2. Origin and Meaning of BED
BED( Biological effective dose) concept introduced by Barendsen in 1982
his concept was
“the total dose that would cause same log cell kill as the schedule under
consideration if it was delivered in infinitely small fractions well spaced out
or at infinitely low dose rate”
The highest total dose required to obtain a specific effect. Nothing but BED.
3. Application of BED
Commonly used for iso effective dose fractionation calculations
Simple fractionation changes
Correction of errors
Conversion to 2Gy/fraction equivalent dose
Effect of change in overall treatment time
Correction for rest periods
4. BED from LQ model
LQ model cell kill given
Suppose d tends to zero
5. BED is the measure of the true biological dose delivered by a particular
combination of dose per fraction and total dose to a given tissue
characterized by a specific a/b ratio.
If d increases BED increase even though the D kept constant
BED is high in low a/b ratio if total dose and d kept constant
BED = total physical dose * Relative effectiveness
6. Tumour BED calculation
A range of a/b ratio available for different tumour type
A repopulation correction factor should be included in the case of tumours
that contain rapidly proliferating clonogens
K is daily BED of repopulation(Gy)
T is the overall treatment duration
Tk is onset of accelerated repopulation (21-28days)
7. Clinical examples 1
Calculate BED for 60Gy/30 fraction schedule for tumour(a/b=10) and normal
tissue(a/b=3)
Normal tissue
d= 2Gy
N=30
a/b=3
Answer= 100Gy
8. Tumour
Equation will be
d=2, n=30, K=0.9Gy/day, T= 42 days, Tk=28days, a/b=10
BED=60(1+2/10) – 0.9(42-28)
Answer will be 59.4Gy BED
9. Example 2:simple fractionation change
Question: what dose/fraction delivered in 25 fractions will give same
probability of late normal tissue damage as 60Gy delivered in 30 fractions of
2Gy/fraction???????????
Answer: Calculate BED for 60Gy/30#.
Equate BED to 25# schedule then calculate d.
d= 2Gy, n=30, a/b=3Gy for normal tissue
Then BED =60*(1+2/3)=100Gy
10. Question1(continue)
Then equate the BED with 25# schedule
100=25*d(1+d/3)
Re arrange and calculate d using quadratic equation
Answer d= 2.27Gy/fraction
practice….
15. BED and EQD2
BED is additive. If treatment done in 2 different phase with different dose
fractionation
Predictions of normal tissue complication probabilities are complex and the
biological bases are not fully understood at present
A more practical alternative for the clinical oncologist is to convert the BED
values to equivalent total doses delivered 2Gy per fraction
Called EQD2
16. EQD2 concept and calculation
2Gy/fraction equivalent dose is very easy to understand with clinical
experience
17. Example for EQD2
Calculate the EQD2 for 45 delivered in 15 fractions
Answer:
Calculate BED for 45Gy/15# ( in case of tumour take a/b=10)
D=45/15=3Gy
BED = 45*(1+3/10)= 58.5Gy
Then EQD2= BED/(1+2/10)
= 58.5/1.2
= 48.75Gy is the EQD2.
18. In clinical trials, the BED concept is useful for the matching of isoeffective
doses and the interpretation or prediction of results
19. summary
BED idea works generally well in a fairly wide range of situations
Better understanding of parameters required for applying BED
clinicial oncologist should use BED as a guide, rather than an absolute
indicator
In clinical trials, the BED concept is useful for the matching of isoeffective
doses and the interpretation or prediction of results
