title: Intrafractional Baseline Shift or Drift of Lung Tumor Motion During Gated Radiation Therapy With a Real-Time Tumor-Tracking System.
abstract: OBJECTIVE
  To investigate the frequency and amplitude of baseline shift or drift (shift/drift) of lung tumors in stereotactic body radiation therapy (SBRT), using a real-time tumor-tracking radiation therapy (RTRT) system.
  
  
  METHODS
  Sixty-eight patients with peripheral lung tumors were treated with SBRT using the RTRT system. One of the fiducial markers implanted near the tumor was used for the real-time monitoring of the intrafractional tumor motion every 0.033 seconds by the RTRT system. When baseline shift/drift is determined by the system, the position of the treatment couch is adjusted to compensate for the shift/drift. Therefore, the changes in the couch position correspond to the baseline shift/drift in the tumor motion. The frequency and amount of adjustment to the couch positions in the left-right (LR), cranio-caudal (CC), and antero-posterior (AP) directions have been analyzed for 335 fractions administered to 68 patients.
  
  
  RESULTS
  The average change in position of the treatment couch during the treatment time was 0.45 ± 2.23 mm (mean ± standard deviation), -1.65 ± 5.95 mm, and 1.50 ± 2.54 mm in the LR, CC, and AP directions, respectively. Overall the baseline shift/drift occurs toward the cranial and posterior directions. The incidence of baseline shift/drift exceeding 3 mm was 6.0%, 15.5%, 14.0%, and 42.1% for the LR, CC, AP, and for the square-root of sum of 3 directions, respectively, within 10 minutes of the start of treatment, and 23.0%, 37.6%, 32.5%, and 71.6% within 30 minutes.
  
  
  CONCLUSIONS
  Real-time monitoring and frequent adjustments of the couch position and/or adding appropriate margins are suggested to be essential to compensate for possible underdosages due to baseline shift/drift in SBRT for lung cancers.
authors.affiliation: Department of Medical Physics, Hokkaido University Hospital, Sapporo, Japan.
authors.forename: Seishin
authors.initials: S
authors.lastname: Takao
authors.affiliation: Department of Medical Physics, Hokkaido University Hospital, Sapporo, Japan.
authors.forename: Naoki
authors.initials: N
authors.lastname: Miyamoto
authors.affiliation: Department of Medical Physics, Hokkaido University Hospital, Sapporo, Japan.
authors.forename: Taeko
authors.initials: T
authors.lastname: Matsuura
authors.affiliation: Department of Radiation Medicine, Hokkaido University Graduate School of Medicine, Sapporo, Japan.
authors.forename: Rikiya
authors.initials: R
authors.lastname: Onimaru
authors.affiliation: Department of Radiation Oncology, Hokkaido University Hospital, Sapporo, Japan.
authors.forename: Norio
authors.initials: N
authors.lastname: Katoh
authors.affiliation: Department of Radiation Oncology, Hokkaido University Hospital, Sapporo, Japan.
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Inoue
authors.affiliation: Department of Medical Physics, Hokkaido University Hospital, Sapporo, Japan.
authors.forename: Kenneth Lee
authors.initials: KL
authors.lastname: Sutherland
authors.affiliation: Department of Medical Physics, Hokkaido University Hospital, Sapporo, Japan.
authors.forename: Ryusuke
authors.initials: R
authors.lastname: Suzuki
authors.affiliation: Department of Radiation Medicine, Hokkaido University Graduate School of Medicine, Sapporo, Japan; Global Station for Quantum Medical Science and Engineering, Global Institution for Collaborative Research and Education, Hokkaido University, Sapporo, Japan.
authors.forename: Hiroki
authors.initials: H
authors.lastname: Shirato
authors.affiliation: Global Station for Quantum Medical Science and Engineering, Global Institution for Collaborative Research and Education, Hokkaido University, Sapporo, Japan; Department of Radiation Oncology, Hokkaido University Graduate School of Medicine, Sapporo, Japan. Electronic address: sshing@med.hokudai.ac.jp.
authors.forename: Shinichi
authors.initials: S
authors.lastname: Shimizu
mesh_terms: D002289:Carcinoma, Non-Small-Cell Lung / Q000473:pathology / Q000601:surgery*; D003199:Computer Systems; D019583:Dose Fractionation, Radiation; D057918:Fiducial Markers; D006801:Humans; D008175:Lung Neoplasms / Q000473:pathology / Q000601:surgery*; D009068:Movement; D056888:Patient Positioning / Q000295:instrumentation* / Q000706:statistics & numerical data; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D061106:Radiotherapy Setup Errors / Q000517:prevention & control; D012119:Respiration; D013997:Time Factors