title: Observations on real-time prostate gland motion using electromagnetic tracking.
abstract: OBJECTIVE
  To quantify and describe the real-time movement of the prostate gland in a large data set of patients treated with radiotherapy.
  
  
  METHODS
  The Calypso four-dimensional localization system was used for target localization in 17 patients, with electromagnetic markers implanted in the prostate of each patient. We analyzed a total of 550 continuous tracking sessions. The fraction of time that the prostate was displaced by >3, >5, >7, and >10 mm was calculated for each session and patient. The frequencies of displacements after initial patient positioning were analyzed over time.
  
  
  RESULTS
  Averaged over all patients, the prostate was displaced >3 and >5 mm for 13.6% and 3.3% of the total treatment time, respectively. For individual patients, the corresponding maximal values were 36.2% and 10.9%. For individual fractions, the corresponding maximal values were 98.7% and 98.6%. Displacements >3 mm were observed at 5 min after initial alignment in about one-eighth of the observations, and increased to one-quarter by 10 min. For individual patients, the maximal value of the displacements >3 mm at 5 and 10 min after initial positioning was 43% and 75%, respectively.
  
  
  CONCLUSIONS
  On average, the prostate was displaced by >3 mm and >5 mm approximately 14% and 3% of the time, respectively. For individual patients, these values were up to three times greater. After the initial positioning, the likelihood of displacement of the prostate gland increased with elapsed time. This highlights the importance of initiating treatment shortly after initially positioning the patient.
authors.affiliation: Department of Radiation Oncology, M. D. Anderson Cancer Center Orlando, Orlando, FL 32806, USA. katja.langen@orhs.org
authors.forename: Katja M
authors.initials: KM
authors.lastname: Langen
authors.forename: Twyla R
authors.initials: TR
authors.lastname: Willoughby
authors.forename: Sanford L
authors.initials: SL
authors.lastname: Meeks
authors.forename: Anand
authors.initials: A
authors.lastname: Santhanam
authors.forename: Alexis
authors.initials: A
authors.lastname: Cunningham
authors.forename: Lisa
authors.initials: L
authors.lastname: Levine
authors.forename: Patrick A
authors.initials: PA
authors.lastname: Kupelian
mesh_terms: D003199:Computer Systems; D055590:Electromagnetic Phenomena; D006801:Humans; D008297:Male; D011471:Prostatic Neoplasms / Q000175:diagnosis* / Q000532:radiotherapy*; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity