First Demonstration of Combined kV/MV Image-Guided Real-Time Dynamic Multileaf-Collimator Target Tracking
Received 30 September 2008; received in revised form 28 January 2009; accepted 7 February 2009.
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 ∼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.
Reprint requests to: Department of Radiation Oncology, Stanford University, 875 Blake Wilbur Drive, Stanford, CA94305-5847. Tel: (650) 498-5878; Fax: (650) 498-5008
Conflicts of interest: Per R. Poulsen has financial support from Varian Medical Systems, Alex Sloutsky is an employee of Varian Medical Systems, and Paul J. Keall has financial support from the National Institutes of Health/National Cancer Institute (Grant Nos. P01CA116602 and R01CA93626) and Varian Medical Systems.
ABSTRACT