title: Use of maximum intensity projections (MIP) for target volume generation in 4DCT scans for lung cancer.
abstract: OBJECTIVE
  Single four-dimensional CT (4DCT) scans reliably capture intrafractional tumor mobility for radiotherapy planning, but generating internal target volumes (ITVs) requires the contouring of gross tumor volumes (GTVs) in up to 10 phases of a 4DCT scan, as is routinely performed in our department. We investigated the use of maximum intensity projection (MIP) protocols for rapid generation of ITVs.
  
  
  METHODS
  4DCT data from a mobile phantom and from 12 patients with Stage I lung cancer were analyzed. A single clinician contoured GTVs in all respiratory phases of a 4DCT, as well as in three consecutive phases selected for respiratory gating. MIP images were generated from both phantom and patient data, and ITVs were derived from encompassing volumes of the respective GTVs.
  
  
  RESULTS
  In the phantom study, the ratio between ITVs generated from all 10 phases and those from MIP scans was 1.04. The corresponding center of mass of both ITVs differed by less than 1 mm. In scans from patients, good agreement was observed between ITVs derived from 10 and 3 (gating) phases and corresponding MIPs, with ratios of 1.07 +/- 0.05 and 0.98 +/- 0.05, respectively. In addition, the center of mass of the respective ITVs differed by only 0.4 and 0.5 mm.
  
  
  CONCLUSIONS
  MIPs are a reliable clinical tool for generating ITVs from 4DCT data sets, thereby permitting rapid assessment of mobility for both gated and nongated 4D radiotherapy in lung cancer.
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands.
authors.forename: René W M
authors.initials: RW
authors.lastname: Underberg
authors.forename: Frank J
authors.initials: FJ
authors.lastname: Lagerwaard
authors.forename: Ben J
authors.initials: BJ
authors.lastname: Slotman
authors.forename: Johan P
authors.initials: JP
authors.lastname: Cuijpers
authors.forename: Suresh
authors.initials: S
authors.lastname: Senan
mesh_terms: D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D009068:Movement; D019047:Phantoms, Imaging; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012119:Respiration; D012189:Retrospective Studies; D013679:Technology, Radiologic / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods*