What is the primary reason for the field size dependence of PDD?

The primary reason for the field size dependence of Percentage Depth Dose (PDD) is increased electron scatter. As the size of the radiation field increases, the number of electrons produced by the photon interactions also increases. These secondary electrons can travel further from their point of origin and contribute to the dose delivered at varying depths in the tissue.

In larger fields, the scattering of electrons becomes more significant. This is because a wider area leads to additional interactions and scattering events, allowing more electrons to contribute to the dose at deeper levels. Therefore, as the field size increases, the PDD will generally be higher at a given depth, as more scatter and interaction events enhance the dose distribution.

In contrast, increased air absorption, higher energy photon production, and increased tissue density have less direct impact on the field size dependence of PDD compared to electron scattering. Air absorption primarily affects photon attenuation and resultant dose but does not account for the phenomenon of increased scattering with larger fields. Higher energy photons and tissue density also influence dose distributions but are not the primary reasons for the field size dependence observed in PDD. Thus, the influence of increased electron scatter is the most critical factor in understanding how PDD changes with field size.