Which of the following does NOT influence PDD for photon beams?

The concept of Percent Depth Dose (PDD) is crucial in understanding how radiation dose distribution changes with depth in tissue during radiation therapy. Factors such as field size, beam quality, and beam divergence play significant roles in determining how the dose is delivered to the target tissue.

Field size affects the PDD because larger fields can increase scatter radiation and enhance the dose delivered at a certain depth. Additionally, beam quality, which refers to the energy of the photons within the beam, directly impacts the PDD; higher energy beams generally exhibit higher PDD values due to reduced attenuation in tissue.

Beam divergence also influences PDD. As the beam travels deeper, it spreads out due to divergence, which can lead to a decreased dose per unit area at greater depths. As such, for different depths, the intensity and distribution of the photon beam change due to these factors.

In contrast, patient positioning primarily impacts the treatment’s geometric aspects, like alignment and the area receiving radiation, but does not inherently change the PDD characteristics of the beam itself. PDD is a property tied to the beam and the medium it penetrates rather than the positioning of the patient.