title: A comparison of internal target volume definition by limited four-dimensional computed tomography, the addition of patient-specific margins, or the addition of generic margins when planning radical radiotherapy for lymph node-positive non-small cell lung cancer.
abstract: OBJECTIVE
  Radical radiotherapy for stage II/III non-small cell lung cancer (NSCLC) includes the primary tumour and positive mediastinal lymph nodes in the clinical target volume (CTV). These move independently of each other in magnitude and direction during respiration. To prevent a geographical miss, a generic margin is usually added to the CTV to create an internal target volume (ITV). Previous studies have investigated the use of additional breath-hold computed tomography to generate patient-specific ITVs for primary tumours alone. We used a similar technique to investigate the generation of patient-specific and generic ITVs for CTVs that include mediastinal lymph nodes.
  
  
  METHODS
  Thirteen patients with node-positive NSCLC had two limited end-tidal breath-hold computed tomography scans in addition to their planning computed tomography. The CTV was segmented in each scan and a rigid registration was carried out on the vertebral columns to align them. Different methods for generating an ITV were then analysed.
  
  
  RESULTS
  Generic margins provided >95% mean coverage of the reference ITV. However, with the exception of 1cm expansion margins, there were cases of inadequate coverage (<95%) for each ITV. With increasing ITV margins there was a small increase in reference ITV coverage, but at the expense of a large increase in the volume of normal tissue within the ITV.
  
  
  CONCLUSIONS
  For stage II/III NSCLC, ITV generation by the addition of a generic margin is not optimal. It can result in both geographical miss and excessive irradiation of normal tissue in the same treatment plan. A simple method for producing a patient-specific ITV is to co-register end-tidal breath-hold computed tomography scans to the planning scan.
  
  
  CONCLUSIONS
  Further work is required to determine whether end-tidal breath-hold scans are representative of the anatomy at the limits of tidal respiration. Planning strategies are also needed to account for breathing cycle variation during a course of radiotherapy.
authors.affiliation: Division of Imaging Sciences, King's College London, London, UK. simon.hughes@uclh.nhs.uk <simon.hughes@uclh.nhs.uk>
authors.forename: S
authors.initials: S
authors.lastname: Hughes
authors.forename: J
authors.initials: J
authors.lastname: McClelland
authors.forename: A
authors.initials: A
authors.lastname: Chandler
authors.forename: M
authors.initials: M
authors.lastname: Adams
authors.forename: J
authors.initials: J
authors.lastname: Boutland
authors.forename: D
authors.initials: D
authors.lastname: Withers
authors.forename: S
authors.initials: S
authors.lastname: Ahmad
authors.forename: J
authors.initials: J
authors.lastname: Blackall
authors.forename: S
authors.initials: S
authors.lastname: Tarte
authors.forename: D
authors.initials: D
authors.lastname: Hawkes
authors.forename: D
authors.initials: D
authors.lastname: Landau
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D002289:Carcinoma, Non-Small-Cell Lung / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D005260:Female; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D008207:Lymphatic Metastasis; D008297:Male; D008482:Mediastinum; D008875:Middle Aged; D011859:Radiography; D011880:Radiotherapy Planning, Computer-Assisted; D047368:Tumor Burden