Author ORCID Identifier

0000-0003-1086-6978

Date of Graduation

8-2019

Document Type

Thesis (MS)

Program Affiliation

Medical Physics

Degree Name

Masters of Science (MS)

Advisor/Committee Chair

Paige Taylor

Committee Member

David Followill

Committee Member

Stephen Kry

Committee Member

Dershan Luo

Committee Member

Christine Peterson

Abstract

Small field dosimetry presents complications and uncertainties that could be circumvented by using detectors which are smaller than the radiation field. This study evaluates the reproducibility and accuracy of TLD micro-cubes for use in stereotactic radiosurgery (SRS) remote auditing quality assurance (QA) for treatment centers participating in clinical trials. This study tested the hypothesis that TLD micro-cubes could be commissioned to evaluate small field dosimetry, and provide reproducibility within 3%, as well as assure agreement between measured dose and calculated doses to within 5%.

The aims of this thesis were to characterize and commission TLD micro-cubes as well as to develop guidelines for handling micro-cubes. Additionally the micro-cubes were commissioned to evaluate standard single field output dosimetry. Further aims were to adapt IROC Houston SRS head phantoms to use TLD micro-cubes for anthropomorphic phantom quality assurance and to test this design on linac, Gamma Knife, and CyberKnife treatment delivery machines. The final aim was to use TLD micro-cubes to evaluate photon fields which are smaller than 1.25 cm in diameter.

This study was designed by first defining the handling process, including: selection of micro-cubes, annealing parameters, and readout techniques. The micro-cubes were then characterized based on correction factors for element sensitivity, signal fading, dose response, and energy response.

To test the reproducibility and accuracy of the dosimeters, they were first evaluated under a single small field beam in a simple geometric configuration, then in anthropomorphic SRS head phantoms. Agreement between calculated dose and measured dose was evaluated. Following satisfactory results of these experiments, the micro-cubes were used to evaluate single small fields down to 5 mm fields on the same basis as single field output checks.

TLD micro-cubes showed good accuracy and agreement when compared to beam output, treatment planning system (TPS) dose, and measurements made with TLD powder. For all experiments conducted in this study, measured dose was within 4.1%. For SRS experiments, the average difference in measured and expected dose was within 3.4% with an average difference of 1.0% and an average coefficient of variation of 0.9%. For single field experiments, all measurements were within 4.1% with an average of 2.1% and an average coefficient of variation of 0.6%. These results give us confidence in our ability to accurately measure dose in radiation fields as small as 5 mm in diameter, as well as obtain excellent reproducibility.

Keywords

Thermoluminescent dosimeter, micro-cube, small field dosimetry, quality assurance, radiotherapy, TLD

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