The dosimetric consequences of weighted fields using the same isocenter in radiosurgery

One handicap in achieving ideal dosimetry with the Gamma Knife is the limited number of standard collimator sizes (4, 8, 14 and 18 mm). We report an approach that makes possible the creation of 50% or higher isodose lines, corresponding to collimator sizes from 4 to 18 mm. This technique, which uses the same icocenter, but with different sized and weighted collimators, improves matching for given isodose lines to the diameter of the lesion in a defined plane. However, when combination fields, designed to reproduce the 50% isodose diameter of the 8- and 14-mm collimators, are compared to the standard sizes, there are significant differences in the three-dimensional dosimetry.