Simple Treatment Plan: Acoustic Schwannoma
Typically an X-Knife radiosurgery system will include 12 to 14 collimators ranging in size from 5 to 50mm in bore diameter. An acoustic schwannoma represents a typical spherical or near-spherical target and therefore a lesion requiring a simple, usually single isocenter treatment plan. An acoustic tumor and typical treatment plan are featured in Figure 1a-e. After the lesion is contoured and reformatted in X-Knife space, X-Knife will, at the designation of the treatment planner, autoposition a default target isocenter in the geometric center of the lesion. A collimator size is chosen from a pop-up menu based on its depiction in X-Knife space as the smallest sphere which covers the broadest diameter of the lesion (Figure 1a). The treatment planner must then designate a family of non-coplanar and non-overlapping arcs of radiation which intersect at target in a way which creates a highly conformal plan. An arc is a single angular sweep of the LINAC gantry through space at a fixed couch angle.
Different couch angles for each arc creates a family of non-coplanar arcs (Figure 1a). Since only a single isocenter will be employed, hotspots from overlapping additional isocenters will not be an issue thus obviating a concern of dose inhomogeneity. The resulting 3-dimensional dose is featured as a translucent “dose cloud” as featured in Figure 1b. If the target is near a dose-limiting structure such as the optic apparatus, arc beams should be placed to avoid passage through these structures. X-Knife provides a means of visualizing the radiation path through the brain with a tool called “beam’s eye view” (BEV) which allows the treatment planner an efficient means of avoiding dose-limiting structures with the appropriate gantry start and stop angles for each arc. As an efficient starting point, the treatment planner may also elect to start a plan with an X-Knife option called “autoplan” which automatically configures a family of arcs to optimize dose to target yet minimize dose to contiguous critical structures using parameters designated by the treatment plannner. These tools increase the efficiency of the treatment planning process and allow the radiosurgery team time to refine the plan with adjustments to each arc.