Purpose: The authors have developed a novel technique using an electronic portal imaging device (EPID) to verify the geometrical accuracy of delivery of dose-rate-regulated tracking (DRRT). This technique, called verification of real-time tracking with EPID (VORTE), can potentially be used for both on-line and off-line quality assurance (QA) of MLC-based dynamic tumor tracking.
Methods: The shape and position of target as a function of time, which is assumed to be known, is projected onto the EPID plane. This projected sequence of apertures as a function of time (target motion) is then used as the reference. The accuracy of dynamic MLC tracking can then be assessed by how well the delivered beam follows this projected target motion without the use of a physical moving phantom. The beam apertures controlled by DRRT (aperture motion) is detected by the EPID as a function of time. The aperture motion is compared to the target motion to evaluate tracking error introduced by DRRT. The accuracy of VORTE was measured using film measurements of ten static fields. The VORTE for dynamic tumor tracking was tested with several target motions, including (1) rigid-body two-dimensional (2-D) cyclic motion in the superior-inferior direction with various period and amplitude; (2) the above 2-D cyclic motion plus cyclic deformation; and (3) 2-D cyclic motion with both deformation and rotation. For each target motion, the controlled aperture motion resulting from DRRT was acquired at using EPID in the continuous-acquisition mode. Leaf positions in all captured frames were measured from the EPID and compared to their expected positions. The passing rate of 2 mm criteria for all leaves from all frames was calculated for each of the four patterns of tumor motion. Additionally, the root-mean-square (RMS) deviations of the centroid of the apertures between the designed and delivered beams were calculated for all three cases.
Results: The accuracy of MLC-leaf position determination by VORTE is 0.5 mm (1 standard deviation) by comparison to film measurements. With DRRT, the passing rates using the 2 mm criteria for all acquired frames are 100% for the 2-D displacement, 99% for the 2-D displacement with deformation, and 88% for the 2-D displacement combined with both deformation and rotation. The RMS deviations are 0.6 mm for the 2-D displacement, 1.0 mm for the 2-D displacement with deformation, and 1.1 mm for the 2-D displacement combined with both deformation and rotation.
Conclusions: The VORTE can measure the accuracy of MLC-based tumor tracking without the necessity of employing a moving phantom. Moreover, it can be used for complex target motion (i.e., 2-D displacement combined with deformation and rotation) that is difficult to create with physical moving phantoms. Therefore, the VORTE and the novel QA process illustrated by this study have a great potential for verifying real-time tumor tracking.