R3.3: Relative Biological Effectiveness (RBE)

Relative Biological Effectiveness (RBE) allows comparison of a test radiation with a standard radiation. It is the ratio of dose between the radiations to give a certain biological effect, for instance a grade of skin reaction or death of mice. Standard radiation is either 250 keV x-rays or 1.17/1.33 MeV 60Co gamma rays.

The formula is:

(1)
\begin{align} \text{RBE}=\frac{\text{Dose of Standard Radiation}}{\text{Dose of Test Radiation}} \text{for a given effect} \end{align}

For example, if 10 Gy of 60Co gamma rays kills 50% of the mice in a group, and 1 Gy of heavy ion radiotherapy kills the other group, the RBE would be:

(2)
\begin{align} \text{RBE}=\frac{10}{1} = 10 \end{align}

## Factors influencing RBE

Aside from the test and standard radiations, the RBE depends on a number of other factors:

• The chosen biological effect. It may be that to kill 100% of the mice, a different ratio of doses is required
• Fractionation of each dose. Fractionation may split the survival curves of the mice, meaning that 20 Gy of gamma rays versus 1.5 Gy of heavy ions are required, giving a RBE of 13.3.
• Changes in dose rate can also impact on the survival of cells. This can therefore alter the RBE.

## LET and RBE

As discussed in the LET section, there is a relationship between LET and RBE. As LET increases towards 100 keV/μm, RBE also increases, up to 4 - 5 when compared with low LET radiations. This increase is also dependent on the other factors which influence RBE (such as dose rate, fractionation and the chosen biological effect). Over 100 keV/μm, the RBE falls as LET increases. This is due to cell overkill - too much dose is deposited in individual cells, wasting radiation which could be used elsewhere.