When does the collimator scatter factor Sc depend on the collimator jaws or the MLCs? Pg. 443
If the MLC in the linac head is installed closer to the patient surface than the collimator jaws (as in Varian accelerators), the Sc factor depends predominately on the jaw opening and not on the MLC opening.
In the use of static MLC in voncentional radiotherapy, Sc for a given jaw opening is affected very little by the MLC setting for fields larger than 4 x 4cm. However, as the MLC aperture is reduced to much smaller openings, the Sc factor could drop significantly (e.g., by 5% for a 1 x 1-cm field). The reduction is caused by the MLC aperture approaching the geometric penumbra (radiation source has a finite size).
On the other hand, if the MLC is located above the collimator jaws, the head scatter would be affected more by the MLC setting than the jaw opening.
In either case, the treatment-planning algorithm must account for the Sc factors depending on the MLC geometry and the IMRT technique used.