title: Tumor, lymph node, and lymph node-to-tumor displacements over a radiotherapy series: analysis of interfraction and intrafraction variations using active breathing control (ABC) in lung cancer.
abstract: OBJECTIVE
  To estimate errors in soft tissue-based image guidance due to relative changes between primary tumor (PT) and affected lymph node (LN) position and volume, and to compare the results with bony anatomy-based displacements of PTs and LNs during radiotherapy of lung cancer.
  
  
  METHODS
  Weekly repeated breath-hold computed tomography scans were acquired in 17 lung cancer patients undergoing radiotherapy. PTs and affected LNs were manually contoured on all scans after rigid registration. Interfraction and intrafraction displacements in the centers of mass of PTs and LNs relative to bone, as well as LNs relative to PTs (LN-PT), were calculated.
  
  
  RESULTS
  The mean volume after 5 weeks was 65% for PTs and 63% for LNs. Systematic and random interfraction displacements were 2.6 to 4.6 mm and 2.7 to 2.9 mm, respectively, for PTs; 2.4 to 3.8 mm and 1.4 to 2.7 mm, respectively, for LNs; and 2.3 to 3.9 mm and 1.9 to 2.8 mm, respectively, for LN-PT. Systematic and random intrafraction displacements were less than 1 mm except in the superoinferior direction. Interfraction LN-PT displacements greater than 3 mm were observed in 67% of fractions and require a safety margin of 12 mm in the lateral direction, 11 mm in the anteroposterior direction, and 9 mm in the superoinferior direction. LN-PT displacements displayed significant time trends (p < 0.0001) and depended on the presence of pathoanatomic conditions of the ipsilateral lung, such as atelectasis.
  
  
  CONCLUSIONS
  Interfraction LN-PT displacements were mostly systematic and comparable to bony anatomy-based displacements of PTs or LNs alone. Time trends, large volume changes, and the influence of pathoanatomic conditions underline the importance of soft tissue-based image guidance and the potential of plan adaptation.
authors.affiliation: Department of Radiation Oncology, Virginia Commonwealth University, Richmond, VA 23298, USA. eweiss@mcvh-vcu.edu
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Weiss
authors.forename: Scott P
authors.initials: SP
authors.lastname: Robertson
authors.forename: Nitai
authors.initials: N
authors.lastname: Mukhopadhyay
authors.forename: Geoffrey D
authors.initials: GD
authors.lastname: Hugo
mesh_terms: D059925:Anatomic Landmarks / Q000000981:diagnostic imaging*; D001842:Bone and Bones / Q000000981:diagnostic imaging; D019583:Dose Fractionation, Radiation; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging; D008175:Lung Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy; D008198:Lymph Nodes / Q000000981:diagnostic imaging*; D008482:Mediastinum / Q000000981:diagnostic imaging; D009068:Movement; D011446:Prospective Studies; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061106:Radiotherapy Setup Errors; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D012119:Respiration; D014057:Tomography, X-Ray Computed / Q000379:methods; D047368:Tumor Burden