title: Positional stability of electromagnetic transponders used for prostate localization and continuous, real-time tracking.
abstract: OBJECTIVE
  To determine the relative positional stability of implanted glass-encapsulated circuits (transponders) used in continuous electromagnetic localization and tracking of target volumes during radiation therapy. Ideally, the distances between transponders remains constant over the course of treatment. In this work, we evaluate the accuracy of these conditions.
  
  
  METHODS
  Three transponders were implanted in each of 20 patients. Images (CT scan or X-ray pair) were acquired at 13 time points. These images occurred from the day of implant (2 weeks before simulation) to 4 weeks posttreatment. The distance between transponders was determined from each dataset. The average and standard deviation of each distance were determined, and changes were evaluated over several time periods, including pretreatment and during therapy.
  
  
  RESULTS
  Of 60 transponders implanted, 58 showed no significant migration from their intended positions. Of the two transponders that did migrate, one appears to have been implanted in the venous plexus, and the other in the urethra, with no clinical consequences to the patients. An analysis that included the planning CT scan and all subsequent distance measurements showed that the standard deviation of intertransponder distances was < or =1.2 mm for up to 1 month after the completion of therapy.
  
  
  CONCLUSIONS
  Implanted transponders demonstrate the same long-term stability characteristics as implanted gold markers, within statistical uncertainties. As with gold markers, and using the same implant procedure, basic guidelines for the placement of transponders within the prostate help ensure minimal migration.
authors.affiliation: Department of Radiation Oncology, University of Michigan, 1500 East Medical Center Drive, Ann Arbor, MI 48109, USA. litzen@umich.edu
authors.forename: Dale W
authors.initials: DW
authors.lastname: Litzenberg
authors.forename: Twyla R
authors.initials: TR
authors.lastname: Willoughby
authors.forename: James M
authors.initials: JM
authors.lastname: Balter
authors.forename: Howard M
authors.initials: HM
authors.lastname: Sandler
authors.forename: John
authors.initials: J
authors.lastname: Wei
authors.forename: Patrick A
authors.initials: PA
authors.lastname: Kupelian
authors.forename: Alexis A
authors.initials: AA
authors.lastname: Cunningham
authors.forename: Andrea
authors.initials: A
authors.lastname: Bock
authors.forename: Michele
authors.initials: M
authors.lastname: Aubin
authors.forename: Mack
authors.initials: M
authors.lastname: Roach
authors.forename: Katsuto
authors.initials: K
authors.lastname: Shinohara
authors.forename: Jean
authors.initials: J
authors.lastname: Pouliot
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D055590:Electromagnetic Phenomena / Q000295:instrumentation*; D005548:Foreign-Body Migration; D006801:Humans; D008297:Male; D008875:Middle Aged; D011467:Prostate; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D019736:Prostheses and Implants / Q000592:standards*; D011829:Radiation Dosage; D011859:Radiography; D020266:Radiotherapy, Conformal / Q000295:instrumentation* / Q000592:standards