Compton Scattering

Why Compton scattering dominates at treatment energies.

Now that you’ve seen which interactions occur at low energies, let’s look at the one that really matters for radiation therapy — Compton scattering.

In Compton scattering, a photon interacts with an outer-shell electron, transferring part of its energy to that electron and changing direction. The scattered photon continues with reduced energy, and the ejected electron (called a Compton electron) goes on to deposit energy in tissue.

We know that electrons in the outer shells are loosely bound to the atom, so it doesn’t take much energy to remove them. Because of this, they only "steal" a portion of the photon’s energy — just enough to be ejected — while the rest of the energy stays with the scattered photon, which continues traveling in a new direction.

This partial energy transfer is what makes Compton scattering so important in therapy. It explains how photons gradually lose energy as they pass through tissue, rather than being completely absorbed in one go. It is the most important interaction in soft tissues in radiation therapy.

Compton scattering dominates in the megavoltage range, which is why it’s the most important interaction for dose calculations in therapy.

The probability of Compton scattering depends mainly on the electron density of the material, which is why soft tissue, muscle, and water behave so similarly in therapy. This makes water phantoms ideal for dose measurement and calibration.

In the next lesson, we’ll explore pair production, which becomes significant only at very high photon energies.

Keep the photons flowing,

Sara