PDD is normalized to 100% at which depth?

PDD, or percent depth dose, is a crucial concept in radiotherapy, representing the percentage of the dose delivered to a specific depth in tissue compared to the dose at a reference depth. In this case, PDD is normalized to 100% at Dmax, which is the depth at which the dose from a therapeutic radiation beam reaches its maximum value.

Dmax is significant because it represents the point where the radiation intensity is greatest and where the dose is most adequately delivered to the tumor while minimizing exposure to surrounding healthy tissue. As you move deeper into the tissue beyond Dmax, the dose gradually decreases due to factors like tissue attenuation and scatter, resulting in a lower PDD percentage at greater depths. By normalizing PDD at Dmax, clinicians can accurately assess how much dose is delivered at various depths within the treatment area, which is critical for treatment planning and ensuring effective radiation therapy.

Other choices, such as normalization at a uniform depth like 10 cm or at the surface of the skin, do not correspond with the standard practice in dosimetry where Dmax serves as the reference point for dose calculations.