title: An image-based method to quantify biomechanical properties of the rectum in radiotherapy of prostate cancer.
abstract: BACKGROUND
  Gastrointestinal morbidity after radiotherapy (RT) for prostate cancer may be related to the biomechanical properties of the rectum. In this study we present a magnetic resonance imaging (MRI)-based method to quantitate the thickness and elasticity of the rectal wall in prostate cancer patients treated with RT.
  
  
  METHODS
  Four patients previously treated with RT for prostate cancer underwent an MRI session with stepwise rectal bag deflation (from a maximum tolerable volume to 0 ml, in 50 ml steps), with a probe inserted inside the bag to monitor the internal rectal pressure. MRIs were acquired using Dixon sequences (4 mm axial slice thickness) at each deflation step. Rectal walls were defined from the recto-sigmoid junction to 3 cm above the anal canal as the space between the inner and outer wall surfaces. The wall thickness was determined and biomechanical properties (strain and stress) were calculated from the pressure measurements and the MRI-segmented rectal walls.
  
  
  RESULTS
  The integral rectal pressure varied for the maximum tolerable volume (range 150-250 ml) across patients and ranged from 1.3 to 4.0 kPa (SD = 1.2 kPa). Wall thickness was found to vary between patients and also across different rectum segments, with a mean (SD) thickness for the different segments at the 50 ml distension volume of 1.8-4.0 (0.6) mm. Stress showed larger variation than strain, with mean (SD) values for the different segments ranging between 1.5 and 7.0 (1.5) kPa.
  
  
  CONCLUSIONS
  We have developed a method to quantify biomechanical properties of the rectal wall. The resulting rectal wall thickness, strain and stress differed between patients, as well as across different rectal wall sections. These findings could provide guidance in future predictive outcome modelling in order to better understand the rectal dose-volume response relationship.
authors.affiliation: a Department of Medical Physics , Aarhus University Hospital/Aarhus University , Denmark.
authors.forename: Oscar
authors.initials: O
authors.lastname: Casares-Magaz
authors.affiliation: b Department of Medical Physics , Memorial Sloan Kettering Cancer Center , New York , USA.
authors.forename: Maria
authors.initials: M
authors.lastname: Thor
authors.affiliation: c GIOME Academia, Department of Clinical Medicine , Aarhus University , Denmark.
authors.forename: Donghua
authors.initials: D
authors.lastname: Liao
authors.affiliation: d Department of Clinical Medicine , Aalborg University Hospital , Denmark.|e Department of Radiology , Aalborg University Hospital , Denmark.
authors.forename: Jens B
authors.initials: JB
authors.lastname: Frøkjær
authors.affiliation: f Department of Oncology , Aarhus University Hospital/Aarhus University , Denmark.
authors.forename: Pia
authors.initials: P
authors.lastname: Kræmer
authors.affiliation: g Department of Hepatology and Gastroenterology , Aarhus University Hospital/Aarhus University , Denmark.
authors.forename: Klaus
authors.initials: K
authors.lastname: Krogh
authors.affiliation: d Department of Clinical Medicine , Aalborg University Hospital , Denmark.|h Department of Gastroenterology , Aalborg University Hospital , Denmark.
authors.forename: Asbjørn M
authors.initials: AM
authors.lastname: Drewes
authors.affiliation: i GIOME, College of Bioengineering, Chongqing University , Chongqing , China.
authors.forename: Hans
authors.initials: H
authors.lastname: Gregersen
authors.affiliation: j Department of Radiation Medicine and Applied Science , University of California San Diego , San Diego , USA.
authors.forename: Vitali
authors.initials: V
authors.lastname: Moiseenko
authors.affiliation: f Department of Oncology , Aarhus University Hospital/Aarhus University , Denmark.
authors.forename: Morten
authors.initials: M
authors.lastname: Høyer
authors.affiliation: a Department of Medical Physics , Aarhus University Hospital/Aarhus University , Denmark.
authors.forename: Ludvig P
authors.initials: LP
authors.lastname: Muren
mesh_terms: D001696:Biomechanical Phenomena; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011832:Radiation Injuries / Q000503:physiopathology*; D012007:Rectum / Q000503:physiopathology* / Q000528:radiation effects*