Purpose: Few methods exist for streamlined MR-Linac 3D imaging-radiation isocenter coincidence measurements which can be practically performed on a daily basis in line with TG-142 guidelines. A recently developed Cherenkov imaging-based solution for daily MR-linac isocenter coincidence and output measurement has been in use for 16 months in our clinic, and in this study, we present a longitudinal assessment of the performance of this system.
Methods: A custom MR-Linac phantom developed by our group was implemented for daily output checks and MR-RT isocentricity measurements. This phantom consisted of a watertight acrylic cylinder containing a radioluminescent conical structure with fiducial marks aligned to isocenter, allowing for 3D isocenter localization. Recently, optical data has also been collected during the output check to observe the sensitivity and linearity of the Cherenkov emission with dose, in order to further streamline the daily check procedure by leveraging optical emissions.
Results: 260 out of 280 measurements for total 3D isocenter coincidence were below 2 mm (92.9%), with a mean of 1.5 ± 0.4 mm and a maximum of 2.6 mm. For isocenter coincidence along the individual dimensions, the pass rates were 100%, 99.3%, and 100% for x, y, and z, respectively. Measurements above 2 mm were attributed to setup difficulties and were concentrated towards the beginning of the study and during a training period. A strong linear correlation (R² = 0.99, NRMSE = 0.2%) was observed between cumulative Cherenkov counts and ion chamber readings, highlighting the potential for performing daily output check with this technique.
Conclusion: This study addresses the successful long-term performance of a novel Cherenkov-imaging based technique for MR-Linac daily QA, representing the first solution for 3D isocenter coincidence measurement that can be practically implemented on a daily basis.
Funding Support, Disclosures, and Conflict of Interest: This work has been funded in part by NIH grants R01 EB023909 and R44 CA232879 and the Norris Cotton Cancer Center shared irradiation resources through P30 CA023108. DA reports receiving consulting fees from DoseOptics LLC.
MRI, Quality Assurance, Optical Imaging
IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined Quality Assurance