1. INTERACTION OF PARTICLES WITH MATTER
Radiation Damage

2. ENERGY LOSS OF PARTICLE RADIATION IN MATTER
Energy loss and dose are correlated with each other and help to formulate the interaction of internal and external radiation with matter to predict the affectivity of the radiation treatment and the possible damage to adjacent body tissue.
Radiation treatment is based on different kind of radiation and depends on the different kind of interaction between the radiation and matter (body tissue).

3. 1. Light charged particles (electrons)
• excitation and ionization of atoms in absorber material (atomic effects)
• interaction with electrons in material (collision, scatter)
• deceleration by Coulomb interaction (Bremsstrahlung)
2. Heavy charged particles (Z>1)
• excitation and ionization of atoms in absorber material (atomic effects)
• Coulomb interaction with nuclei in material (collision, scatter)
(long range forces)
3. Neutron radiation
• interaction by collision with nuclei in material (short range forces)

4. The interaction between radiation particles and absorber material determines the energy loss of the particles and therefore the range of the particles in the absorber material.
Each interaction process leads to a certain amount of energy loss, since a fraction of the kinetic energy of the incoming particle is transferred to the body material by scattering, excitation, ionization or radiation loss.
The sum over all energy loss events along the trajectory of the particle yields the total energy loss.

5. Electrons are light mass particles, electrons are therefore scattered easily in all directions due to their interactions with the atomic electrons of the absorber material. This results into more energy loss per scattering event.

6. The multiple scattering results in a very limited spatial resolution of the electron beam within the absorber material.
The energy loss of the electrons is dominated by excitation and ionization effects (dE/dx)exc and by bremsstrahlung losses (dE/dx)rad,
the energy loss components depend sensitively on the charge number Z and the average ionization potential I  11.5Z [eV] of the absorber material, the number density N, the relativistic velocity of the electrons v (  = v/c ) with mass m0.

7. RBE and LET
Relative Biological Effectiveness (RBE): factor for a given type of radiation is the number of rads of x radiation or gamma radiation that produces the same biological damage as 1 rad of the radiation being used.
Dose in Gy from 250 keV X-rays / Dose in Gy from another radiation source to produce the same biologic response
RBE =
The RBE for different kinds of radiation can be expressed
in terms of energy loss effects LET.

8. Deposition of energy of different particles
Photon
The electrons set in motion
if X-rays are absorbed are
very light, negatively charged particles. X-rays are sparsely ionizing
Neutron
By contrast, the particles set
in motion if neutrons are absorbed are heavy and densely ionizing. As the density of ionization increases, the probability of a direct interaction between the particle track and the target molecule
(possibly DNA) increases
Alpha particle

10. Linear Energy Transfer (LET): the energy deposited per unit track.
Linear Energy Transfer (LET) is the rate at which energy is deposited as a charged particle travels through matter by a particular type of radiation
Linear Energy Transfer (LET):
the energy deposited per unit track.
Unit is keV/m.
It is determined by:
quality of radiation
quantity of radiation
received dose of radiation
exposure conditions (spatial distribution)
The different kinds of radiation have different energy loss effects LET.
The linear energy transfer (LET) of charged particles in the medium is the quotient of dE/dx, where dE is the average energy locally imparted to the medium by a charged particle of specified energy in traversing a distance of dx.

11. air tissue high LET (, n, ~) incident radiation low LET (, x, ~)
As you will see from the diagram, alpha particles impart a large amount of energy in a short distance. Beta particles impart less energy than alpha, but are more penetrating. Gamma rays impart energy sparsely and are the most penetrating. Remember, gamma and x-rays vary widely in energy. The diagram shows a high energy gamma ray.
dispersion of energy
air
tissue
high LET (, n, ~)
greater radiotoxicity
incident radiation
low LET (, x, ~)
LET = linear energy transfer

12. The optimal LET
LET of about 100 keV/μm is optimal in terms of producing a biologic effect.
At this density of ionization, the average separation in ionizing events is equal to the diameter of DNA double helix which causes significant DSBs. DSBs are the basis of most biologic effects.
The probability of causing DSBs is low in sparsely ionizing radiation such as x-rays that has a low RBE.

13. For low LET radiation,  RBE  LET, for higher LET the RBE increases to a maximum, the subsequent drop is caused by the overkill effect.
These high energies are sufficient to kill more cells than actually available!
In the case of sparsely ionizing X-rays the probability of a single track causing a
DSB is low, thus X-rays have a low RBE. At the other extreme, densely ionizing
radiations (ex. LET of 200 keV/ μm) readily produce DSB, but energy is “wasted”
because the ionizing events are too close together. Thus, RBE is lower than
optimal LET radiation.

14. Effect of LET on cell survival
Survival curves for cultured cells of human origin exposed to 250-kV X-rays,
15-MeV neutrons, and 4-MeV alpha-particles. As the LET of the radiation increases, the survival curve changes: the slope of the survival curves gets steeper and the size of the initial shoulder gets smaller.
A more common way to represent these data is on the next slide.