title: Long-term stability of the Leksell Gamma Knife® Perfexion™ patient positioning system (PPS).
abstract: OBJECTIVE
  To assess the long-term mechanical stability and accuracy of the patient positioning system (PPS) of the Leksell Gamma Knife(®) Perfexion™ (LGK PFX).
  
  
  METHODS
  The mechanical stability of the PPS of the LGK PFX was evaluated using measurements obtained between September 2007 and June 2011. Three methods were employed to measure the deviation of the coincidence of the radiological focus point (RFP) and the PPS calibration center point (CCP). In the first method, the onsite diode test tool with single diode detector was used together with the 4 mm collimator on a daily basis. In the second method, a service diode test tool with three diode detectors was used biannually at the time of the routine preventive maintenance. The test performed with the service diode test tool measured the deviations for all three collimators 4, 8, and 16 mm and also for three different positions of the PPS. The third method employed the conventional film pin-prick method. This test was performed annually for the 4 mm collimator at the time of the routine annual QA. To estimate the effect of the patient weight on the performance of the PPS, the focus precision tests were also conducted with varying weights on the PPS using a set of lead bricks.
  
  
  RESULTS
  The average deviations measured from the 641 daily focus precision tests were 0.1 ± 0.1, 0.0 ± 0.0, and 0.0 ± 0.0 mm, respectively, for the 4 mm collimator in the X (left/right of the patient), Y (anterior/posterior of the patient), and Z (superior/inferior of the patient) directions. The average of the total radial deviations as measured during ten semiannual measurements with the service diode test tool were 0.070 ± 0.029, 0.060 ± 0.022, and 0.103 ± 0.028 mm, respectively for the central, long, and short diodes for the 4 mm collimator. Similarly, the average total radial deviations measured during the semiannual measurements for the 4, 8, and 16 mm collimators and using the central diode were 0.070 ± 0.029, 0.097 ± 0.025, 0.159 ± 0.028 mm, respectively. The average values of the deviations as obtained from the five annual film pin-prick tests for the 4 mm collimator were 0.10 ± 0.06, 0.06 ± 0.09, and 0.03 ± 0.03 mm for the X, Y, Z stereotactic directions, respectively. Only a minor change was observed in the total radial deviations of the PPS as a function of the simulated patient weight up to 202 kg on the PPS.
  
  
  CONCLUSIONS
  Excellent long-term mechanical stability and high accuracy was observed for the PPS of the LGK PFX. No PPS recalibration or any adjustment in the PPS was needed during the monitored period of time. Similarly, the weight on the PPS did not cause any significant disturbance in the performance of the PPS for up to 202 kg simulated patient weight.
authors.affiliation: Department of Radiation Oncology, Medical Physics Division, University of Pittsburgh Cancer Institute, Pittsburgh, Pennsylvania 15213; Department of Medical Physics, Na Homolce Hospital, Prague 150 30, Czech Republic; Institute of Biophysics and Informatics, First Faculty of Medicine, Charles University in Prague, Prague 120 00, Czech Republic; Department of Dosimetry and Application of Ionizing Radiation, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague 115 19, Czech Republic; and Department of Radiation Oncology, University Hospital Motol, Prague 150 06, Czech Republic.
authors.forename: J
authors.initials: J
authors.lastname: Novotny
authors.affiliation: Department of Radiation Oncology, Medical Physics Division, University of Pittsburgh Cancer Institute, Pittsburgh, Pennsylvania 15213.
authors.forename: J P
authors.initials: JP
authors.lastname: Bhatnagar
authors.affiliation: Department of Radiation Oncology, Medical Physics Division, University of Pittsburgh Cancer Institute, Pittsburgh, Pennsylvania 15213.
authors.forename: Y
authors.initials: Y
authors.lastname: Xu
authors.affiliation: Department of Radiation Oncology, Medical Physics Division, University of Pittsburgh Cancer Institute, Pittsburgh, Pennsylvania 15213.
authors.forename: M S
authors.initials: MS
authors.lastname: Huq
mesh_terms: D002138:Calibration; D003198:Computer Simulation; D004867:Equipment Design; D019544:Equipment Failure Analysis / Q000295:instrumentation; D005373:Film Dosimetry / Q000379:methods; D006801:Humans; D007854:Lead; D056888:Patient Positioning / Q000379:methods*; D011786:Quality Control; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D015203:Reproducibility of Results
chemical_list: D007854:Lead