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Medical Physics: Radiotherapy > Radiobiology > Flashcards

Radiobiology Flashcards

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1
Q

What does alpha represent?

A

Alpha - mean number of lethal double-strand DNA breaks per cell per unit dose caused by one ionisation track.

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2
Q

What does beta represent?

A

Beta - Mean number of lethal double-strand DNA breaks per cell per unit dose caused by two ionisation tracks.

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3
Q

What does the alpha-beta ratio represent?

A

The dose at which double-strand break damage caused by single tracks equals those caused by two independent tracks.

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4
Q

What does a low alpha-beta ratio mean for a tissue?

A

Greater fractionation sensitivity and repair capability

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5
Q

Define the Tumour Control Probability (TCP).

A

The fraction of tumours that given a certain radiotherapy regime will have local control (i.e. there are no active clonogenic cells).

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6
Q

Define the Normal Tissue Complication Probability (NTCP).

A

The relative probability of a late effect occurring after a given time delay following radiotherapy.
Usually quoted as TD(50/5) i.e. the 50% probability of an effect after five years.

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7
Q

Define the Biologically Effective Dose (BED) or the Extrapolated Response Dose (ERD).

A

A function that represents the effectiveness of a radiotherapy regime taking into account the total dose, dose per fraction, total time for the course, alpha-beta ratio, and cell repair kinetics.

It is equal to the dose to give the same effects using an infinite number of fraction and an infinitesimally small dose per fraction.

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8
Q

What is the ion responsible for the majority of the non-repairable lesions induced on the DNA strand?

A

Hydroxyl (OH-).

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9
Q

What is the T(1/2) for normal tissue?

A

1.5 hours.

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10
Q

What is the T(1/2) for tumour cells?

A

0.5 hours.

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11
Q

What percentage of single strand breaks are repaired between fractions?

A

100%

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12
Q

What is the equation for the survival fraction for a single fraction?

A

SF = EXP[-(alphadose + Betadose^2)]

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13
Q

What is the equation for the survival fraction for n fractions?

A

SF(n) = EXP[-ndose(alpha + Beta*dose)]

= EXP[-total dose(alpha + Betadose)]

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14
Q

How can the alpha-beta ratio be found graphically?

A

Extrapolate a line from the tangent of the initial survival fraction curve.
The dose at which the distance to the extrapolated line from the x-axis is the same as the distance from the extrapolated line to the survival fraction curve gives the alpha-beta ratio.

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15
Q

What is the alpha-beta ratio for tumours?

A

10

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16
Q

What is the alpha-beta ratio for most normal tissue?

A

3

17
Q

What is the alpha-beta ratio for the Central Nervous System?

A

2

18
Q

What is the alpha-beta ratio for lung tissue?

A

4.5

19
Q

Why are tumour cells more suspectable to small fractions?

A

Tumour cells are early responding tissues.

20
Q

What type of tissue (early or late reacting) has a quicker gradient change with increasing dose on a survival fraction curve?

A

Late reacting tissue as it is more sensitive to fraction size.

21
Q

What is the equation for the Biologically Effective Dose?

A

BED = -ln(SF)/alpha = D[1+(d/(alpha/beta))]

22
Q

What is the equation fro Tumour Control Probability?

A

TCP = EXP[-k(0)EXP(-alphaBED)]

k(0) is the initial number of clonogenic cells)

23
Q

How does k(0) affect the shape of a TCP curve?

A

As k(0) increases the curve moves to the right.

24
Q

How does alpha affect the shape of a TCP curve?

A

As alpha increase the curve shifts to the left and becomes steeper.

25
Q

What is the cause of the proliferation of tumour cells following a radiotherapy fraction?

A

More space and resources (i.e. improved environment) for tumour cells to grow following fraction.

26
Q

What is the function of proliferation correction?

A

Applied to the survival fraction to take into account BED loss due to tumour proliferation.

27
Q

What is the equation for the proliferation correction?

A

PC = EXP[ln(2)/Tp]*T
Tp is the potential clonogenic doubling time (i.e. the time taken for a tumour to double under ideal conditions)
[ln(2)/Tp] is often expressed as gamma.
T is the time elapsed since the start of the treatment.

28
Q

What is the equation for BED taking into account the proliferation correction?

A

BED=D[1+(d/(alpah/beta))]-([ln(2)/Tp]*T)/alpha)

29
Q

What is gamma/alpha for normal tumour?

A

gamma/alpha ~ 0.5Gy/day

30
Q

What is gamma/alpha for head and neck tumour?

A

gamma/alpha ~ 0.9Gy/day

31
Q

What is the time delay after which proliferation starts?

A

28 days.

32
Q

What is the equation for BED taking into account the proliferation correction and time delay??

A

BED=D[1+(d/(alpah/beta))]-([ln(2)/alpha]*[(T-T(delay))/Tp)]

Medical Physics: Radiotherapy (7 decks)

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