Evaluation of radixact motion synchrony for 3D respiratory motion: Modeling accuracy and dosimetric fidelity

Abstract

The Radixact® linear accelerator contains the motion Synchrony system, which tracks and compensates for intrafraction patient motion. For respiratory motion, the system models the motion of the target and synchronizes the delivery of radiation with this motion using the jaws and multi-leaf collimators (MLCs). It was the purpose of this work to determine the ability of the Synchrony system to track and compensate for different phantom motions using a delivery quality assurance (DQA) workflow. Thirteen helical plans were created on static datasets from liver, lung, and pancreas subjects. Dose distributions were measured using a Delta4® Phantom+ mounted on a Hexamotion® stage for the following three case scenarios for each plan: (a) no phantom motion and no Synchrony (M0S0), (b) phantom motion and no Synchrony (M1S0), and (c) phantom motion with Synchrony (M1S1). The LEDs were placed on the Phantom+ for the 13 patient cases and were placed on a separate one-dimensional surrogate stage for additional studies to investigate the effect of separate target and surrogate motion. The root-mean-square (RMS) error between the Synchrony-modeled positions and the programmed phantom positions was <1.5 mm for all Synchrony deliveries with the LEDs on the Phantom+. The tracking errors increased slightly when the LEDs were placed on the surrogate stage but were similar to tracking errors observed for other motion tracking systems such as CyberKnife Synchrony. One-dimensional profiles indicate the effects of motion interplay and dose blurring present in several of the M1S0 plans that are not present in the M1S1 plans. All 13 of the M1S1 measured doses had gamma pass rates (3%/2 mm/10%T) compared to the planned dose > 90%. Only two of the M1S0 measured doses had gamma pass rates > 90%. Motion Synchrony offers a potential alternative to the current, ITV-based motion management strategy for helical tomotherapy deliveries.

Keywords: intrafraction motion; radixact; synchrony; tomotherapy; tracking.

Fig. 1

(a) A photograph of the Radixact system at UW‐Madison with the cover removed. The MV source is hidden by the couch in the photograph. (b) An illustration of the setup for a patient Synchrony treatment for respiratory motion. The light‐emitting diode (LED) camera (A) is mounted to the ceiling and monitors the position of LED's on the patient’s chest (B) and on the couch (C). Image courtesy of Accuray, Inc.

Fig. 2

(a) The modified Phantom+ with the ball‐cube removed. The ball‐cube holds fiducials. (b) The Phantom+ on the Hexamotion stage. The three patient light‐emitting diodes (LEDs) were either placed on the separate surrogate stage (shown here), or on the Phantom+, and one couch LED was placed on the couch or a static object on the couch.

Fig. 3

(a) Examples of treatment‐length phantom motion traces for three subject cases. (b) 30‐s samples of the phantom motion traces. (c) Cumulative tracking error plots showing the probability of observing a tracking error greater than the specified value throughout the treatment.

Fig. 4

(a) Full‐treatment and 30‐second sample phantom motion trace for Lung 5a in the X, Y, Z, and surrogate directions. The X, Y, Z motion of the target was not changed for cases Lung 5b‐Lung 5i (other than fitting parameter change for Lung 5h and Lung 5i). (b) Cumulative tracking error plots showing the probability of observing a tracking error greater than the specified value throughout the treatment for various cases.

Fig. 5

Profile analysis for Liver 2 and Lung 2. Profiles were acquired through diodes passing through the geometric center of the Phantom+. Liver 2 was chosen to demonstrate effects of interplay and dose blurring for the M1S0 plan, that are not present for the M1S1 plan. Lung 2 was chosen to demonstrate that although the gamma pass rates for the M1S0 delivery were high (>98% in Table 5), the shape of the dose distribution for M1S0 did not match the planned or static measured distributions as well as for M1S1.

Fig. 6

30‐s sample of Synchrony‐modeled motion (“Tracking”) vs phantom motion in the IEC‐Y direction for Liver 1 with four images per gantry rotation (top) and five images per gantry rotation (bottom). Multicolored points indicate the phase at times which the kV images were acquired (four colors in the top and five colors in the bottom). When imaging with four images per gantry rotation, the imaging frequency was found to alias with the breathing frequency and the model was not accurately built (hence there is no tracking curve in the top image).