title: Modeling set-up error by daily MVCT for prostate adjuvant treatment delivered in 20 fractions: Implications for the assessment of the optimal correction strategies.
abstract: OBJECTIVE
  To optimise image-guided radiotherapy (IGRT) approaches in set-up error correction for hypofractionated adjuvant treatment of prostate cancer using helical tomotherapy (HT).
  
  
  METHODS
  HT unit provides megavoltage - CT (MVCT) imaging capability for daily patient set-up. The intrinsic uncertainty of the available image registration techniques was first investigated on a pelvic phantom. To optimise different set-up correction strategies, MVCT data of 37 patients treated with 58 Gy delivered in 20 fractions were analyzed. 620 MVCT images were globally available for the analysis. Daily on-line set-up correction without any action level was applied for 27/37 patients. The residual and the effective set-up error were assessed. For 10/37 patients an off-line correction strategy with an action level was applied.
  
  
  RESULTS
  Phantom measurements showed an intrinsic uncertainty in MVCT imaging and registration around 0.6 mm (+/-0.5 mm) for each of the three main axes. The minimum value of the residual systematic error was found at 7th-10th session, with values between 0.7 and 1.1 mm (1 SD); a systematic residual error <2 mm for all directions was in any case found at 3rd-4th session. This result was also confirmed by the effective systematic set-up error analysis, with a minimum value within 1.5mm nearly at the 4th or 5th fraction.
  
  
  CONCLUSIONS
  Although the minimum systematic residual error is reached at 7th-10th treatment session, the effective systematic set-up error analysis confirmed that an off-line correction at the 4th fraction without any action level may be a robust compromise with a large sparing of time and resources compared to on-line correction.
authors.affiliation: Medical Physics, IRCCS S. Raffaele, Milano, Italy. broggi.sara@hsr.it
authors.forename: Sara
authors.initials: S
authors.lastname: Broggi
authors.forename: Cesare
authors.initials: C
authors.lastname: Cozzarini
authors.forename: Claudio
authors.initials: C
authors.lastname: Fiorino
authors.forename: Eleonora
authors.initials: E
authors.lastname: Maggiulli
authors.forename: Filippo
authors.initials: F
authors.lastname: Alongi
authors.forename: Giovanni Mauro
authors.initials: GM
authors.lastname: Cattaneo
authors.forename: Nadia
authors.initials: N
authors.lastname: Di Muzio
authors.forename: Riccardo
authors.initials: R
authors.lastname: Calandrino
mesh_terms: D019583:Dose Fractionation, Radiation; D006801:Humans; D008297:Male; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D018714:Radiotherapy, Adjuvant; D036542:Tomography, Spiral Computed / Q000379:methods*