As linear energy transfer (LET) decreases, how is relative biologic effectiveness (RBE) affected?

Relative biologic effectiveness (RBE) is a measure used to compare the biological effectiveness of different types of ionizing radiation. It is influenced significantly by linear energy transfer (LET), which quantifies the energy deposited per unit length of tissue traveled by the radiation.

As LET decreases, the biological effectiveness of the radiation on tissues tends to diminish. This is because lower LET radiation, such as X-rays and gamma rays, tends to produce indirect ionization by interacting with electrons rather than direct ionization of biological molecules. This results in fewer direct interactions with DNA and cellular structures that can cause damage compared to high LET radiation, such as alpha particles, which deposit energy more densely and cause more significant damage.

Consequently, as the energy transfer per unit distance decreases, the ability of the radiation to cause biological damage also decreases, leading to a lower RBE. This relationship emphasizes the importance of LET in determining how effective different types of radiation are in causing biological effects, particularly in the context of radiation therapy and safety assessments in radiation exposure scenarios.