Abstract
The ArcCHECK‐MR diode array utilizes a correction system with a virtual inclinometer to correct the angular response dependencies of the diodes. However, this correction system cannot be applied to measurements on the ViewRay MR‐IGRT system due to the virtual inclinometer's incompatibility with the ViewRay's multiple simultaneous beams. Additionally, the ArcCHECK's current correction factors were determined without magnetic field effects taken into account. In the course of performing ViewRay IMRT quality assurance with the ArcCHECK, measurements were observed to be consistently higher than the ViewRay TPS predictions. The goals of this study were to quantify the observed discrepancies and test whether applying the current factors improves the ArcCHECK's accuracy for measurements on the ViewRay. Gamma and frequency analysis were performed on 19 ViewRay patient plans. Ion chamber measurements were performed at a subset of diode locations using a PMMA phantom with the same dimensions as the ArcCHECK. A new method for applying directionally dependent factors utilizing beam information from the ViewRay TPS was developed in order to analyze the current ArcCHECK correction factors. To test the current factors, nine ViewRay plans were altered to be delivered with only a single simultaneous beam and were measured with the ArcCHECK. The current correction factors were applied using both the new and current methods. The new method was also used to apply corrections to the original 19 ViewRay plans. It was found the ArcCHECK systematically reports doses higher than those actually delivered by the ViewRay. Application of the current correction factors by either method did not consistently improve measurement accuracy. As dose deposition and diode response have both been shown to change under the influence of a magnetic field, it can be concluded the current ArcCHECK correction factors are invalid and/or inadequate to correct measurements on the ViewRay system.
Highlights
Magnetic resonance imaging-guided radiation therapy has tremendous potential for real-time image guidance during treatment delivery
The ViewRay system presents a unique challenge to IMRT QA devices due to its ability to deliver up to three simultaneous beams in the presence of a significant magnetic field
The ArcCHECK device (Sun Nuclear Corporation, Melbourne, FL, USA) is a helical diode array consisting of 1386 diodes encased within a cylindrical polymethyl methacrylate (PMMA) shell at a water-equivalent depth of 3.3 g cmÀ2
Summary
Magnetic resonance imaging-guided radiation therapy has tremendous potential for real-time image guidance during treatment delivery. The ViewRay system (ViewRay, Inc., Oakwood Village, OH, USA) is the world’s first MRI-guided delivery system, and uses a ring gantry with three cobalt-60 (60Co) treatment heads mounted 120° apart, each with an independent double-focused multileaf collimator (MLC).[1] The system is capable of delivering a variety of treatment options, including adaptive and intensity-modulated radiation therapy, all with simultaneous image guidance using its integrated 0.35 T MRI system. The ArcCHECK-MR model is one of the few IMRT QA devices with claimed MRI-compatibility up to magnetic field strengths of 0.35 T. This is accomplished by placing the device’s power source outside the 5 gauss line. Previous studies of the ArcCHECK system by Kozelka et al and Li et al found it to have an accuracy and precision acceptable for clinical IMRT and VMAT quality assurance.[2,3] it must be noted that these studies did not use the MRI-compatible ArcCHECK and all measurements were performed without the presence of a significant magnetic field
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