International Leksell Gamma Knife Calibration Survey – Preliminary Results

Keywords: gamma knife, dosimetry, physics, calibration, Radiation

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       Different calibration procedures can be used in radiosurgery.
     The purpose of this project is to survey between 50 to 100 Leksell Gamma Knife (LGK) units worldwide and: 1) gather detailed information about calibration procedures used for the LGK and 2) measure output of the surveyed LGK units using alanine dosimeter and compare these results with treatment planning system calibration value.
     Each participant of the project received a LGK calibration questionnaire seeking the following information on LGK calibration: LGK model, calibration protocol used, phantom used, ion chamber (manufacturer, model, volume and etc.) used for calibration, LGK calibration personnel (i.e., on-site physicist, ELEKTA personnel, other), whether independent verification of calibration was performed, and collimator relative output factors used (ELEKTA default values, values measured by on-site physicist, other). Alanine dosimeters were used to measure the dose rate of the each surveyed LGK unit and results compared with calibration data. Alanine dosimeters were evaluated with a Bruker ECS106 Electron Paramagnetic Resonance spectrometer using the protocol described in the NIST Ionizing Radiation Division Quality System Manual.
     To date, 17 LGK units have participated in this project (8 from the USA, 3 from Europe and 6 from Asia).The calibration protocols used for 17 surveyed sites were: AAPM TG21 8 sites, AAPM TG51 1 site, IAEA TRS277 1 site and IAEA TRS398 7 sites. ELEKTA ABS spherical phantom was used in 15 cases and ELEKTA solid water phantom in 2 cases. Following ion chambers were used for LGK calibration: PTW 31010 (0.125 cm3) 8 times, Exradin A16 (0.007 cm3) 4 times, PTW 31014 (0.015 cm3) 1 time, Wellhoffer IC-10 (0.125 cm3) 1 time and Capintec PR-05P (0.070 cm3) 3 times. Calibration of the LGK unit was performed by an on-site physicist in 11 cases and by ELEKTA personnel in 6 cases. Independent verification was done only in 3 cases out of total 17 surveyed LGK units. All LGK units surveyed except one are currently using the ELEKTA default values for collimator relative output factors.
     Not all active centers were surveyed.
     The average dose ratio of planned to measured (by alanine dosimetry) was 1.003 (range 0.975 – 1.030) demonstrating very good LGK calibration consistency.
     This project is ongoing and more information will be provided at its conclusion.


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