title: Cone-beam computed tomography for on-line image guidance of lung stereotactic radiotherapy: localization, verification, and intrafraction tumor position.
abstract: OBJECTIVE
  Cone-beam computed tomography (CBCT) in-room imaging allows accurate inter- and intrafraction target localization in stereotactic body radiotherapy of lung tumors.
  
  
  METHODS
  Image-guided stereotactic body radiotherapy was performed in 28 patients (89 fractions) with medically inoperable Stage T1-T2 non-small-cell lung carcinoma. The targets from the CBCT and planning data set (helical or four-dimensional CT) were matched on-line to determine the couch shift required for target localization. Matching based on the bony anatomy was also performed retrospectively. Verification of target localization was done using either megavoltage portal imaging or CBCT imaging; repeat CBCT imaging was used to assess the intrafraction tumor position.
  
  
  RESULTS
  The mean three-dimensional tumor motion for patients with upper lesions (n = 21) and mid-lobe or lower lobe lesions (n = 7) was 4.2 and 6.7 mm, respectively. The mean difference between the target and bony anatomy matching using CBCT was 6.8 mm (SD, 4.9, maximum, 30.3); the difference exceeded 13.9 mm in 10% of the treatment fractions. The mean residual error after target localization using CBCT imaging was 1.9 mm (SD, 1.1, maximum, 4.4). The mean intrafraction tumor deviation was significantly greater (5.3 mm vs. 2.2 mm) when the interval between localization and repeat CBCT imaging (n = 8) exceeded 34 min.
  
  
  CONCLUSIONS
  In-room volumetric imaging, such as CBCT, is essential for target localization accuracy in lung stereotactic body radiotherapy. Imaging that relies on bony anatomy as a surrogate of the target may provide erroneous results in both localization and verification.
authors.affiliation: Department of Radiation Physics, Princess Margaret Hospital, Toronto, ON, Canada. Tom.Purdie@rmp.uhn.on.ca
authors.forename: Thomas G
authors.initials: TG
authors.lastname: Purdie
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Bissonnette
authors.forename: Kevin
authors.initials: K
authors.lastname: Franks
authors.forename: Andrea
authors.initials: A
authors.lastname: Bezjak
authors.forename: David
authors.initials: D
authors.lastname: Payne
authors.forename: Fanny
authors.initials: F
authors.lastname: Sie
authors.forename: Michael B
authors.initials: MB
authors.lastname: Sharpe
authors.forename: David A
authors.initials: DA
authors.lastname: Jaffray
mesh_terms: D002289:Carcinoma, Non-Small-Cell Lung / Q000000981:diagnostic imaging / Q000601:surgery*; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000601:surgery*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D013238:Stereotaxic Techniques; D036542:Tomography, Spiral Computed