Dosimetric characteristics and quality control tests for the collimator sectors of the Leksell Gamma Knife® PerfexionTM
(a) The total Flash-radiation dose (D F )tot,16 and (b) the corresponding ETD, (T F )tot,16 for the 16 mm collimator (all sectors open) as measured during the monthly calibrations over the consecutive 40 months. The solid-line curve in Fig. (a) is drawn to represent the 60Co decay with the initial value of 3.9 cGy at time zero as a “guide” to the eye. The (D F )tot,16 doses are seen to decrease exponentially with time as expected. The (T F )tot,16 values stay constant with time, with an average of 642 ± 10 ms. The ETD is a characteristic of the machine design and represents the movement of the sectors sliding over the tungsten collimator ring. With the newly introduced version 9.0 of the control software of the MCU in the LGK PFX, the flash-radiation dose has now become practically zero due to the fact that the treatment time is now corrected to compensate for the flash radiation dose.
The flash-radiation dose (D F ) i ,1 6 and the corresponding ETD, (T F ) i ,1 6 for the 16 mm collimator and the transit-radiation dose (D T ) i , 8 and (D T ) i, 4 and the corresponding ETD, (T T ) i , 8 and (T T ) i ,4 for the 8 and 4 mm collimators, respectively. Due to the thrice repeated attenuation of some of the 16 mm beams emerging from sectors 3 and 7 by the ABS phantom holder in the one-shot/ n-shot exposure technique with n = 4, the (D F ) i ,1 6 values for the sectors 3 and 7 are seen to be less by a factor of 0.893 as compared with the average of the rest of the other sector values (sectors1, 2, 4–6, and 8). For a single time attenuation of the 16 mm beams from sectors 3 and 7, the attenuation factor is 0.964. The calibration dose rate on the day of measurement was 280.8 cGy per minute.
Annually measured total D F and D T from all eight sectors and the corresponding T F and T T for the 16, 8, and 4 mm collimators. For reference purposes, the LGK PFX calibration dose rates (D cal)16, (D cal)8, and (D cal)4 for the 16, 8, and 4 mm collimators on the day of measurements are also included. The values of (T F )tot,1 6, (T T )tot, 8, and (T T )tot, 4 are seen to stay practically constant with an average of 649 ± 19, 295 ± 23, and 230 ± 14 ms, respectively. The 8 and 4 mm collimator outputs are related to the 16 mm collimator output by the ROF of the 8 and 4 mm collimators.
The LGK PFX Sector output uniformity, U s , for the 16, 8, and 4 mm collimators using the Elekta ABS phantom with two identical side mounting Lucite brackets without 5 mm thick Lucite quarter-circle plate used to adjust the gamma angle on the LGK model 4C. For the 16 mm collimator, the charge collected for the sectors 3 and 7 is adjusted for the ABS phantom-holder attenuation of some of the 16 mm beams. For comparison purpose, the U s for the 16 mm collimator is also measured in the Elekta solid-water phantom.
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