The Transmission-to-Air Ratio (TAR) is typically not preferred with which type of beams?

The Transmission-to-Air Ratio (TAR) is a measure used in radiation therapy to quantify how much of the irradiation beam penetrates a given medium compared to the amount that passes through air. TAR is particularly useful for assessing fields from photon beams and electron beams because it accounts for the collision and scattering effects that are more prominent in these types of radiation.

Accelerator photon beams, which are commonly used in external beam radiation therapy, produce more predictable and well-characterized relationships between the radiation dose delivered to a patient and the control of the beam parameters like energy and field size. The interactions of these beams with various media are well understood, allowing for accurate modeling using TAR.

In contrast, neutron and particle beams (including protons) have different interaction properties compared to photon beams. Neutrons, for instance, interact with matter predominantly through nuclear reactions rather than electromagnetic interactions, leading to less straightforward energy deposition profiles which make TAR less applicable. Proton beams also exhibit a unique depth-dose characteristic due to the Bragg peak effect, which is not accurately represented using TAR.

Thus, while TAR is effectively applied to photon beams, the peculiarities of neutron and particle beams make TAR less suitable, with accelerator photon beams being the least compatible choice in this context. This is