated dose, plans with deviations of 4 to 8% in the maximum dose values for critical structures were discovered. Since spectrum information is used by TPS in evaluation of the scatter dose, the anatomical sites with large scatter contributions are the most vulnerable to inaccuracies in the modeled spectrum. The developed approach of using readily available attenuators and build-up caps in combination with streamlined data processing confirmed feasibility of acquisition of megavoltage linac beam spectra in a typical radiation oncology clinic, offering an independent check of the TPS model.
Diana Shvydka, PhD (Committee Chair)
E. Parsai, PhD (Committee Member)
David Pearson, PhD (Committee Member)
Physics; Radiation

Recommended Citations

Citations

  • Leheta, D. (2014). Acquisition and Analysis of Megavoltage Linac Beam Transmission Data for Direct Verification of Photon Spectra Models in a Treatment Planning System [Master's thesis, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=mco1416566156

    APA Style (7th edition)

  • Leheta, Dimitra. Acquisition and Analysis of Megavoltage Linac Beam Transmission Data for Direct Verification of Photon Spectra Models in a Treatment Planning System. 2014. University of Toledo, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=mco1416566156.

    MLA Style (8th edition)

  • Leheta, Dimitra. "Acquisition and Analysis of Megavoltage Linac Beam Transmission Data for Direct Verification of Photon Spectra Models in a Treatment Planning System." Master's thesis, University of Toledo, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=mco1416566156

    Chicago Manual of Style (17th edition)

mco1416566156
809
© , some rights reserved.Creative Commons License Acquisition and Analysis of Megavoltage Linac Beam Transmission Data for Direct Verification of Photon Spectra Models in a Treatment Planning System by Dimitra Leheta is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by University of Toledo Health Science Campus and OhioLINK.