Relative biological effectiveness

The relative biological effectiveness, RBE short (English: relative biological effectiveness, RBE ), is in radiation biology, a distinguishing factor for radiation types in terms of their biological effects.

The same physical dose of radiation can exert different biological efficacy in various types of radiation. Reasons may be the different nature of the fabric, the different temporal dose rate or the different local dose distribution ( linear energy transfer, LET) and ionization density of the radiation.

Definition

The relative biological effectiveness is defined as the ratio of the absorbed dose D ref a reference radiation, which causes a particular biological effect to dose DY other radiation that is f for the occurrence of the same effect under the same conditions on the same biological object required:

The RBW is generally dependent on the dose administered, the LET and the type of injury observed.

Function of the linear energy transfer

Radiation with large linear energy transfer radiation so heavy, charged particles, produced more ionizations per volume ( ionization cluster ) than low- LET radiation. This results in the human body to a greater probability of double-strand breaks in the DNA, which can only with difficulty or not be eliminated by the cell's own repair mechanisms.

The relative biological activity increases as a function of the linear energy transfer at the same dose at first continuously up to a maximum of about to be strong decrease. Reason for this is that radiation with very high LET energy deposited in a cell than is required for their inactivation; it comes to the so-called overkill.

Depending on the observed effect

Common direct radiation effects are local damage to the DNA.

• X-rays produced about 1,000 single-strand breaks per cell and Gray, alpha radiation, however, about 250 single-strand breaks per cell and Gray. Thus, the value is the RBW alpha radiation for the generation of single-strand breaks

• X-rays generated about 35 double-strand breaks per cell and Gray, alpha radiation about 63 double-strand breaks per cell and Gray. The RBE value of alpha radiation for the production of double-strand breaks

Determination and results

The RBE is determined experimentally. For this, the effects of several types of radiation on the survival rates of different organisms such as bacteria, eukaryotes, or rats are compared. As reference radiation, low-LET radiation, such as 250 keV X-rays or gamma rays with energies of 60 Co of 1.17 MeV and 1.33 MeV is usually used.

Similar doses of photons and beta radiation usually lead to similar effects in the irradiated organism, so therefore have a similar biological activity during irradiation with neutrons, ions or alpha particles caused significantly more serious damage. Depending on the radiation energy is the biological activity of these particles is between two and twenty times as high as that of X-rays.

Related to dose equivalent

The relative biological activities of different tissues and organisms are generally not comparable, therefore need to be determined or estimated separately. Because this approach but usually is not practical and ethical human experiments were not particularly viable, radiation weighting factors wR are defined in radiation protection based on the known efficiencies that allow a dose- independent comparison of different types of radiation. With the introduction of the uniform equivalent dose limits can be set in consideration of the effectiveness of all kinds of rays.

The radiation weighting factors do not represent physical quantities, but are merely standards for simplified handling in radiation protection. In general, they overestimate the actual biological activity. In Germany they are defined normatively in the Radiation Protection Ordinance.