Acceptance And Commissioning Of The Extend System For The Gamma Knife? Perfexion

Keywords: Extend, technique, gamma knife, physics, radiosurgery

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     The Extend System is an extension to the Gamma Knife Perfexion by using a special patient fixation system and new software system applications. For normal Gamma Knife radiosurgery, a Leksell G-frame is rigidly attached to the patient’s skull using pins and poles. However, the Extend System could replace the Leksell G-frame for patients that planned to have fractionated treatments or treatments in stages (e.g. large tumour size, tumour near critical organ).  As the Extend system used a new patient fixation system, acceptance and commissioning procedures were performed to ensure the reliability of the system. 
     Issues of the positioning accuracy of the Extend system, image co-registration uncertainty and beam accuracy were confirmed before the system came to service.
     1. A plastic head phantom was used throughout the treatment set up process. In essence, the head phantom was fixed with the new fixation system and reference positions were taken. CT imaging was done for defining stereotactic co-ordinates for treatment planning. Verification of the reference positions were done on the treatment couch. 2. Accuracy of the treatment procedure was verified by putting GafChromic film at the phantom and then a 4mm shot was aimed at the centre of the film holder. The distance between the radiological center of the 4mm shot and the central pin mark on the GafChromic film was then calculated. 3. Accuracy of the image co-registration was checked from test images. 4. Assessment of the safety aspects of the system was also performed.
     The reposition checking of the reference positions yielded a vector difference of less than 1mm.  The GafChromic film pin test with the 4mm collimator yielded a Root Mean Square (RMS) error of 0.43mm for the simulated treatment process.
     This is a retrospective study.
     The acceptance and commissioning procedures were performed following the treatment set up procedures for a patient. The accuracy of the patient fixation system was within positioning tolerances of 1mm.
     Moreover, the beam accuracy was within 0.5mm. This uncertainty may due to the errors in repositioning and the accuracy of the imaging processes. A comprehensive protocol for using the Extend System as well as a rigorous QA program for the system should be implemented to ensure the reliability of the system.


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