Given a PDD of 70.5% in the patient midplane due to a single field, what is the total PDD at the tumor center in a treatment involving POP fields?

In radiation therapy, the percentage depth dose (PDD) is a critical parameter representing the amount of dose delivered at a specific depth relative to the dose at a reference point, often the surface. The PDD of 70.5% at the midplane indicates that the dose delivered at that point is 70.5% of what would be delivered at the reference point (typically at the surface).

For a treatment plan utilizing opposing parallel (POP) fields, it is important to understand that the doses from each field overlap at the center of the tumor. This overlap effectively adds the doses contributing from each field.

In this case, if we consider the PDD of 70.5% to represent the dose delivered from one field, the same PDD would be assumed for the second field given the same treatment parameters are applied. Since there are two fields contributing to the overall dose at the tumor center, the total dose delivered can be calculated by adding the contributions from each field. Therefore, this results in:

PDD from the first field + PDD from the second field = 70.5% + 70.5% = 141%.

This additive dose effect at the tumor center due to the presence of the two fields illustrates why