In a constant SSD treatment using POP fields, what is the total PDD at D-Max in the interior direction when the PDD due to the PA field is 47%?

In a constant Source-Surface Distance (SSD) treatment with Parallel Opposed (POP) fields, the total Percent Depth Dose (PDD) at the depth of maximum dose (D-Max) can be derived from the contributions of both fields. When dealing with POP fields, the anterior-posterior (AP) and posterior-anterior (PA) fields work together, creating an additive effect on the dose delivered at the D-Max.

Given that the PDD from the PA field is 47%, this means that at D-Max, the dose from the PA field is 47% of the maximum dose that would be delivered at a reference point in a specific geometry (like at the surface). In a POP configuration, the field contributions from both the anterior and posterior fields at D-Max are summed. Since a typical setting assumes that the dose contribution from the AP field equates to an approximately equivalent or similar level, this leads to an expected PDD at D-Max that is roughly the sum of both contributions.

When the PA field contributes 47%, the AP field would also contribute similarly in many typical scenarios, such as in a balanced POP field treatment. Therefore, assuming another contribution around the same value can lead to a total PDD that would be