Clinical Assignment 7 Biology Question 1

Define linear energy transfer (LET).
Describe the patterns of deposition of energy from beams of varying LET, using photons, protons and neutrons as examples.
Include the quality factors for each of these radiation types.
A table is expected.

1. LET is the average amount of energy imparted to a material per unit distance
2. For radiobiology we use keV/um
3. It is not actually very useful as it varies widely according to particle energy
4. The 'ideal' LET is 100 keV/um (highest cell kill per unit energy as per relative biological effectiveness)
5. In general:
1. Particles with low mass have a lower LET
2. Particles with charge have a higher LET
3. Except neutrons which are complicated because they don't interact like any other particles
 Photons 10 keV - ??? 250 keV 1.17 - 1.33 MeV (Cobalt 60) 3 MeV (Linac) 18 MeV (Highest possible output from Varian Linac) ??? 2 keV/um 0.3 keV/um 0.3 keV/um [taken from this blog] ??? Electrons 1 keV 10 keV 1 MeV 20 MeV 12.3 keV/um 2.3 keV/um 0.25 keV/um ??? Neutrons Neutrons are tricky because the LET depends on whether you view it as the mean energy deposited over a set distance (track average) or as the average distance over which a specific energy is deposited (energy average). Neutrons also behave differently at energies below or above 6 MeV - above 6 MeV they may cause carbon atoms to disintegrate into several alpha particles. Therefore, I guess it would be important to know both the track average LET and energy average LET for neutrons with energies above and below 6 MeV. Hall gives us 14 MeV. Track Average: Under 6 MeV 14 MeV ??? 12 keV/um Energy Average: Under 6 MeV 14 MeV ??? 100 keV/um Protons Again, Hall only gives us one example, of 10 MeV protons. Also, protons typically deposit their energy in a Bragg Peak, so I don't know how LET would even relate to them! In the chapter on protons (chapter 24), Hall talks about the initial LET of protons to be 0.5 keV/um. As they come to rest, the LET increases to about 100 keV/um (the 'ideal' LET). I can't tell if this is also dependent on beam energy (although one would assume that higher energies would have proportinally lower LET). 10 MeV Average LET - 4.7 keV/um Protons in general: LET on entering phantom - 0.5 keV/um LET on stopping - 100 keV/um
page revision: 9, last edited: 03 Aug 2010 12:02