To verify the positional accuracy of a novel x-ray-image-based dynamic tumor-tracking (DTT) irradiation technique using the gimbaled MV x-ray head of a Vero4DRT (MHI-TM2000).
Verification of the x-ray-image-based DTT was performed using three components: a three-dimensional moving phantom with a steel ball target, a laser displacement gauge, and an orthogonal kV x-ray imaging subsystem with a gimbaled MV x-ray head and the system controller of the Vero4DRT. The moving phantom was driven based on seven periodic patterns [peak-to-peak amplitude (A): 20–40 mm, breathing period (T): 2–5 s] and 15 patients’ aperiodic respiratory patterns (A: 6.5–22.9 mm, T: 1.9–5.8 s). The target position was detected in real time with the orthogonal kV x-ray imaging subsystem using the stereo vision technique. Subsequently, the Vero4DRT predicted the next position of the target, and then the gimbaled MV x-ray head tracked the corresponding orientation of the target. The displacements of the target were measured synchronously using the laser displacement gauge. The difference between the target positions predicted by the Vero4DRT and those measured by the laser displacement gauge was computed as the prediction error (EP), and the difference between the target positions tracked by the gimbaled MV x-ray head and predicted target positions was computed as the mechanical error (EM). Total tracking system error (ET) was defined as the difference between the tracked and measured target positions.
The root mean squares (RMSs) of EP, EM, and ET were up to 0.8, 0.3, and 0.7 mm, respectively, for the periodic patterns. Regarding the aperiodic patterns, the median RMSs of EP, EM, and ET were 1.2 (range, 0.9–1.8) mm, 0.1 (range, 0.1–0.5) mm, and 1.2 (range, 0.9–1.8) mm, respectively. From the results of principal component analysis, tracking efficiency, defined as the ratio of twice the RMS of ET to A, was improved for patients with high respiratory function (R = 0.91; p < 0.01).
The present study demonstrated that the Vero4DRT is capable of high-accuracy x-ray-image-based DTT. ET was caused primarily by EP, and EM was negligible. Furthermore, principal component analysis showed that tracking efficiency could be improved with this system, especially for patients with high respiratory function.
The authors express their appreciation to the entire technical staff at MHI for providing detailed information on the gimbaled MV x-ray head tracking system and acquiring experimental data. This research was funded by the Japan Society for the Promotion of Science (JSPS) through the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program), initiated by the Council for Science and Technology Policy (CSTP). This research was in part sponsored by Mitsubishi Heavy Industries Ltd., Japan. Takashi Mizowaki, Masaki Kokubo, and Masahiro Hiraoka have consultancy agreement with Mitsubishi Heavy Industries Ltd., Japan. This research was presented, in part, at the 2011 Joint American Association of Physicists in Medicine (AAPM) and the Canadian Organization of Medical Physicists (COMP) Meeting in Vancouver, 31 July–4 August 2011.
II. MATERIALS AND METHODS
II.A. System description of the DTT function in the Vero4DRT
II.B. Sequential prediction model
II.C. Experimental system to verify tracking accuracy
II.D. Characteristics of the motion patterns
III. RESULTS AND DISCUSSION
III.A. Periodic patterns
III.B. Aperiodic patterns
III.C. Comparison with other DTT irradiation techniques
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