title: Modeling positioning uncertainties of prostate cancer external beam radiation therapy using pre-treatment data.
abstract: OBJECTIVE
  To investigate the influence of treatment plan data and image guidance (IG) on positioning uncertainty during prostate cancer (PCa) radiotherapy (RT).
  
  
  METHODS
  Body mass index (BMI), planning target volume (PTV), bladder volume (BV), and rectal cross section area (RCS) were collected for 267 consecutive PCa patients undergoing daily IGRT. Radiographic isocenter corrections to intra-prostatic fiducials for 12,490 treatment fractions were used to derive random (RE) and systematic (SE) inter-fraction uncertainties for the cardinal axes. These data were used to simulate RE and SE for weekly IG and Action Level (AL)-IG treatment protocols.
  
  
  RESULTS
  SE and RE were 2-5 and 3-4mm in the cardinal axes, respectively, during simulation of no IG. Without IG, positive correlations (p<0.01) were noted for (1) anterior-posterior RE vs. RCS and BV and (2) cranio-caudal RE vs. RCS, BV and BMI. The RE increase was 3mm for the highest quartile of RCS, BV and BMI. Daily IGRT eliminated this relationship. 3D IG corrections of 1cm or more occured in 27% of treatment fractions and in 97% of patients.
  
  
  CONCLUSIONS
  PCa patients with elevated pre-treatment BV, RCS and BMI have increased inter-fractionation positioning uncertainty and appear the primary candidates for daily IGRT.
authors.affiliation: Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, USA; Department of Radiation Oncology, Radiation Medicine Research Center, Copenhagen, Denmark; Niels Bohr Institute, University of Copenhagen, Denmark.
authors.forename: Per
authors.initials: P
authors.lastname: Munck af Rosenschöld
authors.affiliation: Department of Radiation Oncology, Memorial Sloan-Kettering Cancer Center, USA.
authors.forename: Neil B
authors.initials: NB
authors.lastname: Desai
authors.affiliation: Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, USA.
authors.forename: Jung Hun
authors.initials: JH
authors.lastname: Oh
authors.affiliation: Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, USA.
authors.forename: Aditya
authors.initials: A
authors.lastname: Apte
authors.affiliation: Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, USA.
authors.forename: Margie
authors.initials: M
authors.lastname: Hunt
authors.affiliation: Department of Radiation Oncology, Memorial Sloan-Kettering Cancer Center, USA.
authors.forename: Abraham
authors.initials: A
authors.lastname: Kalikstein
authors.affiliation: Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, USA.
authors.forename: James
authors.initials: J
authors.lastname: Mechalakos
authors.affiliation: Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, USA.
authors.forename: Laura
authors.initials: L
authors.lastname: Happersett
authors.affiliation: Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, USA.
authors.forename: Joseph O
authors.initials: JO
authors.lastname: Deasy
authors.affiliation: Department of Radiation Oncology, Memorial Sloan-Kettering Cancer Center, USA. Electronic address: zelefskm@mskcc.org.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Zelefsky
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D057918:Fiducial Markers; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D015999:Multivariate Analysis; D056888:Patient Positioning; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061089:Radiotherapy, Image-Guided; D012189:Retrospective Studies; D035501:Uncertainty
keywords: IGRT; Image guided radiotherapy; Interfraction motion; Prostate cancer