For electron beam energy of E (in MeV), how does the PDD depend on field size W?

When discussing the percentage depth dose (PDD) for electron beams, the relationship between PDD and field size is quite important. For electron beam therapy, PDD is influenced by field size, particularly in relation to the energy of the electrons.

The correct choice highlights that the PDD is affected by field size only when the field size is less than the energy of the electron beam expressed in centimeters (E cm). This is because, at smaller field sizes, the scattered radiation produced from the edges of the field significantly contributes to the dose delivered at depth. As the field size increases and approaches or exceeds the effective range of the electrons, the impact of scatter becomes more consistent, and thus the PDD stabilizes.

If the field size is less than E cm, the scattering effect is pronounced, leading to variations in the PDD as the field size changes. However, once the field size exceeds that threshold, increases in field size have diminishing returns on the PDD, as there’s sufficient electron scattering, and the effect of increasing the size is less significant.

This understanding is essential for dosimetry, as it informs the treatment planning process and ensures that the appropriate dose is delivered to the tumor while sparing surrounding healthy tissues.