Evaluation Of Dosimetric Characteristics Of The Leksell Gamma Knife Perfexion Collimator Sectors

Keywords: gamma knife, dosimetry, physics, Robotic, dose delivery

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       The perfexion unit has collimator sectors that must be maintained.
     The purpose of this study was to evaluate the dosimetric characteristics of each sector of Leksell Gamma Knife Perfexion (LGK PFX) and to determine tests that can be done for routine quality control of sectors of LGK PFX.
     Following tests were done to evaluate dosimetric characteristics of sectors: (1) Sector output uniformity, (2) Individual sector relative isodose distribution, (3) Total transit doses for 16, 8, and 4 mm collimators, (4) Uniformity of individual sector transit doses, (5) Sector leakage measurements. For all measurements, both solid water and ABS phantoms from Elekta, AB were used with Exradin A16 ion chamber placed at the center of phantoms.
     The sector output uniformity was determined to be within + 0.9% of the average dose output for all sectors. Preliminary results on comparison of isodoses obtained from Gafchromic film dosimetry and Leksell GammaPlan show good agreement between measured and calculated dose distribution for each sector. The total transit doses for 16, 8 and 4 mm collimators were measured to be 1.0%, 0.5%, and 0.3% of the dose rates measured with respective collimators. The transit dose for each sector varied very little and was measured to be 0.15% of 16 mm collimator dose rate. The leakage radiation from each sector also varied very little and was measured to be 0.4% of 16 mm collimator dose rate.
     This was tested at only one center.
     In conclusion, our measurements show that the measured dosimetric characteristics of all sectors are satisfactory.
     It is our recommendation that the sector performance of LGK PFX be determined initially at the time of installation of LGK PFX and thereafter, sector output uniformity be verified on an annual basis.


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