Title page for ETD etd-04052011-102551

Type of Document Master's Thesis
Author Mathews, Bobby Chon
Author's Email Address bcmathews84@gmail.com
URN etd-04052011-102551
Title "Development of a Monte Carlo Based Correction Strategy for a TG-43 Based Brachytherapy Treatment Planning System to Account for Applicator Inhomogeneities"
Degree Master of Science (M.S.)
Department Physics & Astronomy
Advisory Committee
Advisor Name Title
Price, Michael Committee Chair
Clayton, Geoffrey Committee Member
Dowling, Jonathan Committee Member
Fontenot, Jonas Committee Member
Gibbons, John Committee Member
Matthews, Kenneth Committee Member
Wood, Charles Committee Member
  • Monte Carlo
  • Correction
  • AAPM TG-43
  • Tandem and Ovoids
  • Treatment Planning System
  • Dwell time gradient
  • Model Validation
Date of Defense 2011-02-11
Availability unrestricted
Purpose: The purpose of this work was to investigate the use of pre-calculated Monte Carlo (MC) ovoid and source-based attenuation-correction factors that would correct a commercially-available brachytherapy treatment planning system (TPS) generated plan in order to account for any dosimetric effects due to the presence of intracavitary brachytherapy (ICBT) applicators during treatment delivery.

Methods: A MC model of an ICBT CT-MR compatible ovoid applicator set was confirmed utilizing radiochromic film (RCF). MC was used to simulate dose distributions resulting from eight source-dwell-positions within the ICBT applicator. Also, the American Association of Physicist in Medicine Task Group 43 Report (AAPM TG-43) was utilized to calculate absolute dose rate around a microSelectron version 2 192Ir source contained in water. With these dose distributions, a library of ovoid and source-based 3D attenuation-correction factor datasets characterizing the dosimetric effects of the ICBT applicator was developed. Appropriate attenuation-correction factors were then applied to correct a brachytherapy TPS-calculated plan. Several plans with different maximum dwell-time gradients (dt) were compared to evaluate the effectiveness of both correction methods with respect to criteria of acceptability being within +/- 2% absolute dose or +/- 2 mm distance-to-agreement (DTA).

Results: RCF confirmation measurements from 3 active dwell-positions in a single ovoid agreed with MC simulated planes with over 96% of points agreeing within 2% or 2 mm DTA. Plans generated by Oncentra TPS can be corrected utilizing either the ovoid-based or source-based correction methods to agree with full simulated Monte Carlo datasets to within +/- 2% or +/- 2mm DTA. Although, dwell-time combinations utilized in this study with a maximum dwell-time gradient above 10% is a threshold for the ovoid- based correction scheme to correct the TG-43 calculation. The source- based correction method consistently results in 100% agreement between a corrected plan and the equivalent MC generated plan.

Conclusions: The MC model is sufficient to predict measured RCF dose distributions accurately. Source- based correction factors can be applied to correct a TG-43 based treatment plan to match a full MC simulation.

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