In a parallel opposed, equally weighted, constant SSD field treatment, what is the dose at D-Max depth due to each field?

In a parallel opposed, equally weighted, constant source-to-skin distance (SSD) field treatment, the concept of depth and dose distribution is critical in understanding how radiation dose is delivered to the target area.

At D-Max depth, which is the depth at which the dose delivered by the radiation beam is at its maximum, each field contributes to the dose in a significant way. For radiation treatments, particularly in parallel opposed fields, the dose at the D-Max is typically additive from both fields due to their equal weighting and opposing orientations. This means that both fields deliver the same dose to the target area simultaneously, enhancing the overall dosage at that depth.

In this scenario, if each field contributes 100% of its prescribed dose at the D-Max, and since there are two fields, the total effective dose at D-Max depth effectively doubles, leading to a 100% dose contribution from each field combined. Thus, at D-Max, the patient receives the full intensity of both beams, resulting in a cumulative dose at that depth of 100%. This rationale aligns with the principles of radiation therapy where overlapping fields produce additive effects.

Therefore, the correct answer reflects the standard of care in delivering effective radiation treatment through parallel opposed fields.