* RBP - 08: LET and RBE Flashcards
What is LET?
Linear Energy Transfer - The amount of energy that is transferrred per unit length of the track
What is RBE?
Radiation Biology Effect - looking at how different radiations interact with the same body cell
Give an example of high LET
Neutrons, alpha, beta (loe E), protons
Give an example of low LET
x-rays, gamma, beta (high E)
What does LOW LET mean?
Low LET = less transfer of energy = less ionisation
What is the opposite of sparse ionisation?
Densely ionising radiation
What is the formula for LET?
L = dE/dl (energy/dstance)
What is LET measured in?
keV/microM
What is the difference between LET and “stopping power”?
LET is the amount of energy deposited per unit track. Stopping power is 100% energy loss - like alpha cannot penetrate a piece of paper
What is the difference between Track Average and Energy Average (LET)?
Track is measured energy per unit distance, Energy average is measured per unit of energy
What would it mean if the distance of energy average is smaller?
means that the medium in which the radiation is deposited in needs more energy to be able to penetrate it
Is keV/microm LET of cobalt-60 gamma rays higher or lower than 250kV x-rays?
LOWER!!!
What is the trend for LET charged particles? What are the charged particles?
Energy increase, LET decrease. Electrons, protons, alpha
Order: Electron, proton and alpha in smallest to largest in LET
electron, proton and alpha
At 100kEv, what is the LET for electrons, protons and alpha?
0.42, 90, 260
Which is more penetrating: photons or charged particles?
Photons!
LET of photons: What is the trend? What are photons???
Photons referring to x-rays: in diagnostic range (less than 1MeV), energy increase and LET increase. ABOVE this, (therapeutic range), it becomes normal again: energy increase, LET decrease
What are photons???
x-ray and gamma!!!
Describe the path/track for alpha vs. beta
alpha - path is straight and short because they have high LET - transfer a low of energy so their ionisation is deposited very CONCENTRATED. Beta has low LET, so it travels further; as it releases energy, its path keeps changing
What is the range for alpha particles? Why is this important?
0.037mm - means it cannot penetrate a piece of paper
What is the spatial resolution for human eye?
0.2mm
Which one has higher LET? x-rays or gamma? What does this mean?
x-rays have higher LET - means it travels shorter distance but it transfers energy more
WHAT IS THE MOST IMPORTANT EXCEPTION!!!!! OF LET!!!
more energy is less let.. exception is x-rays in diagnostic range… UP TO 1000meV, as energy increase, LET increase (DIAGNOSTIC!!!)
Energy absorbed per unit mass is…?
J/kg - absorbed dose, gray
What is the unit of exposure?
Coulomb/kg
Define RBE - What does it mean by R?
Relative Biological Effect. RELATIVE because it depends what you’re comparing with each other! i.e. neutron compared to xray or neutron compared to alpha
Textbook definition of RBE?
Ratio of the standard dose to the test dose for equal biological effect
RBE: What do you use to compare (like, since tou are comparing the same amount of biological effect)
We are using human cells - LD50: Lethal dose 50, so how much each dose of each type of radiation needed to kill 50%
RBE: What is the formula?
Standard dose / test radiation
What is the unit of RBE?
no unit! comparing 2 doses!
What does RBE equal to 1 mean?
Means that both types of radiation has the same biological effect
Can RBE be <1?
Yes, of course!
What factors can affect RBE?
Presence of oxygen, fractionation
How does fractionation change RBE?
Depending on what radiation it is, it means the weaker radiation might need more energy to produce the same biological effect as the stronger radiation (i.e. xrays vs, alpha)…
Why does fracitonation increase the RBE (if the less ionising radiation is used as the standard)
because fractionation decreases the effect of i.e. x,rays, NOT, NOT NOT BECAUSE IT INCREASES DAMAGE OF NEUTRONS; NOT!!! - look at graph
Can OER be <1? Can RBE?
OER: no! RBE, yes!
What happens to OER as LET increases?
Decreases!
What is 1cGy equal to?
centrigray; so 100 cGy is = 1Gy
What is 1RAD?
1 RAD = 1 cGy
What is the relationship between RBE and LET?
RBE increases with LET up to A CERTAIN LEVEL - approx 100keV/microM – for ANY cell!! - See curve
The peak LET at a certain RBE is at 100keV/microM for any type of cell
TRUE
What is the peak LET for RBE? Why is it this value?
100keV/ microM. because it is inherit of DNA - DNA diameter… at 100keV/microM, we get two EXACT ionisations AT each strand
Describe the LET/RBE curve
from 10-100 keV/microM LET, RBE increases then rapidly at 100, then right after 100, it drops
Describe the relo between LET, OER, RBE
- High LET = no OER effect
2. High LET = RBE increase UNTIL it gets to 100keV/microM
Cells having a small shoulder in their photon survival curve have a small neutron RBE value. T or F? Explain
TRUE - because if they have a smaller shoulder, they need less energy to produce the same effects of neutron (which have no shoulder) - so the ratio would be smaller (Standard /test). RBE value smaller.
Why does RBE decrease after 100keV/microM?
BECAUSE, 100keV/microM is the optimal amount of energy given to give 2 ionisations/interactions at each strand of the DNA - if you give more dose, you are giving more ionisations however this is not at the strands - it is like a “Waste”; RBE is not as effective.
T or F: More densely ionising radiation is just as effective per track length, but less effective per unit dose
T
Does OER increase or decrease as LET increases?
It DECREASES - higher LET = more ionising = less time for the effect of oxygen to show
For RBE, its peak is at 100keV/microM. What is the relo between LET and OER?
Declines at 30keV/microM, reaches unity at 160keV/microM
Up to what MeV do x-rays return to the normal increase in energy = decrease in LET
1.0 MEV
