Collision indicator charts for gantry-couch position combinations for Varian linacs.

The use of non-coplanar radiation fields can potentially lead to collisions between the gantry and the couch or patient. The collisions are often not realized until the plan is finished and the fields are checked on the machine, or even later when the patient is already on the table. This paper presents an easy method of gauging if a collision is likely between the gantry and couch or patient during treatment planning. The method involves creating a chart of allowable gantry and couch combinations. The charts contain curves on a polar graph of the gantry and couch angle "plane". The curves display the limits of collisions for each gantry and couch combination for vertical couch positions 10, 15 and 20 cm below isocenter and for couch lateral positions of -10, 0, +10 cm, covering the majority of couch positions encountered in patient treatments. All combinations in the region within the curves (containing the origin) are valid, while all combinations outside the curves will result in a collision. The data for the charts are collected from measurements of the gantry angle that just clears each couch angle. The patient presence was modeled by placing a stereotactic body frame on the top of the couch. Separate charts were created for couch angles between 0° and 90° and between 360° and 270° over all gantry angles. The graphs are easy to create, implement, and use in the clinic and help reduce the time, complications, and uncertainties of planning with non-coplanar fields.