When Slot Collimation is enabled from this menu, or by the Lipped vs Slotted button in the toolbar of the Prescription window, collimation by the gold slot of an EP plaque of primary radiation emitted from a source is included in the dose calculation.
Slot collimation should be enabled for all EP plaques.
When slot collimation is enabled for an EP plaque, there is no need to simultaneously enable Shell Collimation because virtually all primary radiation is collimated at the surface of the gold slot surrounding the source. Including both Slot and Shell collimation greatly lengthens the calculation time because it requires ray tracing the path of primary radiation between a calculation point and each source and testing for collision with the thousands of triangles that model the shell instead of, or as well as, the 10 triangles that model a slot. As illustrated below, enabling Shell collimation instead of Slot collimation results in the same dosimetry but takes much longer to calculate.