Particle Interactions with Matter (LET and RBE)

Question 1

What is collisional energy loss?

Answer 1

Negatively charged electron experiencing repulsive Coulomb force resulting interaction and energy lost from the incident electron.

It’s also a form of dose deposition such as linear energy transfer (LET)

Question 2

What is another type of energy loss other than collisional energy loss?

Answer 2

Radiative energy loss

Question 3

What is radiative energy loss?

Answer 3

any charged particle undergoing acceleration near traveling a nucleus produces radiation in the form of photon which is also called “bremsstrahlung” photons.

this is also the way we produce x-ray or radiation is through the radiative energy loss.

Question 4

What formalism is developed to describe the radiative energy loss that comes from the bremsstrahlung process?

Answer 4

S rad (Radiative Stopping Power)

Question 5

What are the two types of energy loss mechanisms and their examples?

Answer 5

1. Collisional (via Coulomb) –> This is the way you deposit dose ~ LET
2. Radiative (via acceleration) –> Bremsstrahlung (This is the way we provide X-ray)

Question 6

Stopping power is made up for two types of energy loss, what are they?

Answer 6

Total stopping power (S tot = S coll (stopping power from collisional energy loss) + S rad (stopping power from radiative energy loss)

Question 7

Sometimes S coll (stopping power from collisional energy loss) is also called

Answer 7

electronic stopping power since it involves the electrons in the atom

Question 8

S rad (stopping power from radiative energy loss) is also called

Answer 8

nuclear stopping power since it involves an interaction with the nucleus

Question 9

What is the quantity that S coll (collisional stopping power = electronic stopping power) is closely related to?

Answer 9

Linear energy transfer (LET), which is a description of energy deposited locally and is useful b/c it is related to biological effects operating at the level of the cell

Question 10

What two parameters depend on radiative stopping power (S rad)?

Answer 10

Mass and Z

Question 11

Which type of stopping power or energy loss is associated with Bremsstrahlung process?

Answer 11

S rad, or radiative stopping power or radiative energy loss

Question 12

What is the S rad (radiative stopping power) equation and what is the implication between proton vs electron?

Answer 12

S rad = k x (Z/mass)^2

radiative stopping power (S rad) is proportional to the square of the atomic number, Z and to the inverse square of mass

For proton beams, the radiative stopping power will be relatively small whereas for electron, it will be large

Also, the radiative stopping power will be higher in high Z materials

Question 13

Unlike radiative stopping power that depends on Z and mass, the collisional stopping power depends on

Answer 13

the number of electrons per unit gram in the material (higher the Z the electrons per unit gram goes down)

Question 14

Is collisional stopping power high or low in high Z material?

Answer 14

low, because high Z material has lower electron density

Question 15

draw the graph of stopping power vs energy of water and lead for electrons

Answer 15

Question 16

For heavy particle such as proton, almost all of the stopping power is

collisional loss or radiative loss?

Answer 16

Collisional loss as radiative loss for heavy charged particle are very small

Question 17

draw the graph of stopping power of proton in water (stopping power vs energy)

Explain the relationship of the stopping power and the energy of the particle? and explain the reason.

Answer 17

Figure 7.2.2

The stopping power of the proton increases as the energy of the particle decreases and this is b/c as the speed (or energy) decreases the loss of energy is higher

Question 18

what parameter is most relevant to explaining the path of charged particle?

Answer 18

Mass

Therefore, electron which has a very small mass, does not travel in straght line unlike heavy particles such as proton and carbon

Question 19

What is LET (linear energy transfer)?

Why is it useful?

What is collisional stopping power?

Answer 19

A quantity that describes the energy deposited locally near the point of interaction of a particle.

It is useful b/c it can be tied to biological effects operating at the level of the cell.

LET is equal to the collisional stopping power, S coll

Question 20

LET depends on

Answer 20

types of radiation (X-ray vs light charged particle vs heavier charged particle

LET is much higher for more massive charged particles (up to 100 > KeV/um)

Question 21

LET is high-LET modality b/c

Answer 21

the products they produce (protons and various nuclei) are high-LET

Question 22

LET has a strong impact on the biological effectiveness of radiation modalities and the metric used to quantify this is

Answer 22

relative biological effectiveness (RBE)

Question 23

How is RBE defined?

Answer 23

RBE is defined as the ratio of doses two types of radiation that achieve the same biological effect

Question 24

Describes the relationship between LET and RBE

Answer 24

As LET increases RBE also increases up to approximately 100 keV/um where the RBE can be 3 or more.

Question 25

LET is relatively low for

Answer 25

X-rays, gamma rays, and electrons (< few keV/um)

Question 26

LET is much higher for more

Answer 26

massive charged particles (up to > 100 keV/um)

this is due to a z^2 dependence for collisional stopping power (per eq. 7.3 on page 67)

Question 27

Neutrons are a high-LET modality b/c

Answer 27

the products they produce which are protons and various nuclei are high-LET