Shutter Dose With The Gamma Knife® Perfexion™

Keywords: physics, Dose, gamma knife, dosimetry, dose delivery

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     The GammaPlan™ treatment planning system does not fully account for shutter dose from radiation sector motions with the Gamma Knife® Perfexion™. 
     The shutter dose associated with patient repositioning is measured in this study.
     A stereotactic head-frame was attached to a Leksell® 16-cm diameter spherical phantom with a calibrated ion chamber at its center. An absorbed dose of 10 Gy to the 50% isodose-line was prescribed to the target site (phantom center) for all measurements. Plans with shots ranging from one to 50 in a run were generated for the all collimators to determine the relationship of measured dose to the number of repositions of the patient positioning system (PPS) and to the collimator size. Shutter dose to the target periphery is also measured for the 16-mm collimator.
     Measured shutter dose increases with frequency of repositioning and collimator size. Shutter doses of 0.82 ± 0.02, 1.59 ± 0.04, and 3.53 ± 0.04 cGy/reposition (with focus dose rate of 3.425 Gy/minute) to the target site are observed for the 4, 8 and 16-mm collimators, respectively. Relative shutter dose from the 4 and 8-mm collimators are comparable; however, the 16-mm is more substantial. This can be attributed to unintended dose from the sources moving over the open 4-mm collimators from the off position to reach the 16-mm collimator position. The shutter doses for the 16-mm collimators of the Perfexion™ and the 18-mm collimators of the model 4C were compared; greater dose was observed for the Perfexion™ than its predecessor where an analogous effect has doses of 1.58 ± 0.00, 1.94 ± 0.00, and 2.37 ± 0.00 cGy/reposition (with focus dose rate of 2.254 Gy/minute) for the 8, 14 and 18-mm collimators, respectively. 
     This study used a phantom and not actual measurements from patients.
     The radiation sector motions for the Leksell Gamma Knife® Perfexion™ result in an additional dose due to the shutter effect, which may be an important difference (especially around critical structures within the brain). 
     Further characterization of exposure from the radiation sector motions accompanying PPS movement and development of a dose calculation algorithm to account for these doses would improve the accuracy of the delivered plan.


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