Speaker
Description
Objective: Semi-dynamic arc for carbon ion radiotherapy currently uses coplanar arcs. In regions of high organ-at-risk (OAR) density, large portions of coplanar arcs may have occlusions that prevent meaningful delivery to the target. A non-coplanar arc that makes full use of the motion range afforded by upright positioning solutions may overcome this problem by finding a treatment path within the motion envelope that minimises the geometrical overlap of OARs with the target. Current approaches are computationally expensive and compress the volumetric nature of the challenge into 2-dimentional snapshots.
Method: Segmented CT geometries are imported into a Blender scene surrounded by a vertical cylindrical wall with a distance to the isocenter equivalent to the distance of the nozzle. OARs and external structures are assigned volumetric translucency properties and the target is assigned emissive properties. Occlusions from OARs are cast as shadows on the cylindrical wall. The shadows are rendered as a light map using Blender's Cycles ray-traced rendering engine. A path-finding algorithm is applied on the light map to produce an optimal path that minimises the traversal of occluded regions.
Results: An abstract representation of OARs in the head-and-neck region was modeled inside a cylindrical phantom of 20 cm in diameter. A spherical target of 6 cm in diameter was placed in the center of the phantom. A non-coplanar trajectory was developed using the described method. Non-coplanar and coplanar plans delivering 5 Gy to the target were prepared and evaluated inside RayStation. Compared to a coplanar plan, the non-coplanar plan reduced the OAR and external dose by up to 11%, while maintaining target coverage.
Outlook & Discussion: This first prototype of a ray-traced volumetric evaluation method shows promising performance for the avoidance of OARs in the foreground of the target. The involvement of the thickness of the OARs increases the flexibility of the resultant path. This current method does not consider OARs in the background of the target, which are susceptible to overshot. Additional features, such as OAR prioritisation and isocenter selection, could be implemented with adjustments to the volumetric properties. This method can reduce the computational cost of optimal non-coplanar arc paths, and will contribute to increasing the safe deliverable single-fraction dose in carbon ion arc radiotherapy.