title: Endobiliary stent: marker for patient alignment in image-guided radiotherapy in pancreatic and periampullary cancers.
abstract: OBJECTIVE
  The aim of this study was to analyse the possibility of using stent in pretreatment megavoltage computed tomography (MVCT) images with respect to that on planning kilovoltage computed tomography as tumour surrogate during matching for daily registration in cases of pancreatic and periampullary cancer treated on a TomoTherapy Hi-Art system.
  
  
  METHODS
  Planning CT and pretreatment MVCT of the first and then after every three fractions were transferred to a FocalSim workstation for ten patients. Planning CT of each patient was independently fused with each of the seven MVCT images of that patient. The stent was contoured on all of the eight images for each patient. The difference between the three co-ordinates of centre of mass (CM) of the stent on the planning CT and seven MVCT images was found. The difference between CM of the liver and stents on the planning CT as well as on the MVCT for all seven fractions was also calculated. The mean of these differences across all patients was calculated and analysed.
  
  
  RESULTS
  The mean difference in planning and MVCT CMs for stents in the X, Y and Z directions was 0.13 cm (±0.4), 0.16 cm (±2.2) and 0.35 cm (±0.7), respectively. Average difference between CM of the liver and stent on the planning CT in the X, Y and Z directions was found to be 1.832 cm (±1.64), 5.34 cm (±1.33) and 0.54 cm (±0.26), respectively. Average difference between CM of the liver and CM of stent on the MVCT for that day in the X, Y and Z directions was found to be 1.93 cm (±1.5), 4.6 cm (±1.03) and 0.654 cm (±0.35), respectively.
  
  
  CONCLUSIONS
  Endobiliary stents are stable tumour localisation surrogates and can be used to correct for interfraction target motion.
authors.affiliation: Departments of Radiation Oncology and Medical Physics, Tata Memorial Hospital, Tata Memorial Centre, Parel, Mumbai, 400012, India.
authors.forename: Pragya
authors.initials: P
authors.lastname: Shukla
authors.forename: Reena
authors.initials: R
authors.lastname: Engineer
authors.forename: Supriya
authors.initials: S
authors.lastname: Chopra
authors.forename: Shyam Kishore
authors.initials: SK
authors.lastname: Shrivastava
mesh_terms: D014670:Ampulla of Vater / Q000528:radiation effects*; D003138:Common Bile Duct Neoplasms / Q000532:radiotherapy*; D057918:Fiducial Markers; D006801:Humans; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted; D061089:Radiotherapy, Image-Guided; D050397:Radiotherapy, Intensity-Modulated; D015607:Stents; D036542:Tomography, Spiral Computed; D014057:Tomography, X-Ray Computed