1. Vischioni Barbara MD, PhD Centro Nazionale Adroterapia Oncologica
Radiobiology of fractionated treatments: the classical approach and the 4 Rs
Vischioni Barbara MD, PhD
Centro Nazionale Adroterapia Oncologica

3. Radiobiology
It is fundamental in radiation oncology

4. Radiobiology in radiation oncology

5. First fractionation experiments
In multifraction radiotherapy schemes the dayly patients treatment dose is of Gy

6. Radiobiology in radiation oncology
It contributes to the definition of optimal radiotherapy schemes for patients

7. Tumor control Sigmoid curve
Each radiation dose destroys the same proportion of clonogenic cells. The success of a radiotherapy scheme depends on the distruction of all the surviving clonogenic cells within the tumor
100%
Tumor control
probability% 0%
dose

8. Therapeutic gain
Normal tissue complication probability compared to tumor control probability
Therapeutic gain when the 2 curves are separated

9. 4R in radiobiology (Whiters 1975) REPAIR REPOPULATION REDISTRIBUTION
REOXYGENATION

10. Radiation effect
DNA
Liysosomes, endoplasmic reticulum, cytoplasmic and nuclear membrane, etc.)
proteins

11. Radiation effect

12. Radiation effect at the DNA level
Base damage
Nucleotide damage
SSB
DSB
Bulky lesions
1 ÷ 2 Gy
extensive base damage
1000 SSB
50 DSB
Bulky Lesions
Double Strand Breaks
Base Damage
Single Strand Breaks
Appr. 30% of cells dies and the rest has been repired or are able to survive with a damaged genome

13. Cell fate after radiation
Error-free repair
Faulty repair
No repair
The damage causes mutations not lethal or lethal but in the long-term
The damage is totally removed
The damage is lethal for the cell
Cell survival
Neoplasia
Cell death

14. Healthy tissue tolerance
First R: Repair
Damaged DNA is enzimatically repaired after each fraction of a multifraction radiotherapy scheme
Repair mechanisms in normal tissues works much better than in tumor tissues. It is convenient to fractionate the dose since more cells of the healthy tissue than tumor cells will survive after each fraction
Therapeutic index:
Healthy tissue tolerance
dose
Tumor lethal dose

15. Single Strand Break (SSB) repair
Error-free mechanism of repair
Unrepaired SSBs contribute to DBS damage

16. Double strand break (DSB) repair
Homologous
Recombination
Non-Homologous
End joining

17. Clonogenic activity study

18. In vitro test for clonogenic activity

19. Clonogenic activity study
Cell survival curves considers
Radiation dose
Cell clonogenic activity (surviving fraction of irradiated clonogenic cells)
The shape of the curve is characteristic for each cell population and express specific radiosensitivity

20. Clonogenic activity study
Dose response curve depends on:
Cell population type
Radiation quality
Oxygen level and temperature
drugs

21. Cell fate after IR lymphocytes/ endothelial cells
fibroblasts/ pneumocytes
many normal cells
many tumor cells
Reversible arrest and DNA repair
Short-term arrest and attempted DNA-repair
Apoptosis
Permanent arrest
Attempt to resume proliferation
Resumed proliferation OK
Mitotic catastrophe
Gudkov, Nature 2003 / modified

22. Mathematical models of the radiobiological effect
Radiobiological models can help to predict clinical outcomes when treatment parameters are altered
They have assumptions:
Cell death after radiation connected to abrogation of cell reproductive activity
At least one DSB in DNA is responsible for cell death

23. Cell survival curves and the linear-quadratic model

24. Cell survival curves and the linear-quadratic model
 component
Linear variation with dose (Gy-1)
Lethal damage
DSB
Especially for cells with impaired DDR
machinery
Predominant for high LET radiation
 component
Quadratic variation with dose (Gy-2)
Damage can be repaired
SSB
Especially for cells with good DDR machinery

25. / ratio / ratio defines the bending of the survival curve
/ ratio is the dose at which the linear component of the damage is equal to the quadratic component
/ ratio defines the bending of the survival curve
/ ratio high
Lethal damage
Curve linear at origin
/ ratio low
Damage can be repaired
Curve with shoulder at the beginning

26. / ratio / ratio high Early responding normal tissues
Proliferating tissues
skin
Mucosae
Bone marrow
Fast growing tumor
/ ratio low
Late responding normal tissues
Tissues not proliferating
kidney
liver
Central nervous system
Slow growing tumor

27. / ratio and isoeffect relationship
/ ratio high
No fractionation sensitivity
/ ratio low
Fractionation sensitivity

28. Linear-quadratic model and BED
D = dose totale
d = dose per frazione
BED (biologically equivalent dose)
To calculate isoeffect relationship
To compare different fractionation schemes
To sum up doses given to the same patients with different fractionation

29. Cell survival curves and the linear-quadratic model

31. Radiobiological basis of fractionation
/ RATIO
high / Ratio- early reacting tissues
squamous cell ca
acute normal tissues
--total dose
Low / Ratio- Late reacting tissues
late normal tissues
--total dose and dose/fraction

32. Altered fractionation schemes
Hypofractionation
Hyperfractionation  - low dose/fraction
- higher total dose
- more fraction/day (6 h)
- less total time
(accelerated)
Continuous Hyperfractionation

33. Radiobiological basis of fractionation
Large dose/fraction (hypofractionation) increase the RT effect
Less in the tissues with high / RATIO
Less damage can be repaired within each fraction
Large dose/fraction more toxic to tissues with low / ratio compared to tissues with high / ratio

34. Radiobiological basis of fractionation
Small dose/fraction (hyperfractionation) has reduced effect
in the tissues with low / RATIO
More damage can be repaired within each fraction
Small dose/fraction protects tissues with low / ratio compared to tissues with high / ratio

35. Fractionation sensitivity of different tumors in the clinical setting
Tumor fractionation sensitivity
Definition
Optimal fractionation schedule
Clinical level of evidence
Reference
Low
/ ratio of ca higher than that of late responding healthy tissues
More, smaller-sized fr. with higher total dose, or fr. given over a shorter time course-> improves LC, same late tox, more acute tox.
Level I evidence for improved therapeutic ratio in head and neck and lung ca
Nguyen et al.,2002
Overgaard et al., 2003
Saunders et al., 1999
Moderate to high
/ ratio of ca similar or slightly higher than that of late responding healthy tissues
Fewer, larger-sized fractions might achieve same LC and late toxicity as conventional fractionation
Level II evidence for therapeutic ratio equivalent to conventional scheme in BREAST CA
Yarnold et al., 2005
Owen et al., 2006
Whelan et al., 2002
START A, 2008
START B, 2008
High
/ ratio of ca lower than that of late responding healthy tissues
Fewer, larger-sized fr-> improve LC with similar or reduced late and acute tox effects
Level III evidence for therapeutic ratio equivalent to conventional fr. In prostate ca
Fowler, 2005

36. Dose fractionation and the 4 R
1. Repair of the damage
2. Repopulation:
For tumour cells this repopulation partially counteracts the cell killing effect of radiotherapy
The repopulation time of tumour cells appears to vary during radiotherapy - at the commencement it may be slow (e.g. due to hypoxia), however a certain time after the first fraction of radiotherapy repopulation accelerates.
Repopulation must be taken into account when protracting radiation e.g. due to scheduled (or unscheduled) breaks such as holidays.
Also normal tissue repopulate - this is an important mechanism to reduce acute side effects from e.g. the irradiation of skin or mucosa

37. Dose fractionation and the 4 R
3. Redistribution
4. Reoxygenation: at each fraction oxygenated cells will be killed and hypoxic cells will replace the dead cells in more oxygenated parts of the tumor progressively reducing the final tumor mass

38. New frontiers to increase the therapeutic gain:
hadrontherapy
No fractionation sensitivity
Effect not dependent on cell cycle, oxygenation

39. New frontiers to increase the therapeutic gain:
radiogenomics
Research on factors that increase sensitivity to different fraction size and radiation type
Allow to add drugs to treatment