LET:

• is the rate at which energy is lost from different types of radiation while traveling through matter
• measured in keV/u (kev / microns), which is energy deposited per unit path length of an absorber traveled by the particle.
•  is a function of:

•  mass
•  charge
• Low LET radiation: (sparsely ionizing)

Electromagnetic radiation (x and gamma radiation) has no mass or charge. Interacts with matter by producing fast electrons which have a small mass and -1 charge. Because of the electron's fast speed and low mass the interactions that are produced are far apart from each other, for this reason electromagnetic radiation is called low LET radiation.

• travels faster

• Medium LET radiation:

Neutrons even through they have no charge are highly ionizing particles because of their mass.

• High LET radiation: (densely ionizing)

Alpha particles because of their mass and charge are even more highly ionizing particles than neutrons. These types of particles produce many ionizations in a short distance and therefore are called high LET radiation.

• travels slower

 

• The greater the mass and charge the higher the LET will be
• The higher the LET the more chance of direct effect of radiation:
• Very little repair
• As you increase LET the shoulder region gets smaller
• As you increase LET the Dq gets smaller

**Dq can also be seen as the dose required to bring a cell to only 1 hit left.  When the amount of targets needed to kill a cell have thus been eliminated we will now have straight line killing

LET AND THE CELL SURVIVAL CURVE:

As the LET increases the cell survival curve changes in two important respects:

1. the slope of the cell survival curve increases (becomes steeper) and Do becomes smaller.
2. the extrapolation number goes to one (the shoulder becomes smaller - Dq becomes smaller)

RBE (RELATIVE BIOLOGICAL EFFECTIVNESS)

RBE – ratio of the absorbed dose of a standard type of radiation to that of the test radiation required to produce the same degree of stipulated biological effect

(the stipulated biological effect is the endpoint defined)

• Equal doses of different LET radiation do not produce equal biological effects.
•  A term relating the ability of radiations with different LETs to produce a specific biologic response is relative biological effectiveness (RBE).
• In comparing the different LET radiations it is customary to regard 250 kV x-rays as the standard.
• RBE is defined as the comparison of a dose of some test radiation to the dose of 250 kV x-rays that produces the same biologic response.

RBE IS DEPENDENT ON:

• LET : as LET goes up the RBE will go up (direct realationship)
• DOSE : as dose goes up the RBE will go down (indirect relationship)
• ENDPOINT :
• at early endpoints you will have a high RBE and
• at latter endpoints you will have a low RBE (beyond Dq region)
• If the number of fractions is increased this will also increase the RBE

Examples :

"overkill" effect: ( RBE ) :This effect can be explained by looking at the number of "hits" that the particles produce as they travel across the cell.

• With LOW LET radiation the number of "hits" is low per particle, therefore the dose required (the number of particles) to kill the cell is higher as compared to a higher LET radiation.
•  With the LET of the radiation at the optimal level OPTIMAL the number of "hits" per particle is just at the right LET level to kill the cell with no wasted dose.
• With the HIGH LET radiation the number of “hits” per particles is over the number needed to kill a cell.  This is a wasted dose, for a cell that is dead is not more dead with more “hits”