Room: Karl Dean Ballroom C
Purpose: Commissioning of integrated MRI radiotherapy machines requires precise alignment between MRI and radiation treatment (RT) isocenters. An optical measurement system was devised to quantify the displacement of the MRI isocenter with respect to the optically imaged RT isocenter.
Methods: A conical frustum-shaped phantom was machined out of ABS plastic with alignment crosshairs at known positions. The phantom had base radius=7cm, and top face radius=2cm, with cone half-angle of 45Â°. For proof of principle, the phantom was mounted in air on a traditional linac. An intensified CMOS camera (DoseOptics LLC., Hanover, NH) was placed at the foot of the treatment couch, so that the optical axis was parallel to the conical axis. A 20cm long and 1cm wide 6MV FFF beam was delivered to the phantom and the scintillation of the plastic was imaged with gantry at 270Â° and 90Â°, with iterative translations of the couch position ranging from +1.0cm to -2.0cm at down to 1mm increments to assess accuracy of z-axis alignment. The phantom was aligned near the front 2cm radius face, and a 1cmx1cm 6MV partial arc was imaged for tomographic localization of the x-y isocenter. Post processing of images was conducted to determine radiation beam position with respect to the known phantom geometry in 3 dimensions.
Results: The horizontal and vertical ring diameters measured optically had a strong linear correlation to the known translation distance in the Z direction (RÂ²=0.999 and RÂ²=0.998, respectively), with resolution down to 1mm shifts in Z. The X-Y alignment method allows for accurate positioning the of RT isocenter with respect to the central axis of the phantom.
Conclusion: The data supports the hypothesis that described phantom can be placed in a water bath and imaged concurrent with optical and MRI modalities to determine the quantitative alignment of the two isocenters.
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.