The increase of dose with field size in the buildup region is attributed to which factor?

The increase of dose with field size in the buildup region is primarily attributed to increased electron transport. In the buildup region, the dose delivered by radiation is influenced by the behavior of electrons produced by the photon interactions with the tissue. As the field size increases, there are more photons entering the treatment area, which leads to the generation of a larger number of secondary electrons. These secondary electrons contribute to the dose through their scattering and interaction within the tissue.

With a larger field size, more of these electrons can travel into the tumor or target area before they lose energy, thus enhancing the dose delivered. This phenomenon occurs because the increased number of photons and, consequently, the increased number of electrons leads to a greater effective dose in the buildup region compared to a smaller field size where fewer electrons can contribute.

Factors such as decreased scattering, increased collimation, and higher patient density do not directly pertain to the increase of dose due to field size in the buildup region. Instead, they might influence the overall dose distribution but do not specifically explain the observed increase in the buildup region related to field size.