Polarimetric Cherenkov Imaging for MR-Linac Dosimetry In a Heterogeneous Phantom
Abstract
Purpose
With the recent introduction of clinical MR-Linac devices, there is an increased need for dose measurement techniques unaffected by magnetic fields. Polarized Cherenkov imaging is a luminescence-based dosimetry method potentially well suited for MR-Linac measurements. This work aims to measure accurate projected dose distributions using polarized Cherenkov light emitted from a heterogeneous medium irradiated by an MR-Linac.
Methods
The Elekta Unity MR-Linac was used to irradiate a 15×15×20 cm3 water phantom containing a 5×5×5 cm3 air cavity encapsulated in 2 mm of acrylic. The 7 MV irradiation beam delivered 800 MU at a rate of 420 MU/min. A 5×5 cm2 antero-posterior beam was used. Cherenkov emission produced in the phantom was captured using a CMOS charge intensified camera (BeamSite Research by DoseOptics). Polarimetry was achieved using the Predator lens from Patqer to improve image analysis. To validate Cherenkov-based dose maps, comparisons with Monte Carlo-calculated dose distributions were performed. Monte Carlo simulations were computed using TOPAS version 3.9 and a phase space provided by Elekta replicating the beam.
Results
Comparison of Monte Carlo versus polarized Cherenkov emission-based dose distributions in water showed gamma passing rates of 97% and 92% with a 3%/3mm criteria for the sections respectively above and under the air cavity. At the upper horizontal interface, polarized Cherenkov and Monte Carlo results show overdosage peaks of 122±7% and 115±5% respectively. At the lower horizontal interface, both methods resulted in underdosages of 37% (±4% for polarized Cherenkov and ±1% for Monte Carlo).
Conclusion
General good agreement is obtained between polarized Cherenkov and Monte Carlo dose distributions within water and acrylic sections. Furthermore, overdosage and underdosage predicted by the electron return effect are accurately measurable using polarized Cherenkov imaging. This work highlights the potential of polarized Cherenkov dosimetry for heterogeneous mediums in the context of MR-Linacs.