title: Limited role for biliary stent as surrogate fiducial marker in pancreatic cancer: stent and intratumoral fiducials compared.
abstract: OBJECTIVE
  Because of low soft-tissue contrast of cone beam computed tomography (CBCT), fiducial markers are often used for radiation therapy patient setup verification. For pancreatic cancer patients, biliary stents have been suggested as surrogate fiducials. Using intratumoral fiducials as standard for tumor position, this study aims to quantify the suitability of biliary stents for measuring interfractional and respiratory-induced position variations of pancreatic tumors.
  
  
  METHODS
  Eleven pancreatic cancer patients with intratumoral fiducials and a biliary stent were included in this study. Daily CBCT scans (243 in total) were registered with a reference CT scan, based on bony anatomy, on fiducial markers, and on the biliary stent, respectively. We analyzed the differences in tumor position (ie, markers center-of-mass position) among these 3 registrations. In addition, we measured for 9 patients the magnitude of respiratory-induced motion (MM) of the markers and of the stent on 4-dimensional CT (4DCT) and determined the difference between these 2 magnitudes (ΔMM).
  
  
  RESULTS
  The stent indicated tumor position better than bony anatomy in 67% of fractions; the absolute difference between the markers and stent registration was >5 mm in 46% of fractions and >10 mm in 20% of fractions. Large PTV margins (superior-inferior direction, >19 mm) would be needed to account for this interfractional position variability. On 4DCT, we found in superior-inferior direction a mean ΔMM of 0.5 mm (range, -2.6 to 4.2 mm).
  
  
  CONCLUSIONS
  For respiratory-induced motion, the mean ΔMM is small, but for individual patients the absolute difference can be >4 mm. For interfractional position variations, a stent is, on average, a better surrogate fiducial than bony anatomy, but large PTV margins would still be required. Therefore, intratumoral fiducials are recommended for online setup verification for all pancreatic patients scheduled for radiation therapy, including patients with a biliary stent.
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands. Electronic address: a.vanderhorst@amc.uva.nl.
authors.forename: Astrid
authors.initials: A
authors.lastname: van der Horst
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands.
authors.forename: Eelco
authors.initials: E
authors.lastname: Lens
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands.
authors.forename: Silvia
authors.initials: S
authors.lastname: Wognum
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands.
authors.forename: Rianne
authors.initials: R
authors.lastname: de Jong
authors.affiliation: Department of Gastroenterology and Hepatology, Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands.
authors.forename: Jeanin E
authors.initials: JE
authors.lastname: van Hooft
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands.
authors.forename: Geertjan
authors.initials: G
authors.lastname: van Tienhoven
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands.
authors.forename: Arjan
authors.initials: A
authors.lastname: Bel
mesh_terms: D000328:Adult; D000368:Aged; D059925:Anatomic Landmarks / Q000000981:diagnostic imaging*; D001652:Bile Ducts; D059248:Chemoradiotherapy / Q000379:methods; D054893:Cone-Beam Computed Tomography; D019583:Dose Fractionation, Radiation; D005260:Female; D057918:Fiducial Markers; D006046:Gold; D006801:Humans; D008297:Male; D008875:Middle Aged; D009068:Movement; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging* / Q000628:therapy; D012119:Respiration; D012189:Retrospective Studies; D015607:Stents; D014057:Tomography, X-Ray Computed
chemical_list: D006046:Gold