Characterization and validation of an intra‐fraction motion management system for masked‐based radiosurgery

Abstract Purpose Characterize the intra‐fraction motion management (IFMM) system found on the Gamma Knife Icon (GKI), including spatial accuracy, latency, temporal performance, and overall effect on delivered dose. Methods A phantom was constructed, consisting of a three‐axis translation mount, a remote motorized flipper, and a thermoplastic sphere surrounding a radiation detector. An infrared marker was placed on the translation mount secured to the flipper. The spatial accuracy of the IFMM was measured via the translation mount in all Cartesian planes. The detector was centered at the radiation focal point. A remote signal was used to move the marker out of the IFMM tolerance and pause the beam. A two‐channel electrometer was used to record the signals from the detector and the flipper when motion was signaled. These signals determined the latency and temporal performance of the GKI. Results The spatial accuracy of the IFMM was found to be <0.1 mm. The measured latency was <200 ms. The dose difference with five interruptions was <0.5%. Conclusion This work provides a quantitative characterization of the GKI IFMM system as required by the Nuclear Regulatory Commission. This provides a methodology for GKI users to satisfy these requirements using common laboratory equipment in lieu of a commercial solution.

present with the aim to check the IFMM system's quantitative output." Previous work has described commissioning a GKI system, 2 the quality assurance, stability, and performance of the image guidance system, [3][4][5] and described comparisons of the CBCT to IFMM; 6,7 however, the full quantitative characterization of the IFMM system is absent from all of these works. We are currently unaware of any commercial systems or published literature that allow the user to quantitatively test the temporal latency along with the spatial accuracy of the IFMM system as required by the NRC and as is recommended in current published radiation oncology quality assurance guidelines. 8 The goal of this work was to quantitatively test and characterize the IFMM system. This includes the spatial accuracy of the IFMM, the ability of the IFMM to control the radiation unit of the Gamma Knife, the temporal latency of the system, and the temporal performance of the Gamma Knife sector drive unit. Furthermore, this work aims to make these quantifications safely from outside the vault during clinically realistic conditions to give the user confidence that the system will function as intended when treating patients.

2.A | Phantom construction
Using computer aided design (CAD), a model of a phantom created and then constructed using common optical laboratory parts is shown in Fig. 1. The current mask adapter for GKI was used as a F I G . 1. A computer-aided design model of the constructed phantom is shown in (a). A picture of the phantom mounted on the treatment machine is shown in (b). (c) An exploded-view drawing of the phantom consisting of (1) an acrylic plate, (2) optical breadboard, (3) a acrylic spacer, (4) thermoplastic sphere, (5) infrared marker, (6) translation stage, (7) flipper motor, and (8) SubMiniature version A to Bayonet Neill-Concelman adapters. (d) An example IFMM trace during a treatment showing the IFMM marker distance (blue points) on the Y axis as a function of time in seconds on the X axis. The five interruptions due to the phantom motion signaled by the user are shown in yellow. template for cutting a 10 mm thick acrylic base platform. Holes were drilled in the acrylic plate to match the mask registration pegs in the mask adapter [ Fig. 1(b)]. The acrylic was cut to be flush with the outside of the mask adapter as clearance is limited between the CBCT arm and the mask adapter. Holes were then drilled to attach the acrylic plate to an optical breadboard. The optical breadboard was also cut to be flush with the side of the mask adapter and ground to avoid sharp edges. Two holes were then drilled in the 10 mm spacer was used to place the center of the sphere approximately 25 mm above the optical breadboard and close to the center of radiation unit focal point. The sphere was drilled with a 6.5 mm diameter bit for detector placement. An exploded-view diagram of all these components can be seen in Fig. 1(c).

2.B | Characterization and validation of IFMM
In Gamma Knife Leksell stereotactic space, the right posterior superior corner of the frame on a supine patient is (X = 0 mm, Y = 0 mm, Z = 0 mm) and the center of stereotactic space is (X = 100 mm, Y = 100 mm, Z = 100 mm), with XY being the axial plane, XZ being the coronal plane, and YZ being the sagittal plane. An infrared marker was placed at the center of the translation stage. The spatial accuracy of the IFMM was tested by moving each axis of the translation mount a known distance and recording the readout of the IFMM. Each axis has a calibrated micrometer screw that moves the stage a known amount per rotation, 250 μm per rotation of the screw in the X and Y directions, and 500 μm per rotation of the screw in the Z direction.
The displacement according to the IFMM system is given as a magnitude on the treatment console. This value and its fluctuations were observed for each measurement and an average was taken.
Treatment plans were created post capturing a stereotactic reference CBCT of the phantom. Each plan was created to deliver a shot to the center of the detector located in the center of the thermoplastic sphere (X = 100.0, Y = 99.5, Z = 102.5). In the current version of the treatment planning software, the exterior skull definition cannot be completed using the CBCT images. The user must use a helical CT or magnetic resonance imaging (MRI) to define the skull.
For this study, a skull was generated from a helical CT scan. Once   This test for spatial accuracy was completed at installation/full calibration of the Gamma Knife, and thus far has been reproducible during monthly spot check with three displacements (one in each plane)

3.A | Spatial accuracy
being tested every month.

3.B | IFMM latency and temporal performance
Data collected from the electrometer during the irradiation with the 16 mm collimator setting (Fig. 3) show the detector current as a function of time for the GK radiation unit transitioning from a beam on state to beam hold state when a trigger from the remote flipper motor is sent. For ease of analysis, the time of the trigger to the optical flipper was set to zero. One can see the total time for the sector to move from an exposed to a blocked state in Fig. 3.

| DISCUSSION
Current license guidance 1 and quality assurance guidelines 8 mandate that the IFMM gating system should be quantitatively characterized.
These properties include spatial accuracy, temporal accuracy, the ability to interlock the radiation beam, and the overall accuracy of the delivery. As this system is relatively new, to our knowledge, no commercial or vendor guidance is available for testing the system. This work describes a method that was developed to complete these tests using commercially available equipment, at a relatively low cost, in a radiation-safe manner, and under clinically relevant conditions.
Using this method, tests indicate that the IFMM system performance, in terms of spatial accuracy (sub 0.1 mm), its ability to control the beam on/off states of the Gamma Knife radiation unit, and overall system latency (<200 ms), is capable for frameless stereotac- This work shows good agreement with previous works that showed a spatial accuracy of the IFMM to be 0.05 mm on average and within 0.16 mm maximally. 4,5 A limitation of this study is that measurements were performed on a single GKI unit. The performance of other GKI units may vary and would have to be characterized by an individual user following this methodology. Furthermore, these measurements were completed at the time of commissioning and periodically over a 6-month period. At the time of writing the system performance is stable; however, there is no longer term data on the stability of the system's performance. While preliminary data suggest the system is stable, data will continue to be collected on a routine basis throughout the lifetime of the GKI at our institution to ensure this is true.

| CONCLUSION
The IFMM system has been characterized and validated for use in frameless SRS on the GKI. The IFMM can achieve a spatial accuracy better than 0.1 mm and has system latency of less than 200 ms.
Using the methodology presented here one can routinely test the IFMM system fulfilling requirements of the NRC with one phantom,