Calculated lateral penumbra widths (beam width at 80% of maximum dose minus beam width at 20% of maximum dose) for a 65 mm thick brass and a 200 mm SWX-207HD polyethylene collimator (aperture opening diameter 60 mm), using the detailed beamline geometry shown in figure 2, and typical beam and nozzle settings. In the entrance region, the lateral penumbra is worse from the thicker plastic collimator, but in the spread-out Bragg peak (in this case, from 110 to 150 mm), the difference in the lateral penumbra is 0.5 mm.