Two types of phantom scatter correction factors (Sp) for high-energy photons

Two types of phantom scatter correction factors (Sp) are introduced for high-energy photons. One is the traditional Sp correction factor. It is produced using the primary absorbed dose under lateral CPE (charged particle equilibrium). It usually takes a non-zero value at zero-area field. The other is produced using the actual primary absorbed dose, which fails in lateral CPE when the field is small. It takes a value of zero at zero-area field. The two types of Sp correction factors can be connected with the laterally spread primary absorbed dose (LSD).