How does beam divergence and beam attenuation in a phantom affect PDD?

Beam divergence and beam attenuation are critical factors that influence the percentage depth dose (PDD) within a phantom.

As the beam travels through a medium, it spreads out or diverges, which can lead to a reduction in the intensity of the beam reaching deeper layers within the phantom. This phenomenon occurs because the energy of the beam is distributed over a larger area, thereby decreasing the dose delivered to deeper tissues. Furthermore, beam attenuation refers to the absorption and scattering of photons as they pass through the material of the phantom. As photons interact with the material—be it through absorption, scattering, or other interactions—the overall energy of the beam diminishes, resulting in a lower dose delivered at greater depths.

Consequently, both beam divergence and attenuation lead to a decrease in the PDD as the depth increases; the dose received by tissues situated deeper in the phantom diminishes due to these effects. Therefore, it is correct to conclude that PDD decreases as a result of increasing beam divergence and beam attenuation. Understanding these relationships is crucial for accurate treatment planning and dose delivery in medical dosimetry.