Concept of bed in radiobiology

concept of biological effective dose and its clinical use

1. BED and EQD2 concepts in clinic RANJITH C P

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” T he 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….

11. Creating Tables of BED from Ranges of a/b Ratios

12. Dose In homogeneities Within a Planning Target Volume (PTV)

13. BED diff for same physical dose(D)

14. 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

15. EQD2 concept and calculation 2Gy/fraction equivalent dose is very easy to understand with clinical experience

16. 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.

17. In clinical trials, the BED concept is useful for the matching of isoeffective doses and the interpretation or prediction of results

18. 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