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ONPS SEM 2 > * RBP - 08: LET and RBE > Flashcards

* RBP - 08: LET and RBE Flashcards

1
Q

What is LET?

A

Linear Energy Transfer - The amount of energy that is transferrred per unit length of the track

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2
Q

What is RBE?

A

Radiation Biology Effect - looking at how different radiations interact with the same body cell

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3
Q

Give an example of high LET

A

Neutrons, alpha, beta (loe E), protons

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4
Q

Give an example of low LET

A

x-rays, gamma, beta (high E)

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5
Q

What does LOW LET mean?

A

Low LET = less transfer of energy = less ionisation

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6
Q

What is the opposite of sparse ionisation?

A

Densely ionising radiation

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7
Q

What is the formula for LET?

A

L = dE/dl (energy/dstance)

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8
Q

What is LET measured in?

A

keV/microM

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9
Q

What is the difference between LET and “stopping power”?

A

LET is the amount of energy deposited per unit track. Stopping power is 100% energy loss - like alpha cannot penetrate a piece of paper

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10
Q

What is the difference between Track Average and Energy Average (LET)?

A

Track is measured energy per unit distance, Energy average is measured per unit of energy

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11
Q

What would it mean if the distance of energy average is smaller?

A

means that the medium in which the radiation is deposited in needs more energy to be able to penetrate it

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12
Q

Is keV/microm LET of cobalt-60 gamma rays higher or lower than 250kV x-rays?

A

LOWER!!!

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13
Q

What is the trend for LET charged particles? What are the charged particles?

A

Energy increase, LET decrease. Electrons, protons, alpha

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14
Q

Order: Electron, proton and alpha in smallest to largest in LET

A

electron, proton and alpha

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15
Q

At 100kEv, what is the LET for electrons, protons and alpha?

A

0.42, 90, 260

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16
Q

Which is more penetrating: photons or charged particles?

A

Photons!

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17
Q

LET of photons: What is the trend? What are photons???

A

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

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18
Q

What are photons???

A

x-ray and gamma!!!

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19
Q

Describe the path/track for alpha vs. beta

A

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

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20
Q

What is the range for alpha particles? Why is this important?

A

0.037mm - means it cannot penetrate a piece of paper

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21
Q

What is the spatial resolution for human eye?

A

0.2mm

22
Q

Which one has higher LET? x-rays or gamma? What does this mean?

A

x-rays have higher LET - means it travels shorter distance but it transfers energy more

23
Q

WHAT IS THE MOST IMPORTANT EXCEPTION!!!!! OF LET!!!

A

more energy is less let.. exception is x-rays in diagnostic range… UP TO 1000meV, as energy increase, LET increase (DIAGNOSTIC!!!)

24
Q

Energy absorbed per unit mass is…?

A

J/kg - absorbed dose, gray

25
Q

What is the unit of exposure?

A

Coulomb/kg

26
Q

Define RBE - What does it mean by R?

A

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

27
Q

Textbook definition of RBE?

A

Ratio of the standard dose to the test dose for equal biological effect

28
Q

RBE: What do you use to compare (like, since tou are comparing the same amount of biological effect)

A

We are using human cells - LD50: Lethal dose 50, so how much each dose of each type of radiation needed to kill 50%

29
Q

RBE: What is the formula?

A

Standard dose / test radiation

30
Q

What is the unit of RBE?

A

no unit! comparing 2 doses!

31
Q

What does RBE equal to 1 mean?

A

Means that both types of radiation has the same biological effect

32
Q

Can RBE be <1?

A

Yes, of course!

33
Q

What factors can affect RBE?

A

Presence of oxygen, fractionation

34
Q

How does fractionation change RBE?

A

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)…

35
Q

Why does fracitonation increase the RBE (if the less ionising radiation is used as the standard)

A

because fractionation decreases the effect of i.e. x,rays, NOT, NOT NOT BECAUSE IT INCREASES DAMAGE OF NEUTRONS; NOT!!! - look at graph

36
Q

Can OER be <1? Can RBE?

A

OER: no! RBE, yes!

37
Q

What happens to OER as LET increases?

A

Decreases!

38
Q

What is 1cGy equal to?

A

centrigray; so 100 cGy is = 1Gy

39
Q

What is 1RAD?

A

1 RAD = 1 cGy

40
Q

What is the relationship between RBE and LET?

A

RBE increases with LET up to A CERTAIN LEVEL - approx 100keV/microM – for ANY cell!! - See curve

41
Q

The peak LET at a certain RBE is at 100keV/microM for any type of cell

A

TRUE

42
Q

What is the peak LET for RBE? Why is it this value?

A

100keV/ microM. because it is inherit of DNA - DNA diameter… at 100keV/microM, we get two EXACT ionisations AT each strand

43
Q

Describe the LET/RBE curve

A

from 10-100 keV/microM LET, RBE increases then rapidly at 100, then right after 100, it drops

44
Q

Describe the relo between LET, OER, RBE

A
  1. High LET = no OER effect

2. High LET = RBE increase UNTIL it gets to 100keV/microM

45
Q

Cells having a small shoulder in their photon survival curve have a small neutron RBE value. T or F? Explain

A

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.

46
Q

Why does RBE decrease after 100keV/microM?

A

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.

47
Q

T or F: More densely ionising radiation is just as effective per track length, but less effective per unit dose

A

T

48
Q

Does OER increase or decrease as LET increases?

A

It DECREASES - higher LET = more ionising = less time for the effect of oxygen to show

49
Q

For RBE, its peak is at 100keV/microM. What is the relo between LET and OER?

A

Declines at 30keV/microM, reaches unity at 160keV/microM

50
Q

Up to what MeV do x-rays return to the normal increase in energy = decrease in LET

A

1.0 MEV

ONPS SEM 2 (12 decks)

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