The Radiobiology of Radiation Therapy

1. The Radiobiology of Radiation Therapy

2. Type of Injuries • Nuclear DNA is major target • Cellular membrane damage – minor • Nuclear membrane damage – minor • Cellular organelle injury – minor • Mitochondrial DNA ??

3. Mechanism • Two mechanisms of injury • Direct Ionization of the DNA, ≈ 15% • Indirect Ionization of the DNA, ≈ 85% • DNA damaged by free radicals formed in the micro-environment of the DNA • Water is most important source • Oxygen is important in fixating injury • Sulfhydryl compounds promote repair

4. Types of DNA Injury • Base pair injury • Base pair deletion • Base pair cross linkage • Single strand break in backbone • Double strand break in backbone • Gene suppression or activation

5. Base Pair Injury • Damage to one of the pairs of nitrogenous bases in the DNA sequence. • Easily repaired by cellular repair mechanisms. • Repair is error free

6. Base Pair Deletion • Complete destruction of a pair of the nitrogenous bases in the sequence • Rapidly repaired by cellular repair mechanisms • Not necessarily error free repair.

7. Base Pair Crosslinkage Injury • Abnormal pairing of the nitrogenous bases. • May effect conformation of DNA • Repaired efficiently

8. Single Strand Break • Result of ionization of the sugar-phospate rail of the DNA molecule • Most is easily repaired unless base pairs are also lost • Repair is rapid and accurate but some is not repairable.

9. Double Strand Break • Breakage of both strands of the DNA backbone in close proximity to each other. • Difficult to repair • Repair is quite prone to errors. • High dose and High LET event.

10. Gene Suppression or Activation • Radiation injury may result in upregulation of some genes. • Tumor Promoter genes • Tumor Suppressor genes • Radiation injury may result in down regulation of the same genes • Down regulation of genes controlling intracellular repair.

11. Cell Survival Curves • Cell survival curve expressed on a log/linear plot. • Developed through many years of experimentation • Different curves are derived for different types of radiation.

12. Cell survival, neutrons vrs. xrays

13. Single Hit Killing • Lethal damage to DNA by single photon. • Mostly due to double strand breaks • May be due to pro apoptotic gene activation • Represented by the initial straight portion of the photon survival curve

14. Multi-hit Killing • Lethal injury to the DNA following multiple hits of the DNA by photon radiation • Coincident single strand breaks result in a double strand break • Activation of pro apoptotic genes • Increases with dose • Represented by steep part of curve

15. Survival Curve Shoulder • Represents the transition zone between single and multiple hit killing • The shoulder is representative of the repair capability of the cell population • Wider in slowly dividing cells • Narrower in rapidly dividing cells

16. Alpha/Beta Ratio • Really is determined by a dose point • Point on survival curve where single and multi-hit killing are equal • Larger in cell lines with a wider repair shoulder.

17. Alpha/Beta Ratio

18. LET and Effect on Survival • LET = Linear Energy Transfer • Measured in keV/micron • Characteristic of particulate radiation • High LET radiation increase killing per unit energy deposited. • Results in severe repair deficiencies • Effectively removes the repair shoulder

19. LET and Effect on Survival • High LET radiation is densely ionizing • Averages >1 ionization event within the span of a DNA molecule. • High ionization density increases probability of double strand breaks. • Reaches a maximum effect at about 100 keV/micron.

20. LET and Effect on Survival • Photons have an average LET of about 1. • <1 ionization event within the diameter of a DNA Molecule. • Single strand breaks predominate • Repair is permitted

21. LET and Effect on Survival

22. Cell Cycle and Radiation Injury • M phase – mitosis very sensitive to radiation injury • G1 phase – resting phase, moderately resistant • S phase – DNA synthesis, moderately resistant to radiation • G2 resting phase – sensitive • G0 non cycling cells – moderate resistance

23. Cell Cycle and Radiation Injury • Mitosis • Chromosomes are condensed • DNA is closely packed – bigger target • Repair mechanisms are shut down • Very compressed time scale = 1 hr. • Any DNA injury is fixed in place • Cell may loose large segments of DNA • Fragments excluded from nucleus

24. Cell Cycle and Radiation Injury • S phase • Phase of DNA synthesis • Most radiation resistant phase • Cellular repair mechanisms are active • Increases repair of radiation damage • Lasts about 5 hours.

25. Cell Cycle and Radiation Injury • G1 • Functional part of cell cycle • Resistance varies with part of phase • Goes down as cell nears the G1-S interface • Point in cell cycle where apoptosis occurs • Cell death at this point is referred to as interphase death • Longest part of cycle. • Lasts hours to years

26. Cell Cycle and Radiation Injury • G2 • Short rest phase before M • Quite radiation sensitive • Short time allows little for injury repair • Radiation injury incurred in S-phase may be repaired • May result in a mitotic delay in G2 • Apoptosis-like death may also occur

27. The Four R’s • Repair • Reassortment • Reoxygenation • Repopulation

28. Repair • Rapid repair of injury • Initiated within seconds of injury • Complete by 6 hours after injury • Can be modified by environmental conditions • Presence or absence of oxygen or free radical scavengers. • Responsible for shoulder of survival curve

29. Reassortment • When cells killed in sensitive phases it leave a gap in the cell population for those phases. • Within two cycles cells from less sensitive parts of cycle replace them • Some non-cycling cells may be recruited into the cycling pool.

30. Reoxygenation • Most tumors larger than 1 cm have some hypoxic cells in them • Some tumor types have larger % • May be transient or chronic • Radiation preferentially kills oxygenated cells (O2 fixation of injury) • Major contributor to tumor radiation resistance.

31. Reoxygenation

32. Reoxygenation

33. Repopulation • Following killing of cells in a population by any means there is either replacement or repopulation of the cells killed • Usually there is days to weeks delay before this begins • Tissues with large clonogenic populations are able to do this better

34. Repopulation • Tends to be a low dose phenomenon • Usually is most important in rapidly cycling cell population. • This includes tumors • Rapid repopulation may reduce level of repair

35. Tissue Level Radiation Effects • All mammalian cells equally sensitive in cycling populations in cell culture • However, in tissue the rate of cell replacement is variable • Some cell populations turn over every 3-5 days and some never do. • Cell growth fractions and cell death fractions should be in balance.

36. Tissue Effects • Radiation response at tissue level is tied to cell death • Cell death is mostly tied to cell reproduction • Apoptosis • Radiation induction of apoptosis pathways • Mitotic linked death • Reproductive failure due to missing DNA • Long cell cycle times blunt response

37. Tissue Effects • Long cell cycle times promote repair and slow repopulation • Short cell cycle times promote repopulation and blunt repair • Large non-cycling populations blunt radiation response • Dose required to inhibit function is much higher than that for reproductive inhibition or failure.

38. Tissue Effects • At the tissue level the ultimate survival of the tissue depends on: • The number of cycling cells • The ability of the tissue to repair the injury. • The ability of the tissue to repopulate the tissue with the original cell type.

39. Tissue effects • Repopulation is most important at low doses; • Early responding tissues tend to have more repopulation • Late responding tissues tend to have limited repopulation capability • Therefore sensitive to larger doses of radiation.

40. Tissue Effects

41. Radiation Delivery • Treatment with a number smaller doses improves normal tissue response and increases total dose that can be given to a tumor • Reduces hypoxia • Promotes repopulation in late responding tisues • Promote reassortment • Promotes repair of DNA injury

42. Fractionation

43. Fractionation • Optimal dose is that which is just about midway through the repair shoulder. • Usually approximately equal to the Do dose • Must wait at least 6 hours for repair to be complete.