Radiobiology Flashcards
Define α, including its units.
Mean number of lethal double strand DNA breaks,
per cell,
per unit dose,
caused by one ionisation track; [Gy-1 ]
Define β, including its units.
Mean number of lethal double strand DNA breaks,
per cell, per unit dose(squared),
caused by two separate ionisation tracks; [Gy-2 ]
Define α/β ratio, including its units.
The dose at which double strand break damage caused by single events equals double strand break damage by two independent single strand break events. [Gy]
What does a low α/β ratio indicate?
A greater fractionation sensitivity and repair capability; late reacting normal tissue is preferentially spared if many fractions are delivered with a small dose per fraction.
How does ionising radiation damage DNA?
1) Direct - double strand breaks rendering the DNA unable to self repair and replicate as normal
2) Indirect - water molecules are ionised creating an hydroxyl (OH) free radical which is highly reactive and damages DNA; OH has an unpaired electron in the valance shell
What is the mean number of lethal events per cell for a single dose?
d = αd + βd2
On a Survival Curve, does α or β dominate at low doses? Why?
α (double strand breaks due to one event) dominates at low doses due to the linear component of the survival fraction having greater influence at low d.
On a Survival Curve, does α or β dominate at high doses? Why?
β (double strand breaks due to two independant events) dominates at high doses due to the quadratic component of the survival fraction having greater influence at high d.
What is a typical value of α/β ratio for most tumours (early reacting)?
10
What is a typical value of α/β ratio for most healthy (late reacting) tissues?
3
Define Normal Tissue Complication Probability (NTCP).
The relative probability of a late effect occuring after a given time delay following RT treatment.
What is Biological Effective Dose (BED)?
A comparative measure of the effect of different dose fractionation regimes; note, it is not a physical dose.
How can you determine the α/β ratio from a survival curve showing healthy and tumour tissue?
The α/β ratio is the point where both curves cross.
On a servival curve, what effect does increasing the α/β ratio have?
The α/β ratio is the point where both curves cross, therefore, increasing this moves the curves to the right with the crossing point at the value for α/β.
On a servival curve, what effect does increasing the dose per fraction (d) have?
SF gradient increases (in magntitude).
