Dissertation Defence: Quantitative Investigations on Uncertainties in Permanent Breast Seed Implant Brachytherapy

Claire Zhang, supervised by Dr. Michelle Hilts and Dr. Andrew Jirasek, will defend their dissertation titled “Quantitative Investigations on Uncertainties in Permanent Breast Seed Implant Brachytherapy” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Medical Physics.

An abstract for Claire Zhang’s dissertation is included below.

Examinations are open to all members of the campus community as well as the general public. Registration is not required for in person examination.

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ABSTRACT

Permanent breast seed implant (PBSI) brachytherapy is a single-session partial breast irradiation technique for early-stage breast cancer patients who have previously undergone lumpectomy. Despite excellent clinical outcomes and high patient satisfaction rates, this technique has not been widely adopted in clinical practice due to challenges in standardizing and optimizing the planning and delivery of the technique. The goal of this thesis is to quantitatively investigate the uncertainties in PBSI and develop methods and technologies to improve the accuracy and robustness of this technique.

We investigated the uncertainties arising from varying lumpectomy surgical margins in our PBSI patient cohort. We compared the currently used safety margin in PBSI planning to that recommended in Groupe Europ´een de Curieth´erapie-European Society for Therapeutic Radiation and Oncology (GEC ESTRO) guidelines. Our results showed that the current PBSI clinical safety margin provides satisfactory coverage on the GEC ESTRO recommended target. However, implementing GEC ESTRO recommendations in PBSI planning has dosimetric advantages.

We also quantified PBSI seed displacement, as well as seed stability in breast tissue during the first month post implantation. We evaluated the dosimetric impact of seed displacement and movement and demonstrated that PBSI post-implant dosimetry remains within clinical goals even with local seed displacement and movement. Furthermore, we developed a simulation framework to evaluate PBSI plan robustness to seed displacement, which will be a useful tool for evaluating and improving planning methods.

To address the challenges in target visualization and implant imaging guidance in PBSI mentioned in previous studies, we continued the development and implementation of 3D ultrasound technologies in PBSI. We characterized an encoded 3D ultrasound systems developed for PBSI, and further developed a new system with improved clinical usability. Our commissioning and testing results showed good feasibility to implement 3D ultrasound in PBSI and great potential to improve visualization and guidance of PBSI.

In conclusion, our quantitative evaluation of some of the current uncertainties in the PBSI technique and the development of tools to improve its accuracy and user-dependence will facilitate the standardization and optimization of PBSI and promote the clinical adoption of this
technique.