Dosimetric characteristics and quality control tests for the collimator sectors of the Leksell Gamma Knife(®) Perfexion(TM).

Purpose: The purpose of this study was to evaluate the dosimetric characteristics of each sector of the Leksell Gamma Knife Perfexion (LGK PFX) and to develop tests that can be done for the routine quality assurance checks of the sectors of the LGK PFX.
Methods: The following tests were performed to evaluate the dosimetric characteristics of the sectors: (1) Flash-radiation dose for the 16 mm collimator, (2) transit-radiation dose for the 8 and 4 mm collimators, (3) sector leakage within the radiation cavity and, (4) sector output uniformity. In these tests, the Elekta ABS phantom was used. A micropoint ion-chamber Exradin A16 was placed at the center of the phantom for all measurements.
Results: With the version 8.0 of the control software of the MCU in the LGK PFX, the average flash-radiation dose per sector for the 16 mm collimator was measured to be 0.423 ± 0.003 cGy, and the average transit-radiation dose per sector for the 8 and 4 mm collimators was measured to be 0.169 ± 0.0009 and 0.147 ± 0.020 cGy, respectively. The calibration dose rate on the day of measurements was 280.8 cGy/min. Here, the authors have introduced a new concept of "equivalent-time-duration" (ETD) to represent the time duration, during which the flash-radiation or the transit-radiation dose is delivered. The ETD is a quotient of the measured dose of the flash-radiation or the transit-radiation and the respective calibrated dose rate for the 16, 8, or 4 mm collimator. The ETD constancy is an indicator of the constancy of the sector movements. The average value of ETD per sector was measured to be 724 ± 6, 313 ± 2, and 311 ± 45 ms for the 16, 8, and 4 mm collimators, respectively. During monthly spot checks, the authors have been measuring the total ETD for the flash-radiation when all eight sectors are open with the 16 mm collimator. The average value of the total ETD of the last 40 consecutive months was measured to be 642 ± 10 ms. This number is a useful quality parameter for the LGK PFX, which can be used to establish the base-line performance of the collimators of the LGK PFX. The reader is alerted that with the newly introduced version 9.0 of the control software of the MCU in the LGK PFX, the flash-radiation and the transit-radiation doses have become practically zero due to the fact that the treatment time is now corrected to compensate for these radiation doses. The sector output uniformity for the 16, 8, and 4 mm collimators was determined to be 98.9%, 97.3%, and 96.6%, respectively. The total leakage radiation with in the radiation cavity from all sectors was measured to be 0.17% of the dose output for the 16 mm collimator.
Conclusions: Our measurements show that the dosimetric characteristics of all sectors of the LGK PFX are satisfactory. The authors recommend that the sector performance of the LGK PFX be determined initially at the time of installation of the LGK PFX and thereafter, at least annually. Similarly, our recommendation is that the total flash-radiation dose for the 16 mm collimator and the corresponding ETD value be measured on a monthly basis. Should any change in the sector movement occur, it will be revealed by the sector performance and the measurement of ETD.