The Gamma Knife (GK) Perfexion has an innovative collimation system with unified 4, 8, 16 mm collimator unlike the older models. The cobalt-60 source moves automatically along the linear track on the collimator to select the shot size planned in TPS.

     Since there are some block positions to change the collimator size between each shot, we measured the unintended leakage dose by blocked collimator.

      

     With spherical solid water phantom, ion chamber dosimetry with PTW 31010 was carried out for maximum 30Gy treatment plan. First, current dose rate was measured 10 times to determine exact exposure time for each shot. And the treatment plans with 1, 5, 10, 20, 30 16 mm shots were established to deliver 30 Gy to the center of phantom. They were built with physics test function of the LGK console and the total absorbed dose was compared for the treatments.

     The measured dose rate was 3.561 Gy/min. There is about 3cGy overdose for additional one 16 mm shot and it amounts to 1Gy for 30 16 mm shots for the current dose rate. It rises from the unified collimator structure of GK Perfexion, which arranged in 16 mm, 4 mm, 8 mm collimator order. When the source moves from 16 mm shots to off position located in the middle of 4mm and 8 mm collimator, there is a flashing 4 mm shot during the movement and it is known as “shutter dose”. It is independent of prescription dose and only influenced by the number of shots. Also, if the dose rate decreases with source decay, it reduces accordingly.

     This is a retrospective study.

     There exists a shutter dose of 0.8% of current dose rate in GK Perfexion collimator system.

     Though it does not increase significant figure beside prescription dose, cautions are needed for the treatment plans with many 16 mm shots.