SU-E-T-149: Couch and Gantry Rotation Isocenter QA Using Portal Imager

Abstract

Purpose: Propose a method for extending digital image starshot analysis to gantry and couch rotation isocenters using and EPID. Methods: The portal imager can easily be used for creating spoke shots for the collimator rotation, but for the couch and gantry rotation typically films (traditional or GafChromics) are used. (a) For couch rotation a metal rod or a narrow slit in a metal sheet (e.g. an electron block cutout) is placed on the top of the couch. Exposures with several couch angles are obtained with the portal imager to create the spoke shot. (b) For gantry angle measurement a long rod is placed obliquely on the table top from the table axis and exposures are obtained with several gantry angles resulting in a spoke shot. The image summations and spoke shot evaluations were performed with DoseLab quality assurance/image analysis software (developed by Mobius Medical Systems). Results: The resulting spoke shots were all suitable for analysis (see supporting data). For couch rotation isocenter, the slit in an electron block worked better than a solid rod. For the gantry rotation isocenter the best Results was obtained with about ∼60 degree angle between the rod and the long axis of the table, and using 0, 70, 90, 110 and 180 degree gantry angles. The source‐portal imager distance was extended to 180cm, in order to decrease the differences in intensity between the proximal and distal parts of the rod. Conclusion: The method is easy to use and is faster than using films for isocenter analysis. This process also eliminates the need to scan films after exposure. All tests can be performed in one session with three entries into the treatment room. The DoseLab software performed the rotation isocenter tests for all three easily and efficiently.