title: Interfractional position variation of pancreatic tumors quantified using intratumoral fiducial markers and daily cone beam computed tomography.
abstract: OBJECTIVE
  The aim of this study was to quantify interfractional pancreatic position variation using fiducial markers visible on daily cone beam computed tomography (CBCT) scans. In addition, we analyzed possible migration of the markers to investigate their suitability for tumor localization.
  
  
  METHODS
  For 13 pancreatic cancer patients with implanted Visicoil markers, CBCT scans were obtained before 17 to 25 fractions (300 CBCTs in total). Image registration with the reference CT was used to determine the displacement of the 2 to 3 markers relative to bony anatomy and to each other. We analyzed the distance between marker pairs as a function of time to identify marker registration error (SD of linear fit residuals) and possible marker migration. For each patient, we determined the mean displacement of markers relative to the reference CT (systematic position error) and the spread in displacements (random position error). From this, we calculated the group systematic error, Σ, and group random error, σ.
  
  
  RESULTS
  Marker pair distances showed slight trends with time (range, -0.14 to 0.14 mm/day), possibly due to tissue deformation, but no shifts that would indicate marker migration. The mean SD of the fit residuals was 0.8 mm. We found large interfractional position variations, with for 116 of 300 (39%) fractions a 3-dimensional vector displacement of >10 mm. The spread in displacement varied significantly (P<.01) between patients, from a vector range of 9.1 mm to one of 24.6 mm. For the patient group, Σ was 3.8, 6.6, and 3.5 mm; and σ was 3.6, 4.7 and 2.5 mm, in left-right, superior-inferior, and anterior-posterior directions, respectively.
  
  
  CONCLUSIONS
  We found large systematic displacements of the fiducial markers relative to bony anatomy, in addition to wide distributions of displacement. These results for interfractional position variation confirm the potential benefit of using fiducial markers rather than bony anatomy for daily online position verification for pancreatic cancer patients.
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands. a.vanderhorst@amc.uva.nl
authors.forename: Astrid
authors.initials: A
authors.lastname: van der Horst
authors.forename: Silvia
authors.initials: S
authors.lastname: Wognum
authors.forename: Raquel
authors.initials: R
authors.lastname: Dávila Fajardo
authors.forename: Rianne
authors.initials: R
authors.lastname: de Jong
authors.forename: Jeanin E
authors.initials: JE
authors.lastname: van Hooft
authors.forename: Paul
authors.initials: P
authors.lastname: Fockens
authors.forename: Geertjan
authors.initials: G
authors.lastname: van Tienhoven
authors.forename: Arjan
authors.initials: A
authors.lastname: Bel
mesh_terms: D000328:Adult; D000368:Aged; D059925:Anatomic Landmarks / Q000000981:diagnostic imaging*; D054893:Cone-Beam Computed Tomography; D019583:Dose Fractionation, Radiation; D005260:Female; D057918:Fiducial Markers; D005548:Foreign-Body Migration / Q000000981:diagnostic imaging; D006801:Humans; D008297:Male; D008875:Middle Aged; D009068:Movement; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012189:Retrospective Studies; D013997:Time Factors