title: Localization accuracy of the clinical target volume during image-guided radiotherapy of lung cancer.
abstract: OBJECTIVE
  To evaluate the position and shape of the originally defined clinical target volume (CTV) over the treatment course, and to assess the impact of gross tumor volume (GTV)-based online computed tomography (CT) guidance on CTV localization accuracy.
  
  
  METHODS
  Weekly breath-hold CT scans were acquired in 17 patients undergoing radiotherapy. Deformable registration was used to propagate the GTV and CTV from the first weekly CT image to all other weekly CT images. The on-treatment CT scans were registered rigidly to the planning CT scan based on the GTV location to simulate online guidance, and residual error in the CTV centroids and borders was calculated.
  
  
  RESULTS
  The mean GTV after 5 weeks relative to volume at the beginning of treatment was 77% ± 20%, whereas for the prescribed CTV, it was 92% ± 10%. The mean absolute residual error magnitude in the CTV centroid position after a GTV-based localization was 2.9 ± 3.0 mm, and it varied from 0.3 to 20.0 mm over all patients. Residual error of the CTV centroid was associated with GTV regression and anisotropy of regression during treatment (p = 0.02 and p = 0.03, respectively; Spearman rank correlation). A residual error in CTV border position greater than 2 mm was present in 77% of patients and 50% of fractions. Among these fractions, residual error of the CTV borders was 3.5 ± 1.6 mm (left-right), 3.1 ± 0.9 mm (anterior-posterior), and 6.4 ± 7.5 mm (superior-inferior).
  
  
  CONCLUSIONS
  Online guidance based on the visible GTV produces substantial error in CTV localization, particularly for highly regressing tumors. The results of this study will be useful in designing margins for CTV localization or for developing new online CTV localization strategies.
authors.affiliation: Department of Radiation Oncology, Virginia Commonwealth University, Richmond, VA 23298, USA. gdhugo@vcu.edu
authors.forename: Geoffrey D
authors.initials: GD
authors.lastname: Hugo
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Weiss
authors.forename: Ahmed
authors.initials: A
authors.lastname: Badawi
authors.forename: Matthew
authors.initials: M
authors.lastname: Orton
mesh_terms: D016880:Anisotropy; D002289:Carcinoma, Non-Small-Cell Lung / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D009068:Movement; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012074:Remission Induction / Q000379:methods; D012119:Respiration; D055752:Small Cell Lung Carcinoma / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D013997:Time Factors; D036542:Tomography, Spiral Computed / Q000379:methods; D047368:Tumor Burden