Relative Biological Effectiveness

In radiology, the relative biological effectiveness (often abbreviated as RBE) is the ratio of biological effectiveness of one type of ionizing radiation relative to another, given the same amount of absorbed energy. The RBE is an empirical value that varies depending on the particles, energies involved, and which biological effects are deemed relevant. It is a set of experimental measurements.

In dosimetry (the practical attempt to apply RBE realistically to human and animal experience), the RBE is represented in regulations by the radiation weighting factor, (W_R) or formerly the quality factor. The weighting factors, arrived at by consensus of governments, industry, and regulators, converts absorbed dose (measured in units of grays or rads) into biological equivalent dose for radiation exposure (measured in units of sieverts or rem).

The higher the RBE or weighting factor numbers for a type of radiation, the more damaging is the type of radiation, per unit of energy deposited in biological tissues.

Different types of radiation have different biological effectiveness mainly because they transfer their energy to the tissue in different ways. Photons and beta particles have a low linear energy transfer coefficient, meaning that they ionize atoms in the tissue that are spaced by several thousand angstroms apart along their path. In contrast, alpha particles and neutrons leave a denser trail of ionized atoms in their wake, spaced about one angstrom apart.

Radiation weighting factors that go from physical energy to biological effect must not be confused with the tissue weighting factors. The tissue weighting factors are used to convert an equivalent dose to a given tissue in the body, to an effective radiation dose, a number that provides an estimation of total danger to the whole organism, as a result of the radiation dose to part of the body.

The concept of RBE is relevant in medicine, such as in radiology and radiotherapy, and to the evaluation of risks and consequences of radioactive contamination in various contexts, such as nuclear power plant operation, nuclear fuel disposal and reprocessing, nuclear weapons, uranium mining, and ionizing radiation safety. However, the various RBE's are scientific numbers that represent raw data, and they are the data that go into regulatory consensus weighting factors which represent the best guess as to how relatively dangerous various types of radiation are, in practice. The radiation weighting factors will be approximately the same as the RBE's that result from some experiments, but may be quite different from RBE's that result from other experiments.