Room: AAPM ePoster Library
Purpose: To optimize the design of a detector for daily quality assurance (QA) measurements of MRgRT treatment machines while maintaining consistency with current clinical workflow.
Methods: Large variations in ionization chamber response have been demonstrated to be entirely due to air gaps between ion chambers and the solid phantoms used for daily QA measurements. These variations can be minimized if all electrons that deposit dose in the air cavity of the chamber are set in motion in the chamber wall. The EGSnrc Monte Carlo code system is used for simulations to optimize an ionization chamber model by increasing the thickness of a brass (high-density, non-ferromagnetic, easy-to-machine) wall until consistent results are produced for simulations with a 1.5 T magnetic field, with and without a 0.2 mm air gap and varying the placement of the chamber model within the air gap. The source is modeled with a spectrum for the 7 MV Elekta Unity MRgRT treatment machine collimated to 10x10 cm² at the surface of a PMMA phantom with the ionization chamber at a depth of 5 cm.
Results: The optimal thickness of the chamber wall such that all electrons that contribute to dose originate in the wall is 1.1 mm of brass. Using this thickness, the differences between simulations with and without an air gap and with asymmetric placement of the chamber within the air gap are less than 0.2 %.
Conclusion: An ionization chamber design is optimized for accurate QA measurements of MRgRT treatment machines. This design will be used to produce a prototype ionization chamber that has the same outer dimensions as Farmer-type chambers in current clinical use, which will enable the use of existing plastic phantoms in MRgRT systems to minimize disruption to clinical workflow and reduce errors introduced by the use of non-standard equipment and procedures.