The geometry of avoiding beam intersections and blocking tray collisions.

The intersection of a therapy x-ray beam with steel rails beneath or along the side of the patient support couch should be avoided. Collisions of the blocking tray mount with any portion of the couch are also prohibited. Equations are derived to determine the range of gantry angles which must be avoided for specified patient setups. These equations were incorporated into a brief computer program. The forbidden angular ranges for a representative set of field widths and isocenter positions are graphically presented for quick clinical reference.