Room: Karl Dean Ballroom A1
Purpose: This study was designed to evaluate the accuracy of Cherenkov-based projection percent depth dose curves (pPDDs) and projection cross beam profiles (pCBPs) acquired in a water tank for a 6MV FFF beam from an MR-linac for the first time.
Methods: An intensified CMOS camera (DoseOptics LLC., Hanover, NH) was placed at the foot of an MR-linac (ViewRay Inc., Cleveland, OH), treatment couch. A 40cm x 30.5cm x 38cm high water tank was filled with water to a height of 18.8cm, to coincide with an SSD of 90cm (bore isocenter), and doped with 1g/1L of fluorophore quinine sulfate dissolved in HCl and water (0.5-N solution); optical Cherenkov-excited fluorescence was imaged. With the water line and transverse center of the tank at treatment isocenter, it was irradiated with twelve beams of decreasing size, starting with the largest possible beam (24.07cmx27.20cm) and ending with the smallest (0.2cmx0.415cm beam). A novel remote triggering device was used to synchronize image acquisition to the radiation pulses without a physical interface. Images were post-processed in MATLAB (MathWorks Inc., Natick MA), and pPDDs and pCBPs were extracted and compared to summed projection images of dose from the treatment planning system (TPS). No beam hardening corrections were deemed necessary.
Results: The Cherenkov-based pPDDs of the 24.07cmx27.20cm, 14.94cmx14.94cm, 9.96cmx9.96cm and 4.98cmx4.98cm beams exhibited an average error of less than 0.2% between 0cm (water line) and 14.5cm depth when compared to TPS pPDDs; error at dâ‚?â‚€ ranged from -0.04% to 0.89%. The pCBPs measured beam width with error of 1.71mm, 0.586mm, -0.174mm, and -0.695mm, respectively. Small beams (2-16mm in width) required longer acquisition times to achieve acceptable imaging signal to noise ratios.
Conclusion: Cherenkov imaging provides a viable beam measurement method for MR-linacs that is rapid, repeatable, and MR-compatible, with the potential to be expanded to small beam dosimetry.
Funding Support, Disclosures, and Conflict of Interest: This work has been funded by NIH grants F31CA192473 and R01EB023909. Potential conflict of interest arises from contributing author B. Pogue, founder and president of DoseOptics LLC developing Cherenkov imaging systems. However, this work was not financially supported by DoseOptics in any way.