Relative biological effectiveness

In radiobiology, 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. In radiobiology, 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. In ionizing radiation dosimetry RBE is now represented in the recommendations of the International Commission on Radiological Protection (ICRP) by the radiation weighting factor, (WR) for each type of radiation; it was formerly the quality factor (Q). These weighting factors convert absorbed dose (measured in SI units of grays or non-SI rads) into formal biological equivalent dose for radiation exposure (measured in units of sieverts or rem). The higher the RBE or weighting factor for a type of radiation, the more damaging it is, and this is incorporated into the calculation to convert from gray to sievert units (see accompanying diagram). The relative biological effectiveness for radiation of type R on a tissue is defined as the ratio where DX is a reference absorbed dose of radiation of a standard type X, and DR is the absorbed dose of radiation of type R that causes the same amount of biological damage. Both doses are quantified by the amount of energy absorbed in the cells. 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 hundred nanometers (several tenths of a micrometer) apart, along their path. In contrast, the much more massive alpha particles and neutrons leave a denser trail of ionized atoms in their wake, spaced about one tenth of a nanometer apart (i.e., less than one-thousandth of the typical distance between ionizations for photons and beta particles). 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. Radiation weighting factors that go from physical energy to biological effect must not be confused with 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. To bypass the complexity of tissue dependence, the ICRP defined standard radiation weighting factors, independently of tissue type, to be used for risk and exposure assessment in radiology and the nuclear industry. These values are conservatively chosen to be greater than the bulk of experimental values observed for the most sensitive cell types, with respect to external (external to the cell) sources. Values for internal sources for heavy ions, such as a recoil nucleus, have not been developed. The ICRP 2007 standard values for relative effectiveness are given below.