11/07/2025
By Michael Lavelle

The Kennedy College of Sciences, Department of Physics and Applied Physics, invites you to attend a Doctoral Dissertation defense by Michael Lavelle titled, “MRI-Guided Surface Brachytherapy: Sequence Comparison and Clinical Implementation.”

Date: Friday, Nov. 21, 2025
Time: 1:30 to 3:30 p.m.
Location: Virtual defense via Zoom. Please contact michael_lavelle@student.uml.edu for Zoom link

Committee Members:

  • Advisor: Ivan Buzurovic, Ph.D, Associate Professor of Medical Physics in Radiation Oncology, Harvard Medical School
  • Erno Sajo, Ph.D., Professor, Director of Medical Physics, Department of Physics and Applied Physics, UMass Lowell
  • Romy Guthier, Ph.D., Assistant Professor, Department of Physics and Applied Physics, UMass Lowell
  • Timothy Cook, Ph.D., Associate Professor, Department of Physics and Applied Physics, UMass Lowell

Abstract:

Brachytherapy is a form of radiation therapy that delivers high doses of radiation directly to diseased tissue through implanted or surface applicators. In surface brachytherapy (SB), accurate visualization of both applicators and surrounding tissue is essential, as even small positional errors can lead to under- or over-treatment. Conventional SB planning relies on computed tomography (CT), which provides excellent visualization of applicators but limited soft-tissue contrast. As a result, clinicians must often balance reliable source reconstruction with imperfect visualization of target tissue. Magnetic resonance (MR) offers superior soft-tissue contrast and the potential for single-modality MR-only workflows that eliminate the need for CT acquisition and image registration, thereby improving geometric accuracy of dose delivery, reducing demand on hospital resources, and enhancing patient convenience. While prior studies have demonstrated the feasibility of MR-guidance in brachytherapy, this work is the first to systematically evaluate multiple optimized sequences for surface applications and directly compare their geometric and dosimetric performance as well as image assessment against CT.

This dissertation investigates whether optimized MR sequences can replace CT for SB planning. Three MR series were evaluated: pointwise encoding time reduction with radial acquisition (PETRA) and volumetric interpolated breath-hold examination (VIBE) with Dixon in-phase (DIP) and opposed-phase (DOP) reconstructions. Imaging was performed on tissue-equivalent phantoms and on eleven patients undergoing SB for Dupuytren’s contracture. MR-based plans were compared to CT-based plans for geometric and dosimetric accuracy as well as applicator visibility and soft tissue contrast.

The first phase assessed feasibility using the PETRA sequence alone. Submillimeter agreement in dwell position reconstruction and strong dosimetric consistency including dose cloud segmentations and dose difference calculations, supported the viability of MR-only planning. The second phase expanded to all three MR series, incorporating image quality assessments based on signal-to-noise and contrast-to-noise ratios. Of the three, the DOP series provided the best combination of applicator visibility and soft-tissue contrast, and this series was therefore selected for further planning studies.

Across both phantom and patient studies, MR imaging achieved geometric agreement within 1 mm of CT-based dwell positions. Dosimetric comparisons showed dose differences of less than 5% between MR- and CT-based plans. Dice similarity coefficients exceeded 0.9 in phantom studies and 0.8 in patient studies, with volumetric similarity values consistently exceeding 0.98. These metrics confirm that MR-only workflows can meet the same standards of accuracy and reliability currently expected of CT-based workflows.

This dissertation presents the first systematic comparison of multiple MR sequences for SB planning and demonstrates that MR imaging is well suited for clinical implementation. Adoption of an MR-only workflow has the potential to improve treatment accuracy by combining robust applicator reconstruction with superior soft-tissue visualization, while simultaneously reducing reliance on multi-modality imaging, streamlining clinical logistics, and enhancing patient experience by eliminating redundant scans. More broadly, this work highlights the role of MR in enabling safer and more patient-centered brachytherapy with the potential to improve long-term outcomes by reducing underdosing and the need for retreatment.