Author: Ivan M. Buzurovic, Phillip M. Devlin, Evangelia Kaza, Michael John Lavelle 👨🔬
Affiliation: Department of Radiation Oncology, Dana-Farber/Brigham and Women’s Cancer Center, Harvard Medical School, Brigham and Women's Hospital, Harvard Medial School, Dana-Farber Cancer Institute, Department of Radiation Oncology 🌍
Purpose: Recent work in surface brachytherapy (SBT) has demonstrated the feasibility of using magnetic resonance (MR)-guidance in place of Computer Tomography (CT) in SBT planning. The purpose of this study is to demonstrate the MR sequence most suitable for SBT treatment planning by optimizing geometric accuracy of catheter reconstruction and soft tissue contrast.
Methods: An anthropomorphic phantom (True Phantom Solutions, Ontario, Canada) fitted with flap-style (Elekta, Netherlands) applicators on the foot and hand and four patients undergoing SBT treatment for Dupuytren’s Contracture/Palmar fascial fibromatosis were imaged by two optimized MR sequences: i) pointwise encoded time reduction with radial acquisition (PETRA) and ii) volumetric interpolated breath-hold examination (VIBE) obtaining Dixon in-phase (DIP) and opposed-phase (DOP) images. Helical CT scans were acquired for verification.
SBT planning was performed in Oncentra Brachy (Elekta Brachytherapy, The Netherlands) treatment planning software. Geometric accuracy was determined by registering CT-based catheter digitizations obtained in the standard approach to each MR-based digitization. Distances between corresponding dwell positions were calculated to assess geometric accuracy of each MR-based plan. Patient MR images were compared qualitatively for soft tissue visualization.
Results: All three MR series produced sufficient applicator-catheter contrast to produce treatment plans. Distances between dwell positions from the CT-based and MR-based plans were on average 0.8±0.2mm for PETRA, 0.7±0.3mm for DIP, for DOP 0.7±0.1mm for the phantom and 1.3±0.7mm for PETRA, 1.3±1mm for DIP, 1.3±0.8mm for the patients. DOP consistently had higher soft tissue contrast than DIP and PETRA, which is helpful to visualize diseased tissue.
Conclusion: Submillimeter accuracy of dwell positions for each MR sequence for phantom plans indicated that they all have the potential to produce accurate plans. MR-CT distances above 1mm for patients could be attributed to inter-fraction motion. The higher soft tissue contrast on DOP suggests it is best suited for treatment planning and tissue visualization.