RT Radiobiology Flashcards
What is meant by an early and late reacting tissue?
Early reacting tissue which suffers a relatively high damage at low doses, compared to higher doses. Such as a tumour. Steep intial gradient on SF curve but then does not increase fast. Tumour a/b = 10.
Late reacting tissues, do not suffer much damage at lower doses, but then have a higher amount of relative damage as the dose is increased. Tissue a/b = 3.
How would you estimate the a/b?
Draw extrapolate the intial gradient on a SF curve and see where the linear line is equal to the exponential component. This is where the damage from the two is equal.
What is the equation for the BED?
BED = -ln(SF)/alpha = D [1 + d / (alpha/beta) ]
What is the EQD2?
It is a way of comparing the effectiveness of a dose regime compared to if it was given in 2 Gy fractions.
BED = nd [1 + d / (alpha/beta) ]
= EQD2 [1 + 2 / (alpha/beta) ]
What is the BED equation with proliferation correction?
BED = D [1 + d / (alpha/beta) ] - k (T - T_delay)
k is BED lose per day e.g. 2 Gy
T is time in days
T_delay is time taken for proliferation to begin e.g. 28 days
What is the TCP and what affects the shape of the curve?
TCP = e^(k_o x SF) where k_0 is initial number of active cologenic cells.
It is the probability that there are no active cologenic cells following treatment, also known as local control.
Increasing k_o shifts the graph to the right. Increasing the dose per fraction shifts the graph to the left. Increasing alpha shifts the graph to the left and makes it steeper.
