Dissertations and Theses (Open Access)

Author ORCID Identifier

https://orcid.org/0009-0008-0007-6888

Date of Graduation

7-2026

Document Type

Thesis (MS)

Program Affiliation

Medical Physics

Degree Name

Masters of Science (MS)

Advisor/Committee Chair

Dr. Mallory Glenn

Committee Member

Dr. Stephen Kry

Committee Member

Dr. Tina Briere

Committee Member

Dr. Ruitao Lin

Committee Member

Paola Alvarez

Abstract

Abstract

Current practices for stereotactic radiosurgery (SRS) have been advancing to include more lesions in a single treatment. The Radiation Quality Assurance Laboratory (RQA Lab) phantom service provides audits for clinics as an end-to-end test for performance. However, the RQA Lab SRS phantom design tests only single-target SRS, representing a gap in the phantom program. Thus, this study set out to design and test a working multi-lesion brain metastases SRS phantom that was robust and could accurately assess clinical performance using the standards set by RQA Lab of 5% dose agreement for thermoluminescent dosimeters (TLD) and passing film gamma analysis criteria using 5%/3 mm thresholds.

There were three aims to achieve this project’s goals; the first being to set up an equivalent film dosimetry system using a flatbed scanner that could accurately measure dose using radiochromic film that could match the current film dosimetry practice within RQA Lab. This was needed as the targets in the new phantom design were smaller requiring a system that could potentially stand alone when measuring dose. The second aim was to investigate multi-lesion SRS patient features and using these findings create a feasible design that would be later fabricated and tested. The final aim was to incorporate the phantom into the RQA Lab workflows and adjust protocols needed to allow for an eventual smooth transition of the new phantom into the program.

An Epson flatbed scanner was obtained to establish a new film dosimetry program. A commissioning process was performed, checking the effects of scanning film and establishing protocols to ensure that the resulting film dose was accurate. Simultaneously, a study into a patient cohort of multi-lesion SRS cases was performed (n=52) spanning cases treated with a linear accelerator, Gamma Knife, and CyberKnife machines. Using this data, patient specific features were chosen as design goals that shaped the phantom design. Finally, after the design plan was completed, a prototype was fabricated. This prototype was then tested using a reproducibility study on a linear accelerator, a study to check accessibility across modalities using a Gamma Knife machine, and finally, a study using an external institution. From these tests, the dosimeter results were compared with the treatment planning system dose calculation to confirm performance accuracy. The dosimeters were evaluated using RQA Lab protocols for TLD and film gamma analysis criteria.

After the commissioning process, a protocol was set for scanner settings and film orientation and position when scanning. Additionally, a set time frame was established for how long the film could be allowed to be read after being irradiated. Using these procedures, the phantom tests were evaluated. For the testing completed, the TLD results agreed with the treatment plan results within 3% representing that the design was appropriate and clinically achievable. In the reproducibility study alone, the standard deviation was 1.73% showing robustness. Film analysis showed agreement between the profiles taken from the film and the treatment plans; gamma analysis results had 28 out of 30 films among the five cases passing the 85% pixel threshold given the 5%/3 mm gamma criteria.

From this it has been shown that a multi-lesion brain metastasis phantom was created that showed reliability in evaluating the accuracy and consistency of contemporary SRS workflows and reflects the increasing clinical complexity of current practices.

Keywords

Multi-lesion SRS, phantom, radiation therapy quality assurance

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Available for download on Thursday, January 14, 2027

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