title: Intra-fractional uncertainties in image-guided intensity-modulated radiotherapy (IMRT) of prostate cancer.
abstract: OBJECTIVE
  To evaluate intra-fractional uncertainties during intensity-modulated radiotherapy (IMRT) of prostate cancer.
  
  
  METHODS
  During IMRT of 21 consecutive patients, kilovolt (kV) cone-beam computed tomography (CBCT) images were acquired prior to and immediately after treatment: a total of 252 treatment fractions with 504 CBCT studies were basis of this analysis. The prostate position in anterior-posterior (AP) direction was determined using contour matching; patient set-up based on the pelvic bony anatomy was evaluated using automatic image registration. Internal variability of the prostate position was the difference between absolute prostate and patient position errors. Intra-fractional changes of prostate position, patient position, rectal distension in AP direction and bladder volume were analyzed.
  
  
  RESULTS
  With a median treatment time of 16 min, intra-fractional drifts of the prostate were >5 mm in 12% of all fractions and a margin of 6 mm was calculated for compensation of this uncertainty. Mobility of the prostate was independent from the bony anatomy with poor correlation between absolute prostate motion and motion of the bony anatomy (R2=0.24). A systematic increase of bladder filling by 41 ccm on average was observed; however, these changes did not influence the prostate position. Small variations of the prostate position occurred independently from intra-fractional changes of the rectal distension; a weak correlation between large internal prostate motion and changes of the rectal volume was observed (R2=0.55).
  
  
  CONCLUSIONS
  Clinically significant intra-fractional changes of the prostate position were observed and margins of 6 mm were calculated for this intra-fractional uncertainty. Repeated or continuous verification of the prostate position may allow further margin reduction.
authors.affiliation: Department of Radiation Oncology, Julius Maximilian University, Würzburg, Germany.
authors.forename: Buelent
authors.initials: B
authors.lastname: Polat
authors.forename: Iris
authors.initials: I
authors.lastname: Guenther
authors.forename: Juergen
authors.initials: J
authors.lastname: Wilbert
authors.forename: Joachim
authors.initials: J
authors.lastname: Goebel
authors.forename: Reinhart A
authors.initials: RA
authors.lastname: Sweeney
authors.forename: Michael
authors.initials: M
authors.lastname: Flentje
authors.forename: Matthias
authors.initials: M
authors.lastname: Guckenberger
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D008297:Male; D009068:Movement; D010384:Pelvic Bones / Q000000981:diagnostic imaging; D011187:Posture; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012007:Rectum / Q000000981:diagnostic imaging; D013997:Time Factors