title: Effect of body mass index on intrafraction prostate displacement monitored by real-time electromagnetic tracking.
abstract: OBJECTIVE
  To evaluate, using real-time monitoring of implanted radiofrequency transponders, the intrafraction prostate displacement of patients as a function of body mass index (BMI).
  
  
  METHODS
  The motions of Beacon radiofrequency transponders (Calypso Medical Technologies, Seattle, WA) implanted in the prostate glands of 66 men were monitored throughout the course of intensity modulated radiation therapy. Data were acquired at 10 Hz from setup to the end of treatment, but only the 1.7 million data points with a "beam on" tag were used in the analysis. There were 21 obese patients, with BMI ≥ 30 and 45 nonobese patients in the study.
  
  
  RESULTS
  Mean displacements were least in the left-right lateral direction (0.56 ± 0.24 mm) and approximately twice that magnitude in the superior-inferior and anterior-posterior directions. The net vector displacement was larger still, 1.95 ± 0.47 mm. Stratified by BMI cohort, the mean displacements per patient in the 3 Cartesian axes as well as the net vector for patients with BMI ≥ 30 were slightly less (<0.2 mm) but not significantly different than the corresponding values for patients with lower BMIs. As a surrogate for the magnitude of oscillatory noise, the standard deviation for displacements in all measured planes showed no significant differences in the prostate positional variability between the lower and higher BMI groups. Histograms of prostate displacements showed a lower frequency of large displacements in obese patients, and there were no significant differences in short-term and long-term velocity distributions.
  
  
  CONCLUSIONS
  After patients were positioned accurately using implanted radiofrequency transponders, the intrafractional displacements in the lateral, superior-inferior, and anterior-posterior directions as well as the net vector displacements were smaller, but not significantly so, for obese men than for those with lower BMI.
authors.affiliation: Schiffler Cancer Center, Wheeling Hospital, Wheeling, West Virginia 26003-6300, USA. wbutler@wheelinghospital.org
authors.forename: Wayne M
authors.initials: WM
authors.lastname: Butler
authors.forename: Mallory N
authors.initials: MN
authors.lastname: Morris
authors.forename: Gregory S
authors.initials: GS
authors.lastname: Merrick
authors.forename: Brian S
authors.initials: BS
authors.lastname: Kurko
authors.forename: Brian C
authors.initials: BC
authors.lastname: Murray
mesh_terms: D000368:Aged; D001696:Biomechanical Phenomena / Q000502:physiology; D015992:Body Mass Index; D004574:Electromagnetic Fields; D057918:Fiducial Markers; D006801:Humans; D008297:Male; D009068:Movement; D009765:Obesity / Q000150:complications; D009929:Organ Size; D056888:Patient Positioning / Q000379:methods; D011467:Prostate / Q000473:pathology; D011471:Prostatic Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods