Monte Carlo calculations and GafChromic film measurements for plugged collimator helmets of Leksell Gamma Knife unit

The Monte Carlo technique and GafChromic films were employed to verify the accuracy of the dose planning system (Leksell GammaPlan(R)) used in Gamma K nife (type B) radiosurgery when plugged collimator helmets were used. The E GS4 Monte Carlo code was used to calculate the dose distribution along the x, y, and z axes when a single shot was delivered at the center point (unit center point: x=100, y=100, z=100) of a spherical polystyrene phantom, wit h gamma angle of 90 degrees. Two different sizes of the plugged collimator helmets, 4 and. 18 mm, were studied. Two typical. plugged patterns, 51 plug s and 99 plugs along the y direction, were: examined. The results of our Mo nte Carlo trials showed good consistency with GammaPlan calculations and Ga fChromic sim measurements. Furthermore, the Monte Carlo results showed that radiation leakage from the plugs was too small to affect the overall isodo se curve distribution even when the heavily plugged pattern of up to 99 plu gs was employed. The results of this project provide confidence to all Gamm a Knife centers using the Leksell GammaPlan treatment planning system. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)02207-5 ].