title: Time dependence of intrafraction patient motion assessed by repeat stereoscopic imaging.
abstract: OBJECTIVE
  To quantify intrafraction patient motion and its time dependence in immobilized intracranial and extracranial patients. The data can be used to optimize the intrafraction imaging frequency and consequent patient setup correction with an image guidance and tracking system, and to establish the required safety margins in the absence of such a system.
  
  
  METHODS
  The intrafraction motion of 32 intracranial patients, immobilized with a thermoplastic mask, and 11 supine- and 14 prone-treated extracranial spine patients, immobilized with a vacuum bag, were analyzed. The motion was recorded by an X-ray, stereoscopic, image-guidance system. For each group, we calculated separately the systematic (overall mean and SD) and the random displacement as a function of elapsed intrafraction time.
  
  
  RESULTS
  The SD of the systematic intrafraction displacements increased linearly over time for all three patient groups. For intracranial-, supine-, and prone-treated patients, the SD increased to 0.8, 1.2, and 2.2 mm, respectively, in a period of 15 min. The random displacements for the prone-treated patients were significantly higher than for the other groups, namely 1.6 mm (1 SD), probably caused by respiratory motion.
  
  
  CONCLUSIONS
  Despite the applied immobilization devices, patients drift away from their initial position during a treatment fraction. These drifts are in general small if compared with conventional treatment margins, but will significantly contribute to the margin for high-precision radiation treatments with treatment times of 15 min or longer.
authors.affiliation: Department of Radiation Oncology, Erasmus MC - Daniel den Hoed Cancer Center, Rotterdam, The Netherlands. m.hoogeman@erasmusmc.nl
authors.forename: Mischa S
authors.initials: MS
authors.lastname: Hoogeman
authors.forename: Joost J
authors.initials: JJ
authors.lastname: Nuyttens
authors.forename: Peter C
authors.initials: PC
authors.lastname: Levendag
authors.forename: Ben J M
authors.initials: BJ
authors.lastname: Heijmen
mesh_terms: D001932:Brain Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D006801:Humans; D007103:Immobilization / Q000379:methods; D009068:Movement; D016684:Prone Position; D019736:Prostheses and Implants; D011859:Radiography; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012371:Robotics / Q000379:methods; D016683:Supine Position; D013899:Thoracic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D013997:Time Factors