Relative Biological Effectiveness Rbe

an index that reflects the strength of a particular type of ionizing radiation (for example, alpha particles, beta particles, or neutrons) relative to the strength of a standard radiation (for example, roentgen rays) with respect to the radiation effect on a single biological substrate.

RBE is the ratio of the angle of inclination of the test radiation to the angle of inclination of an analogous, standard radiation, provided that the dependence of the biological effect on the radiation dosage is linear to both types of radiation. If the dosage dependence is not linear, RBE is calculated as the ratio of doses of standard and tested radiations that cause the same effect. Since the RBE can vary with dosage and magnitude of the observed effect in this case, the level of effect for which the value of the RBE is derived must be specified. For example, if the RBE of neutrons as compared to that of gamma rays with LD_50/30 (lethal dose 50 at 30 days) is equal to 2 with mice, neutrons are twice as effective as gamma rays in destroying one-half of a population of mice within 30 days after irradiation.

The dependence of RBE on dosage can vary. The RBE depends chiefly on the spatial distribution of absorbed energy in the irradiated substrate, as measured by the linear energy transfer (LET) per unit path length of the ionizing particle. The dependence of RBE on LET varies in different objects and with different biological reactions to radiation. The RBE of rays with low LET is usually similar, but RBE increases as LET increases. The coefficient of RBE for electrons, positrons, roentgen and gamma rays, and rapid protons is usually close to 1; for alpha particles and rapid neutrons, 10; and for recoil nuclei and heavy ions with more than a single charge, up to 20.