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Treatment planning for Gamma Knife requires knowledge of the collimators dose profiles, collimators percentage depth doses, and collimator output factors. The primary method of dose computation in the Leksell GammaPlan is the TMR10 algorithm which does not take into account air cavities or inhomogeneous tissue. To account for inhomogeneous tissues, one can use the convolution algorithm.

Two main elements of the treatment planning are shot coordinates and weighting. Dose distributions evolve from placing shots at a distance that does not overlap significantly and weighting shots that are close to one another lightly to fill in areas of the target not covered. The dose distribution will collapse as more shots are added and not appropriately weighted. Figures 6a and b are two examples of how a dose distribution are developed. The red contour is the target and the blue outlines are the shape of the shots. The yellow line surrounding the target is the 50% isodose line. By adding shots throughout the target, blocking sectors and adjusting weights, one can conform the dose distribution very tightly to the target.

Fig. 6. a Highly conformal dose distribution of the target with multiple shots and blocking. b Four shots close together showing dose distribution collapsing to the center. Red contour, target; blue, outlines of the shots; yellow, 50% isodose line.

A few tools are available for the evaluation of treatment plans: conformity index (CI) (RTOG 1995, abandoned), Paddick conformity index (PCI), plan selectivity (PS), and the gradient index (GI). Dose conformity looks to evaluate how well the dose distribution covers the target volume. The CI = PIV/TV, where PIV is the prescribed isodose volume and TV is the target volume. This definition was supposed to evaluate how much the PIV covers the treatment volume, and if applied incorrectly can lead to a target volume that is equal to the planned isodose volume, but they do not overlap at all, giving a false CI of unity. This was corrected in Gamma Knife by PCI = (TV_PIV)²/(TV × PIV), where the TV_PIV = target volume which overlaps the PIV. This definition removes false positive scores. A PCI = 1 means perfect coverage to the contoured volume, while 0 means that the dose and volume are not overlapped in any fashion.

PS = (TV_PIV)/(PIV) is a measure of the comparative target coverage from different plans. A PS <1 corresponds to a lower coverage of the target, a PS >1 means that the dose distribution is larger, outside the target volume. The last index, GI, is a measure of the dose spillage outside of the target. The GI is defined as PIV_50%/PIV, with the PIV_50% being the volume of the 50% isodose line.