When does output or Percent Depth Dose (PDD) become field size dependent for electron beams?

Output or Percent Depth Dose (PDD) for electron beams becomes field size dependent primarily due to the phenomenon of lateral scatter equilibrium. When the beam is delivered, the dose distribution in the treated volume is influenced by the ability of the electrons to scatter into neighboring areas. In smaller field sizes, the electrons are more likely to miss the edges of the field, resulting in a lesser extent of lateral scattering. As the size of the treatment field increases, the lateral scatter becomes more significant, and the variation in electron energy deposition across different field sizes can lead to differences in output or PDD.

This dependency underscores the importance of considering field dimensions when calculating doses for electron therapies, especially in clinical settings. In particular, for larger fields, electrons that scatter into the edges will not only affect the dose to the target area but may also impact the doses delivered to adjacent tissues or organs at risk, necessitating precise dosimetric calculations to ensure optimal treatment outcomes.

Other factors mentioned, such as the uniformity of longitudinal dose, maximum energy, or the patient's overall health, do not directly influence the field size dependency of PDD in the context of lateral scatter equilibrium. Thus, while those elements may play roles in other areas of treatment planning or clinical considerations,