To verify the accuracy of an infrared (IR) marker-based dynamic tumor-tracking irradiation system (IR tracking) using the gimbaled x-ray head of the Vero4DRT (MHI-TM2000).
The gimbaled 6-MV C-band x-ray head of the Vero4DRT can swing along the pan-and-tilt direction to track a moving target. During beam delivery, the Vero4DRT predicts the future three-dimensional (3D) target position in real time using a correlation model [four-dimensional (4D) model] between the target and IR marker motion, and then continuously transfers the corresponding tracking orientation to the gimbaled x-ray head. The 4D-modeling error (E 4DM) and the positional tracking error (E P ) were defined as the difference between the predicted and measured positions of the target in 4D modeling and as the difference between the tracked and measured positions of the target during irradiation, respectively. For the clinical application of IR tracking, we assessed the relationship between E 4DM and E P for three 1D sinusoidal (peak-to-peak amplitude [A]: 20–40 mm, breathing period [T]: 2–4 s), five 1D phase-shifted sinusoidal (A: 20 mm, T: 4 s, phase shift [τ]: 0.2–2 s), and six 3D patient respiratory patterns.
The difference between the 95th percentile of the absoluteE P ( ) and the mean (μ) + two standard deviations (SD) of absolute E 4DM ( ) was within ±1 mm for all motion patterns. As the absolute correlation between the target and IR marker motions decreased from 1.0 to 0.1 for the 1D phase-shifted sinusoidal patterns, the and increased linearly, from 0.4 to 3.0 mm (R = −0.98) and from 0.5 to 2.2 mm (R = −0.95), respectively. There was a strong positive correlation between and in each direction [(lateral, craniocaudal, anteroposterior) = (0.99, 0.98, 1.00)], even for the 3D respiratory patterns; thus, was readily estimated from .
Positional tracking errors correlated strongly with 4D-modeling errors in IR tracking. Thus, the accuracy of the 4D model must be verified before treatment, and margins are required to compensate for the 4D-modeling error.
The authors express their appreciation to the entire technical staff at MHI for providing detailed information on the gimbaled x-ray head-tracking system and acquiring the experimental data. This research was supported by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program)” and a grant-in-aid for Scientific Research from the Association for Nuclear Technology in Medicine. This research was in part sponsored by Mitsubishi Heavy Industries, Ltd., Japan. Takashi Mizowaki, Masaki Kokubo, and Masahiro Hiraoka have consultancy agreements with Mitsubishi Heavy Industries, Ltd., Japan.
II. MATERIALS AND METHODS
II.A. The 4D model for IR tracking with the Vero4DRT
II.B. Dosimetric and positional verification of IR tracking
II.B.1. Dosimetric and positional verification of IR tracking for 1D sinusoidal patterns
II.B.2. Positional verification of IR tracking for 1D phase-shifted sinusoidal patterns
II.B.3. Positional verification of IR tracking for 3D respiratory patterns
III. RESULTS AND DISCUSSION
III.A. Dosimetric and positional verification of IR tracking for 1D sinusoidal patterns
III.B. Positional verification of IR tracking for 1D phase-shifted sinusoidal patterns
III.C. Positional verification of IR tracking for 3D respiratory patterns
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