R3.2: Linear Energy Transfer (LET)

Linear energy transfer (LET) is the average amount of energy a particular radiation imparts to the local medium per unit length; ie: Energy per Length. For radiotherapy, we are normally concerned about small amounts of energy over small distances, so the units we use are keV/μm.

### Calculating LET

The LET can be calculated by two methods:

• The track average calculates the average energy deposited over a set track length
• The energy average calculates the average distance over which a set amount of energy is deposited

These values are similar for most radiations, but vary greatly with neutrons.

### Examples of LET

Some example LETs are:

Photons Orthovoltage
250 keV
2.0 keV/μm
Photons 60Co Teletherapy
1.17 - 1.33 MeV
0.3 keV/μm
Photons Linac
3 MeV
0.3 keV/μm
Neutrons 14 MeV Track Average
12 keV/μm
Neutrons 14 MeV Energy Average
100 keV/μm
Protons 10 MeV Average
4.7 keV/μm
Protons 10 MeV On Entering Phantom
0.5 keV/μm
Protons 10 MeV At Bragg Peak
100 keV/μm

From the above table, it can be seen that photons have a low LET (as do electrons, not shown above). Neutrons show variability based on the way LET is measured. Protons have a low LET when they enter the phantom, but give up a lot of energy at the end of their track (the Bragg Peak). Caution must be taken when using LET to describe radiation, as it may be misleading in some circumstances (such as neutrons).

### LET and Relative Biological Effectiveness

This will be discussed in a later topic. Relative biological effectiveness compares the effect of a radiation when compared with a test radiation. There is a correlation with LET and RBE; radiation is more effective at causing damage as LET approaches 100 keV/μm, before falling off again. It is thought that over 100 keV/μm, so much energy is deposited in a cell that some of the energy is 'wasted' - also known as overkill. Radiation is wasted on the already dead cell, rather than travelling on to another cell to cause more damage.