In the megavoltage energy range, what is the predominant interaction in the patient?

In the megavoltage energy range, the predominant interaction in the patient is the Compton interaction. This is primarily because, at these higher energy levels, photons have enough energy to displace outer shell electrons from atoms within the tissue without being fully absorbed. As a result, Compton scattering is the most significant interaction occurring when high-energy radiation passes through biological tissue.

Compton scattering involves an incoming photon transferring some of its energy to an electron, resulting in the electron being ejected from the atom while the photon is scattered at a different angle with reduced energy. This process is crucial in medical dosimetry because it contributes to both dose distribution in treatment planning and the biological effects of radiation on tissues.

In contrast, while photoelectric absorption is significant at lower energies, it decreases sharply as energy increases above a few hundred keV. Pair production requires even higher photon energies (typically over 1.022 MeV) and is not prevalent in the megavoltage range until much higher energies are involved. Rayleigh scattering, which involves the elastic scattering of photons by matter, plays a much less significant role in energy deposition compared to Compton interactions in this energy range. Thus, in megavoltage radiation therapy, the impact and relevance of