What is the ratio of dose at depth in tissue to the dose at electronic equilibrium on the beam axis called?

The ratio of dose at depth in tissue to the dose at electronic equilibrium on the beam axis is known as the Percentage Depth Dose (PDD). This metric is crucial in radiation therapy as it provides insight into how radiation is absorbed as it penetrates deeper into tissue.

At electronic equilibrium, the dose remains constant with depth because the dose of radiation being delivered is balanced by the dose being scattered out and/or absorbed. As the beam penetrates tissue, the dose decreases due to absorption and scattering processes, and the PDD allows clinicians to understand how effectively a radiation dose will be delivered to a tumor situated within the tissue. This is especially important for treatment planning, as it assists in determining the necessary dose to achieve therapeutic effects while minimizing exposure to surrounding healthy tissues.

PDD values are also influenced by several factors, including beam energy and field size, making them an essential consideration in radiation therapy protocols.