title: [Discrepancy between the intended isocenter and that from orthogonal linacgrams in the treatment of CT-guided stereotactic body radiotherapy(SBRT)].
abstract: OBJECTIVE
  In our institution a CT scanner was installed in the same room as the linear accelerator. In stereotactic body radiotherapy (SBRT) we confirmed the isocenter position by serial thin-slice and long-scan-time CT images before every treatment as well as in planning. In planning we constructed digitally reconstructed radiography (DRR) of both the anterior and lateral views. At the first treatment we also checked the isocenter with linacgraphy. Then we compared the isocenter positions obtained from the DRR and linacgraphy.
  
  
  METHODS
  Between Feb. 2005 and Oct. 2006, we treated 75 lung and liver tumors with SBRT in this way. Based on bony structures, we measured the differences between in-isocenter positions for SI, LR, and AP directions between DRR and linacgraphy.
  
  
  RESULTS
  The median (min-max) of the differences in-isocenter positions for SI, LR, and AP directions between DRR and linacgraphy were 0.0 mm (0-6.0), 0.0 mm (0-10.0), and 0.0 mm (0-10.0), respectively, as well as 3.2 mm (0-12.3) for 3-dimensional distance. In 28 tumors (37%) the differences exceeded 5 mm in three-dimensional distance. The frequency of differences exceeding 5 mm in upper lung lesions tended to be more than that in liver lesions, and that in left pulmonary lesions was significantly more than that in right ones.
  
  
  CONCLUSIONS
  This result suggests that the relative position of the target volume to the bony structure differ in planning and in every treatment. It was recommended to verify isocenter accuracy in institutions where isocenter position is checked only by orthogonal linacgraphy in SBRT.
authors.affiliation: Department of Radiology, Ofuna Chuo Hospital, Japan.
authors.forename: Yoshikazu
authors.initials: Y
authors.lastname: Oooka
authors.forename: Atsuya
authors.initials: A
authors.lastname: Takeda
authors.forename: Yasunobu
authors.initials: Y
authors.lastname: Sudo
authors.forename: Hideo
authors.initials: H
authors.lastname: Iwashita
authors.forename: Yousuke
authors.initials: Y
authors.lastname: Aoki
authors.forename: Tetsurou
authors.initials: T
authors.lastname: Katayose
authors.forename: Noriaki
authors.initials: N
authors.lastname: Sasajima
authors.forename: Naoko
authors.initials: N
authors.lastname: Sanuki
authors.forename: Toshio
authors.initials: T
authors.lastname: Oohashi
authors.forename: Etsuo
authors.initials: E
authors.lastname: Kunieda
authors.forename: Tomoaki
authors.initials: T
authors.lastname: Nagaoka
mesh_terms: D006801:Humans; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D014057:Tomography, X-Ray Computed