International Leksell Gamma Knife Calibration Survey - Final Results

Keywords: gamma knife, calibration, dose delivery, technique, physics

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     There are different methods of gamma knife calibration.
     Our purpose was to survey approximately 100 Leksell Gamma Knife (LGK) units worldwide to 1) gather detailed information about calibration procedures and to 2) measure output of the surveyed LGK units.
     Each participant of the project received a LGK calibration questionnaire seeking the following information: LGK model, calibration protocol, phantom, and ion chamber used for calibration, LGK calibration personnel, whether independent verification of calibration was performed, and relative collimator output factors used. Alanine dosimeters were used to measure the dose rate of each surveyed LGK unit and these results compared with calibration data.
     To date, 70 LGK units from 66 different centers spanning 15 different countries have participated in this study (35 North America, 13 Europe, 21 Asia and 1 South America). The calibration protocols used are: AAPM TG21 (26), AAPM TG51 (7), IAEA TRS277 (1), IAEA TRS398 (29), NPL Code of Practice (5) and DIN 6800-2 (2). ELEKTA ABS phantom was used in 65 (93%) and ELEKTA solid water phantom in 5 (7%) units. Ion chambers most frequently used for the calibration are: PTW 31010 (0.125 cm3) 26 times (37%), Exradin A16 (0.007 cm3) 12 times (17%) and Capintec PR-05P (0.070 cm3) 11 times (16%). Calibration of the LGK was performed by an on-site physicist in 52 (75%) cases, by ELEKTA physicist in 17 (24%) cases and in 1 (1%) case calibration was done by a consulting physicist. Independent verification was done only in 31 (44%) cases; RPC, IAEA or similar audit was done only in 20 (29%) cases. All LGK units surveyed are currently using the default values for collimator relative output factors. Observed deviations between LGK users reported calibration and alanine dosimetry measurements were of small magnitude with mean value of 1.6%. In total 63 (90%) LGK units were within 3% deviation. None of measured units exceeded 4% deviation in calibration.
     One limitation is that not all active centers were surveyed.
     Different calibration procedures, especially calibration protocols are used worldwide.
     Small but systematic deviation between LGK users reported calibration and alanine dosimetry measurements are observed for LGK centers in Europe and Asia where IAEA TRS398 protocol is used. This can be explained by the fact that the ABS plastic phantom is considered to be water equivalent.


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