What remains the same for the collimator scatter factor, Sc, compared to that of the unblocked field?

The collimator scatter factor, Sc, is defined as the ratio of the dose delivered to a point in a homogeneous medium due to the collimator scattering, compared to the dose delivered by an unblocked field of the same nominal beam energy. This factor is primarily influenced by the geometry of the treatment field and the characteristics of the collimator rather than the patient-specific factors or field size.

In terms of its ratio, Sc remains constant regardless of the field size or the patient-specific anatomy. This means that even if the field size changes, the collimator scatter contributes a consistent relative amount of scatter dose in relation to the unblocked field under the same beam conditions. Thus, the core characteristic of Sc is its stability across different situations, as it is determined primarily by the equipment and treatment settings, rather than by variables like the patient's anatomy or the size of the treatment field. Therefore, it accurately reflects that Sc remains unchanged compared to the unblocked field.