title: Prefraction displacement and intrafraction drift of the prostate due to perineal ultrasound probe pressure.
abstract: OBJECTIVE
  In image-guided EBRT of the prostate, transperineal ultrasound (US) probes exert pressure on the perineum both during planning and treatment. Through tissue deformation and relaxation, this causes target and risk organ displacement and drift. In this study, prefraction shift and intrafraction drift of the prostate are quantified during robotic transperineal 4DUS.
  
  
  METHODS
  The position of the prostate was recorded for different positions of the probe before treatment in 10 patients (16 series of measurements). During treatment (15 patients, 273 fractions), intrafraction motion of the prostate was tracked (total of 27 h and 24 min) with the transperineal probe in place.
  
  
  RESULTS
  Per 1 mm shift of the US probe in the cranial direction, a displacement of the prostate by 0.42 ± 0.09 mm in the cranial direction was detected. The relationship was found to be linear (R² = 0.97) and highly significant (p < 0.0001). After initial contact of the probe and the perineum (no pressure), a shift of the probe of about 5-10 mm was typically necessary to achieve good image quality, corresponding to a shift of the prostate of about 2-4 mm in the cranial direction. Tissue compression and prostate displacement were well visible. During treatment, the prostate drifted at an average rate of 0.075 mm/min in the cranial direction (p = 0.0014).
  
  
  CONCLUSIONS
  The pressure applied by a perineal US probe has a quantitatively similar impact on prostate displacement as transabdominal pressure. Shifts are predominantly in the cranial direction (typically 2-4 mm) with some component in the anterior direction (typically <1 mm). Slight probe pressure can improve image quality, but excessive probe pressure can distort the surrounding anatomy and potentially move risk organs closer to the high-dose area.
authors.affiliation: Department of Radiation Oncology, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany.
authors.forename: Minglun
authors.initials: M
authors.lastname: Li
authors.affiliation: Department of Radiation Oncology, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany.
authors.forename: Nina-Sophie
authors.initials: NS
authors.lastname: Hegemann
authors.affiliation: Department of Radiation Oncology, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany.
authors.forename: Farkhad
authors.initials: F
authors.lastname: Manapov
authors.affiliation: Department of Radiation Oncology, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany.
authors.forename: Anne
authors.initials: A
authors.lastname: Kolberg
authors.affiliation: Department of Radiation Oncology, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany.
authors.forename: Patrick Dominik
authors.initials: PD
authors.lastname: Thum
authors.affiliation: Department of Radiation Oncology, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany.
authors.forename: Ute
authors.initials: U
authors.lastname: Ganswindt
authors.affiliation: Department of Radiation Oncology, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany.
authors.forename: Claus
authors.initials: C
authors.lastname: Belka
authors.affiliation: Department of Radiation Oncology, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany. Hendrik.Ballhausen@med.uni-muenchen.de.|University Hospital of LMU Munich, Lindwurmstrasse 2a, 80337, Munich, Germany. Hendrik.Ballhausen@med.uni-muenchen.de.
authors.forename: Hendrik
authors.identifier: http://orcid.org/0000-0003-4733-1926
authors.initials: H
authors.lastname: Ballhausen
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D016477:Artifacts; D019583:Dose Fractionation, Radiation; D006801:Humans; D007089:Image Enhancement; D008297:Male; D008875:Middle Aged; D058958:Organs at Risk / Q000528:radiation effects; D056888:Patient Positioning; D010502:Perineum / Q000000981:diagnostic imaging; D011312:Pressure; D011467:Prostate / Q000000981:diagnostic imaging* / Q000528:radiation effects*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011785:Quality Assurance, Health Care; D011879:Radiotherapy Dosage; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation*; D014159:Transducers; D014463:Ultrasonography / Q000295:instrumentation*
keywords: External beam radiotherapy; Intrafraction motion; Patient positioning; Quality assurance; Transperineal ultrasound