Dose planning with the Leksell Gamma Knife: the effect on dose volume of more than one shot at the same target point.

A study was made of the interaction between Gamma Knife shots of different diameters, located at the same target coordinates. The effect of the doses was calculated by the Kula program and confirmed by film dosimetry. It could be shown that intermediate collimator sizes can be simulated in this way, so that, for example, a 4- and 8-mm shot with the same target coordinates simulates a 6-mm collimator. A coaxial 8- and 14-mm shot simulates an 11-mm collimator and a coaxial 14- and 18-mm shot simulates a 16-mm collimator. Additional intermediate collimator sizes can be simulated by changing the weighting of the two coaxial shots. The sharpness of the dose fall of the simulated coaxial shots is of the same order as that of single shots, indicating that the technique does not depart from ordinary Gamma Knife technique in any significant way.