title: Target repositioning optimization in prostate cancer: is intensity-modulated radiotherapy under stereotactic conditions feasible?
abstract: OBJECTIVE
  To assess repositioning reproducibility of the prostate when treatment setup conditions before radiotherapy (RT) are optimized and internal organ motion is reduced with an endorectal inflatable balloon.
  
  
  METHODS
  Thirty-two patients were treated with 64 Gy to the prostate and seminal vesicles using a three-dimensional conformal radiotherapy technique, followed by a boost (two fractions of 5-8 Gy, 3-5 days apart) delivered to a reduced prostate volume (the peripheral tumor bearing zone with 3-mm margins) using intensity-modulated RT. A commercially available infrared-guided stereotactic repositioning system and a rectal balloon were used. Further improvement in repositioning could be obtained with a stereoscopic X-ray registration device matching the pelvic bones during treatment with the corresponding bones in the planning computed tomography (CT). To simulate repositioning reproducibility, CT resimulation was performed before the last boost fraction. Prostate repositioning was reassessed, first after CT-to-CT fusion with the stereotactic metallic body markers of the infrared-guided system, and second after CT-to-CT registration of the pelvic bony structures.
  
  
  RESULTS
  Standard deviations of the prostate (CTV) center of mass shifts in the three axes ranged from 2.2 to 3.6 mm with body marker registration and from 0.9 to 2.5 mm with pelvic bone registration. The latter improvement was significant, particularly in the right-to-left axis (3.5-fold improvement). In 10 patients, systematic rectal probe repositioning errors (i.e., >20-mL probe volume variations or >8-mm probe shifts in the perpendicular axes) were detected. Target repositioning was reassessed excluding these 10 patients. An additional improvement was observed in the anteroposterior axis with 1.7 times and 1.5 times reduction of the standard deviation with body markers and pelvic bone registrations, respectively.
  
  
  CONCLUSIONS
  Infrared-guided target repositioning for prostate cancer can be optimized with a stereoscopic X-ray positioning device mostly in the right-to-left axis. An optimally positioned inflatable rectal probe further optimizes target repositioning mostly along the anteroposterior axis. Thus a planning target volume with a margin of 2 (right-to-left), 4 (anteroposteriorly), and 6 (craniocaudally) mm around the CTV can be recommended under optimal setup conditions with pelvic bone registration and optimal repositioning of an inflated rectal balloon.
authors.affiliation: Servei de Radio-Oncologia, Instituto Oncológico Teknon, Barcelona, Spain. Raymond.Miralbell@hcuge.ch
authors.forename: Raymond
authors.initials: R
authors.lastname: Miralbell
authors.forename: Meritxell
authors.initials: M
authors.lastname: Mollà
authors.forename: Raquel
authors.initials: R
authors.lastname: Arnalte
authors.forename: Sandra
authors.initials: S
authors.lastname: Canales
authors.forename: Emma
authors.initials: E
authors.lastname: Vargas
authors.forename: Dolores
authors.initials: D
authors.lastname: Linero
authors.forename: Sonia
authors.initials: S
authors.lastname: Waters
authors.forename: Philippe
authors.initials: P
authors.lastname: Nouet
authors.forename: Michel
authors.initials: M
authors.lastname: Rouzaud
authors.forename: Lluis
authors.initials: L
authors.lastname: Escudé
mesh_terms: D006801:Humans; D008297:Male; D009068:Movement; D010865:Pilot Projects; D011467:Prostate; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000379:methods*; D013238:Stereotaxic Techniques