Experience From Commissioning Four Gamma Knife Perfexion Units

Keywords: gamma knife, commissioning, physics, technique, Perfexion

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     The Leksell Gamma Knife Perfexion poses a challenge to the physicist with respect to commissioning. The absence of a mechanical isocentre that can be located outside of the radiation unit and the capability to produce composite shots produce their respective challenges. In addition, isocentre movement via its patient positioning system (PPS) implies a greater reliance on modern technology for treatment accuracy.
     The experience of verifying the dosimetry and positioning accuracy on four separate units will be shared.
     Dosimetry. GafChromic EBT or EBT2 (high sensitivity) films were irradiated in an Elekta solid water phantom, which allowed multiple film irradiation at different planes in 5mm increments to the z axis. An Epson V750 pro dual lens scanner was used, producing 16 bit images. Images were analysed using ImageJ software and MATLAB. Films were irradiated using multiple composite shots, to test the accuracy of composite isodose calculation. Isodoses generated from these films were compared with those produced by GammaPlan.
     Isodoses produced using GafChromic film matched those generated by GammaPlan within experimental error for all four units. The mean error of the PPS for all 130 positions was 0.083mm, the maximum error was 0.19mm. The traditional GafChromic pin prick test with the 4mm collimator gave errors of 0.05mm, for the four different Gamma Knives at coordinates (100,100,100).
     This is a retrospective study.
     Dosimetrical and geometrical accuracy of the Perfexion can be verified using the above tests.
     The need for these additional tests to verify system performance is all the more essential considering the increased complexity and greater automation of the Perfexion Gamma Knife.


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