First demonstration of combined kV/MV image-guided real-time dynamic multileaf-collimator target tracking

Abstract

Purpose: For intrafraction motion management, a real-time tracking system was developed by combining fiducial marker-based tracking via simultaneous kilovoltage (kV) and megavoltage (MV) imaging and a dynamic multileaf collimator (DMLC) beam-tracking system.

Methods and materials: The integrated tracking system employed a Varian Trilogy system equipped with kV/MV imaging systems and a Millennium 120-leaf MLC. A gold marker in elliptical motion (2-cm superior-inferior, 1-cm left-right, 10 cycles/min) was simultaneously imaged by the kV and MV imagers at 6.7 Hz and segmented in real time. With these two-dimensional projections, the tracking software triangulated the three-dimensional marker position and repositioned the MLC leaves to follow the motion. Phantom studies were performed to evaluate time delay from image acquisition to MLC adjustment, tracking error, and dosimetric impact of target motion with and without tracking.

Results: The time delay of the integrated tracking system was approximately 450 ms. The tracking error using a prediction algorithm was 0.9 +/- 0.5 mm for the elliptical motion. The dose distribution with tracking showed better target coverage and less dose to surrounding region over no tracking. The failure rate of the gamma test (3%/3-mm criteria) was 22.5% without tracking but was reduced to 0.2% with tracking.

Conclusion: For the first time, a complete tracking system combining kV/MV image-guided target tracking and DMLC beam tracking was demonstrated. The average geometric error was less than 1 mm, and the dosimetric error was negligible. This system is a promising method for intrafraction motion management.

Fig. 1

Flowchart of X-ray image-guided real-time DMLC tracking.

Fig. 2

Example of a series of (left) megavoltage (MV) and (right) kilovoltage (kV) image pairs acquired during the elliptical motion of the marker by vertical MV and horizontal kV imaging. On MV images with marker locations as insets, the dash-lined box with central cross was also displayed to guide the MV beam isocenter. The centers of MLC apertures were seen to be lagging behind the marker due to the time delay from image acquisition to MLC adjustment.

Fig. 3

Example of an MV image obtained during the same experiment as Figure 2. The distance between the marker position and the center of the circular aperture represents the tracking error.

Fig. 4

Offsets of kV and MV imager as a function of gantry angle with respect to the MV beam isocenter, along with sinusoidal fits. SID stands for source-to-imager distance.

Fig. 5

The 3D positions of the stationary marker at the MV beam isocenter determined by kV/MV images during the arc delivery. The marker was aligned at the isocenter by acquiring anterior–posterior MV and left-lateral kV images. The mean offset in each direction from the isocenter, indicated by the center of dotted circle, was caused by the uncertainty of this alignment process. The radius of the dotted circle, ~± 0.25 mm (1 pixel), represents the uncertainty of marker extraction.

Fig. 6

Trajectories of the marker positions and the aperture centers on MV images with tracking error. X, Y means the horizontal and vertical directions on the MV images, and the Y direction coincides with the superior-inferior direction.

Fig. 7

Dose distributions of the 10-cm circular MLC aperture (blue) for (a) static, (b) no tracking, and (c) tracking, along with the definitions of CTV, OAR, and margins (d–f) in each case. Assuming that in each case the 10-cm circular MLC aperture was designed to cover its CTV with 95% dose (red), V95 was defined as CT V, while the OAR was defined as the outer region of CTV bound to a square of 13×13 cm².

Fig. 8

Dose-volume histogram of CTVs and OARs for static, no tracking, and tracking.

Fig. 9

Dose difference maps computed by subtracting the dose distributions of the static target from the dose distributions of the moving target (a) without tracking, (b) with tracking, as well as gamma test results with gamma index >1 shown in red for (c) without tracking and (d) with tracking.

Fig. 10

Distribution of target-positioning errors with and without tracking for the elliptical motion in the (a) x and (b) y directions measured using MV images acquired during the film irradiation.