Room: AAPM ePoster Library
Purpose: Emergence of the MRI-guided radiation therapy created the need for a reliable QA tools for small field measurements. SRS-Mapcheck, produced by Sun NuclearCorporation, is a device aimed to satisfy that need, but rigorous tests are required in order to assure that the accuracy of the measurements provided by the device is not affected by the presence of the magnetic field.
Methods: dependency of the central diode reading has been investigated for 2.5x2.5 cm and 5x5 cm fields, measured at the isocenter as well as with 2.5 cm lateral offset for a Mapcheck detector plane parallel to the couch and perpendicular to the couch. All measurements were taken at 6 MV FFF , 100 Mus per reading, at 15o gantry rotation interval. Measurements were taken on a MRidian ViewRay LINAC, with a constant magnetic field of 0.35T. Control sets of measurements were taken at Varian Edge linear accelerator.
Results: A clear angular dependency trend can be observed for the SRS MapCheck for both sets of measurements. The difference in reading with and without the magnetic field becomes more pronounced (add numbers here) with the smaller fields and fields shifted off-center, as well as rotated. Absolute measurement readings for the CAX are lower in the presence of the magnetic field vs in absence (Edge), with possible explanation being the multiple small air pockets in cumulative inside the device having a stronger effect on the readings in the presence of a magnetic field.
Conclusion: Further set of readings with the air pockets in the SRS MapCheck device filled would allow us to understand in greater details the angular dependency of the CAX reading on the presence or absence of the magnetic field, and whether or not certain design changes need to be introduced to match the readings under the same calibration conditions.