With the exception of brain metastases, the shapes of many intracranial targets are infrequently spherical and often not even elliptical, and target size often exceeds the range of collimators available. Target size and shape therefore often require multiple isocenter treatments. A common strategy involves multiple isocenters which create a composite dose distribution approximating the target’s shape and size. This strategy maximizes conformality but minimizes dose homogeneity. Utilizing techniques which include arc beam shaping and differential beam weighting, fewer isocenters can be utilized for complex shapes and sizes while maintaining a high degree of conformality. Skull base meningiomas often represent complex three-dimensional targets requiring complex treatment plans.

An intraorbital meningioma and typical treatment plan are featured in Figure 2a-e. Unlike the single shot treatment discussed above, hotspots from overlapping additional isocenters will now be a concern thus requiring an effort to minimize dose inhomogeneity which can be achieved by weighting isocenters as well as converging arcs differently. In the interactive 3-dimensional X-Knife environment, the evolving treatment plan can be continually assessed with either 3-D depictions (volume dose-continuous dose gradient or surface wash-threshold dose gradient, Figures 2b), 2-D depictions (slice dose with continuous dose gradient, Figure 2d), in graphic form (dose-volume histograms), or in tabular form (dose-volume summaries with average dose, minimum dose, and maximum dose reported for each anatome).