Which of these characteristics is NOT related to beam flatness affected by a flattening filter?

The correct choice refers to energy distribution, which is not inherently a characteristic affected by beam flatness that is influenced by the flattening filter. The role of a flattening filter is to modify the beam profile as it exits the linear accelerator, ensuring that the radiation dose is uniform across the treatment area.

Beam flatness primarily relates to intensity uniformity, which indicates the consistency of dose distribution across the target area. A flattening filter is designed to make the intensity of the radiation beam more uniform, thereby enhancing treatment effectiveness. Additionally, correct positioning is crucial during treatment delivery to ensure that the beam is directed accurately at the target, thus contributing to the proper assessment of beam flatness.

The source diameter, related to the size of the radiation source, also plays a role in beam characteristics, as it affects the beam's profile and quality. Thus, even though energy distribution may influence therapeutic efficacy, it is not directly a characteristic altered by the flattening filter's purpose to achieve a uniform beam profile.