Radiobiology of fractionated treatments: the classical approach and the 4 Rs

1. 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 1.8-2 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. DNA • Liysosomes, endoplasmicreticulum, cytoplasmic and nuclear membrane, etc.) • proteins Radiation effect

11. Radiation effect

12. Single Strand Breaks Base Damage Double Strand Breaks Bulky Lesions Radiation effect at the DNA level • Base damage • Nucleotide damage • SSB • DSB • Bulky lesions 1 ÷ 2 Gy • extensive base damage • 1000 SSB • 50 DSB 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. First R: Repair 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: Damaged DNA is enzimatically repaired after each fraction of a multifraction radiotherapy scheme 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. Cell survival curves considers Radiation dose Cell clonogenic activity (surviving fraction of irradiated clonogenic cells) Clonogenic activity study 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 Permanent arrest Apoptosis 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 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. To calculate isoeffect relationship To compare different fractionation schemes To sum up doses given to the same patients with different fractionation Linear-quadratic model and BED D = dose totale d = dose per frazione BED (biologically equivalent dose)

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

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

35. Fractionation sensitivity of different tumors in the clinical setting

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