How does the surface dose for electron beams compare to that of photon beams?

The surface dose for electron beams is significantly larger than that for photon beams primarily due to the different radiation interaction principles. Electron beams deposit energy closer to the surface of the skin because they have a limited range of penetration. They lose energy through ionization and scattering as they travel, which means that electrons deliver a high dose at the skin surface and then rapidly decrease in dose with depth.

In contrast, photon beams (such as X-rays or gamma rays) penetrate deeper into tissues before reaching their maximum dose, primarily due to their higher energy and penetrating capability. The build-up region for photons is typically longer, meaning the dose at the skin level is less compared to the high surface dose provided by electrons.

Understanding this fundamental distinction is crucial in treatment planning, as it impacts dosimetry in radiation therapy, particularly when treating superficial tumors. This reason emphasizes why the correct choice reflects a much larger surface dose for electron beams compared to photon beams.