An Investigation Of The Accuracy And Reproducibility Of The Patient Positioning System On The Perfexion Gamma Knife Under Various Simulated Patient Loads

Keywords: gamma knife, robotics, technique, radiosurgery, stereotactic assisted

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     One of the major differences between the Perfexion Gamma Knife unit and previous models (U, B, C and 4C) is the treatment couch, known as the patient positioning system (PPS). It is not only used to support the patient during treatment but also to accurately move the patient into the proper position for treatment. 
     We tested the accuracy and reproducibility of the PPS under various simulated patient loads.
     All tests were conducted on the Perfexion Gamma Knife unit located at the Wake Forest University School of Medicine Gamma Knife Center. Patient loads were simulated using 250 lbs (98 kg) of weights. The weights were placed at various positions on the couch which created different amounts of additional torque on connection between the PPS and the treatment unit. Measurements were made using the specifically designed Diode Test Tool manufactured by Elekta which came with the Perfexion unit. Baseline tests were performed without weights both before and after the weighted tests to check the PPS reproducibility.
     The Diode Test Tool scans along the intersection of the treatment beams produced by the 4 mm collimators in the x, y and z axes to determine any deviations from the PPS isocenter and the radiative isocenter. Tests performed without weights both before and after the weighted tests indicated maximum deviations in any one axis to be 0.1 mm.  The weighted tests indicated similar results with the maximum deviation in any one axis being 0.1 mm. The rms deviation was greater for the weighted runs, although the difference was insignificant. The position of the weights on the treatment couch had no noticeable effect on the measurements.
     This is a retrospective study.
     The patient positioning system (pps) on the Perfexion Gamma Knife unit performed reproducibly and accurately under various simulated patient loads. 
     The total weight utilized for the tests was well below the specified weight limit for the pps, but the strategic positioning of the weights produced more additional torque on the pps than a typical patient. Further tests are planned to investigate the accuracy and reproducibility of the pps while delivering multiple shots to a film in a phantom under various weight loads.


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