When Shell Collimation is enabled from this menu, or by the Shell button in the toolbar of the Prescription window, collimation by the plaque's outer shell of primary radiation emitted from a source is included in the dose calculation.
For plaque shells with spherical curvature and a circular perimeter, such as the COMS plaques, collimation can be estimated analytically. For plaques with notches and arbitrary shapes, collimation is estimated using a much slower ray-trace method. The choice and priority between these algorithms is set as an intrinsic property of the plaque shell.
Shell collimation should be enabled for all COMS plaques because primary radiation is emitted into a silicone seed carrier and will reach the outer shell of the plaque. Shell collimation is generally disabled for EP slotted plaques in favor of the faster Slot Collimation method because nearly all radiation emitted from a source will be collimated by its immediately surrounding slot and never reaches the outer shell.