title: Accuracy verification of infrared marker-based dynamic tumor-tracking irradiation using the gimbaled x-ray head of the Vero4DRT (MHI-TM2000).
abstract: OBJECTIVE
  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).
  
  
  METHODS
  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 (E4DM) and the positional tracking error (EP) 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 E4DM and EP 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.
  
  
  RESULTS
  The difference between the 95th percentile of the absolute EP (EP (95)) and the mean (μ) + two standard deviations (SD) of absolute E4DM (E4DM (μ+2SD)) 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 E4DM (μ+2SD) and EP (95) 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 E4DM (μ+2SD) and EP (95) in each direction [(lateral, craniocaudal, anteroposterior) = (0.99, 0.98, 1.00)], even for the 3D respiratory patterns; thus, EP (95) was readily estimated from E4DM (μ+2SD).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Radiation Oncology and Image-applied Therapy, Graduate School of Medicine, Kyoto University, Kyoto 606-8507, Japan.
authors.forename: Nobutaka
authors.initials: N
authors.lastname: Mukumoto
authors.forename: Mitsuhiro
authors.initials: M
authors.lastname: Nakamura
authors.forename: Akira
authors.initials: A
authors.lastname: Sawada
authors.forename: Yasunobu
authors.initials: Y
authors.lastname: Suzuki
authors.forename: Kunio
authors.initials: K
authors.lastname: Takahashi
authors.forename: Yuki
authors.initials: Y
authors.lastname: Miyabe
authors.forename: Shuji
authors.initials: S
authors.lastname: Kaneko
authors.forename: Takashi
authors.initials: T
authors.lastname: Mizowaki
authors.forename: Masaki
authors.initials: M
authors.lastname: Kokubo
authors.forename: Masahiro
authors.initials: M
authors.lastname: Hiraoka
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D057918:Fiducial Markers; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000295:instrumentation*; D007259:Infrared Rays; D009369:Neoplasms / Q000175:diagnosis* / Q000532:radiotherapy*; D010363:Pattern Recognition, Automated / Q000379:methods; D019047:Phantoms, Imaging; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013817:Thermography / Q000295:instrumentation*; D014057:Tomography, X-Ray Computed / Q000295:instrumentation*