At a given depth, the scatter contribution to total dose does what as incident photon energy increases?

As photon energy increases, the scatter contribution to the total dose generally decreases at a given depth. This phenomenon is primarily related to how higher energy photons interact with matter.

At lower photon energies, the likelihood of compton scattering events occurring is greater, which leads to a higher proportion of the total dose being contributed by scatter. As the energy of the photons increases, interactions such as Compton scattering become less frequent, and the photons are more likely to pass through tissues with less interaction. Consequently, as energy increases, more of the dose comes from the primary radiation rather than from scattered photons.

In summary, the decrease in the scatter contribution as photon energy increases is a fundamental aspect of radiation interaction with matter, influencing dosimetry calculations and treatment planning in medical applications.