Why does scatter generally increase PDD?

The increase in scatter contributing to a higher percentage depth dose (PDD) is primarily due to its role in enhancing the dose delivered at deeper tissue levels, particularly along the central axis (CAX) of the radiation beam. As radiation travels through tissue, some of it scatters in various directions. This scattered radiation adds to the dose that tissues at greater depths receive.

When a treatment beam is delivered, the primary beam is attenuated as it passes through the skin and into the body. However, scatter from surrounding tissues results in secondary radiation that can reach these deeper tissues, effectively increasing the total dose at those depths compared to what would be delivered by the primary beam alone. This increase in the dose at depth, due to the contribution from scatter, leads to higher values of PDD, enhancing the effectiveness of the radiation treatment in targeting tumors situated deeper within the body.

In summary, scatter increases the PDD because it raises the dose that is delivered to tissues deeper along the CAX, improving the overall treatment efficacy by ensuring that enough radiation reaches the targeted treatment area deeper within the body.