Relative biological effectiveness

Relative Biological Effectiveness (RBE) is a health physics concept introduced in the 1950s, after it was noted that different types of radiation might affect living organisms differently. A higher RBE equates with greater biological damage for equivalent radiation exposure.

The four primary types of ionizing radiation that were of concern to scientists working in laboratories were photon radiation, beta radiation, neutron radiation, and alpha radiation.

Sources that produced those types of radiations were used to irradiate various types of living cells grown in culture medium. Each type was given a known and measured amount of radiation exposure, delivering an exact amount of ionizing energy to each culture dish.

Early experiments

The cells grown were prokaryotic cells such as bacteria, simple eukaryotic cells such as single celled plants, and advanced eukaryotic cells derived from organisms such as rats. The cells were irradiated until they each reached the LD-50 point, that is, the point at which a lethal dose was delivered to half of the cells. A lethal dose was defined as one which created the inability to engage in mitotic division (or, for bacteria, binary fission), effectively sterilizing the cell, even if it still engaged in some cellular functions.

It was found that for all cell types, photon radiation and beta radiation were essentially equivalent, and they were assigned the base value of 1 for their RBE.

For neutron radiation it was found that it took 2–3 times more beta or photon radiation to cause an LD-50 compared to neutron radiation in bacteria; about 4–6 times more beta or photon radiation to cause an LD-50 compared to neutron radiation in simple eukaryotic cells; and about 6–8 times more beta or photon radiation to cause an LD-50 compared to neutron radiation for the higher eukaryotic cells. Accordingly, to be safe, the RBE for neutron radiation was given the value of 10 in governmental regulations.

For alpha radiation it was found that it took 4–6 times more beta or photon radiation to cause an LD-50 compared to alpha radiation in bacteria, about 8–12 times more beta or photon radiation to cause an LD-50 compared to alpha radiation in simple eukaryotic cells; and about 12–16 times more beta or photon radiation to cause an LD-50 compared to alpha radiation for the higher eukaryotic cells. Accordingly, to be safe, the RBE for alpha radiation was given the value of 20 in governmental regulations.

These RBE values are applied by health physicists so that they can compare various types of radiation exposure in a more meaningful way.Thus, one rad is one rem (röntgen equivalent in man) if delivered from, for example, an X-ray procedure involving photons. However, one rad of neutron exposure is thus 10 rem of exposure, and one rad of internally deposited alpha exposure is 20 rem of exposure. In the more current SI usage, the units are not the rad and rem, but the gray (Gy) and sievert (Sv), though the RBE values remain the same for calculating sieverts from grays of exposure.

While photon, beta and neutron are all relatively low LET (linear energy transfer) radiations, in which the ionizations caused by the radiation are separated by many thousands of ångströms (Å), alpha radiation is a high LET radiation, with the ionizations occurring essentially about every ångström of travel of the alpha particle.

Conclusion

In the studies conducted in the 1950s for determination of these RBE values, the sources of radiation were all external to the cells that were irradiated. However, in real life, external irradiation of tissue is essentially impossible from alpha radiation, because the alpha particle cannot traverse the dead layer of skin that surrounds people. Accordingly, alpha radiation is only 'meaningful' if it comes from internally deposited alpha emitters, which then allows for intimate proximity to the alpha radiation. The range of an alpha particle is typically about the diameter of a single eukaryotic cell.

This distinction has raised a serious concern that the RBE for alpha-emitters might be substantially underestimated [Winters-TH, Franza-JR, Radioactivity in Cigarette Smoke, New England Journal of Medicine, 1982; 306(6): 364–365] , as it neglects the small, but potentially significant, ionization caused by the recoil of the parent nucleus during the alpha decay. While this energy of the recoil nucleus is typically only about 2% of the energy of the alpha particle, the range of the recoil nucleus is extremely short, i.e. about 2–3 Å, due to its high electric charge and high mass. Thus, all of the ionization energy is deposited in an extremely small volume wherever the parent nucleus happens to be located, which is often on the chromosomes because alpha emitters are typically heavy metals which preferentially collect on (stain) chromosome material. This has the effect of 'wiping out' that region of the chromosome, whereas the alpha particle only causes a few ionizations, losing most of its ionization energy in the cytoplasm. Studies conducted with intratracheal instillation of polonium-210, an alpha emitter, in hamsters have yielded RBEs as high as 1,000 in some studies.

Due to the uncertainty associated with the RBE for alpha radiation, and the lack of neutron exposure to personnel in almost all settings, it is strongly discouraged to use rems or sieverts as measures of comparative radiation exposure, and instead to use the actual exposure in rads or grays, followed with a description of the type of exposure (almost always from photon or beta in which the RBE is 1 anyway, though rarely from alpha).

Notes and references