4 rs of radiobiology

1. 4 R’ s of Radiobiology By Dr. Deepa Gautam 1st yr resident, Radiotherapy 1

2. The four R’s of radiobiology are the concepts to explain the rationale behind the fractionation of radiotherapy. 2

3. Repair (few hrs) Reassortment /Redistribution (few hrs) Repopulation (5-7 wks) Reoxygenation (hrs to few days) 3

4. Repair 4

5. Direct and Indirect Actions of Radiation 5

6. Types of DNA damage: Single strand break Double strand break 6

7. Types of radiation induced damage Lethal damage Potentially lethal damage Sublethal damage 7

8. Lethal damage : irreversible and irreparable damage that leads to cell death. Eg . Dicentric chromosome Ring chromosome Anaphase bridge 8

9. Potentially lethal damage : Causes cell death under ordinary circumstances but can be modified by postirradiation environmental conditions. If cells are prevented from dividing by creating suboptimal growth conditions for 6 hrs after irradiation, the damage can repair. Invitro : by keeping cells in saline or plateau phase 9

10. Sublethal damage : Repairable in hours under ordinary circumstances unless additional sublethal damage is added Repair of sublethal damage reflects the repair of DNA breaks before they can interact to form lethal chromosomal abberations 10

11. Repair Base Excision Repair Nucleotide Excision Repair DNA DSB Repair: Homologous Recombination Repair Nonhomologous End Joining 11

12. Base Excision Repair 12

13. Nucleotide excision repair 13

14. Homologous Recombination Repair Occurs in late S/G2 phase Undamaged sister chromatid acts as template slow process 14

15. Nonhomologous End Joining Occurs in G1 phase of cell cycle Fast but error prone and thus potentially mutagenic 15

16. By splitting radiation into small parts, cells are allowed to repair the sublethal damage Damage repair depends upon the ability of cells to recognise the damage and activate the repair pathways and cell cycle arrest Malignant cells often have suppressed these pathways Normal tissues are able to repair by the time next fraction is given 16

17. Reassortment /Redistribution 17

18. Cells may be in different phases of cell cycle during irradiation( S-phase being radioresistant and M-phase being most radiosensitive) Resistance and sensitivity depends upon the level of sulfhydryl compounds( radioprotector ) in the cell. A small dose of radiation given over a short period will kill a lot of sensitive cells and less of resistant cells Surviving cells continue the cycle and may reach sensitive phase when second dose of radiation is given 18

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20. Repopulation 20

21. Repopulation is the process of increase in cell division seen in normal and malignant cells after irradiation 21

22. Repopulation in normal tissues The time to onset of repopulation after irradiation and the rate at which it proceeds vary with the tissue Acute-responding tissues (stem cells, progenitor cells, GI epithelium, oropharyngeal mucosa,skin ) begin repopulation early. Late-responding tissues (Renal tubular epithelium, oligodendrocytes , schwann cells, endothelium, fibroblasts) begin repopulation after completion of conventional course of radiation. 22

23. Repopulation of malignant tissues The mechanism applies to malignant tissues as well. Some tumours exhibit accelerated repopulation , a marked increase in their growth fraction and doubling time and decrease in cell cycle time, at 4 - 5 wks. Eg . SCC of head and neck, cervix. It is a dangerous phenomenon that is countered if treatment time extends over 5 wks. It is mediated through radiation-induced receptor activation and cellular growth stimulation that occur after a single radiation exposure of 2 Gy . 23

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25. The current standard treatment times confer a benefit by allowing regeneration of acute-responding tissues, which reduces toxicity. Attempts made to deliver the therapy more quickly has caused the acute responses to become more severe and dose-limiting. Growth factors like hematopoietic growth factors( G-CSF, GM-CSF, erythropoietin, IL-11), keratinocyte growth factor protect the tissues from radiation injury 25

26. Reoxygenation 26

27. Tumours under 1mm size are fully oxic but beyond this size they develop the region of hypoxia. Hypoxia in tumours can result from two different mechanisms. Acute Hypoxia Chronic Hypoxia 27

28. Acute Hypoxia Develop in tumour as a result of the temporary closing or blockage of a particular blood vessel owing to the malformed structure which lacks smooth muscle and often has incomplete endothelial lining and basement membrane At the moment when a dose of radiation is delivered, a proportion of the tumor cells may be hypoxic, but if the radiation is delayed until a later time, a different group of cells may be hypoxic. 28

29. Chronic Hypoxia It results from the limited diffusion distance of oxygen in respiring tissue that is actively metabolizing oxygen The distance oxygen can diffuse in respiring tissue is about 70µm Cells that are hypoxic for long periods become necrotic and die 29

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31. Reoxygenation The phenomenon by which hypoxic cells become oxygenated after a dose of radiation. 31

32. Process of Reoxygenation Tumors contain a mixture of aerated and hypoxic cells. A dose of x-rays kills a greater proportion of aerated than hypoxic cells. The pre-irradiation pattern tends to return because of reoxygenation of hypoxic cells. If the radiation is given in a series of fractions separated in time sufficient for reoxygenation to occur, the presence of hypoxic cells does not greatly influence the response of the tumor. 32

33. Time Sequence of Rexygenation 33

34. Mechanism of Reoxygenation Reoxygenation in tumours have: Fast component : seen in acute hypoxia occurs within hours reoxygenation occurs when temporarily closed vessels reopen Slow component: seen in chronic hypoxia occurs within days reoxygenation occurs when the tumor shrinks in size and the surviving cells that were previously beyond the range of oxygen diffusion, come closer to a blood supply 34

35. The concept of reoxygenation applies mostly to animal tumours that are experimentally studied The human tumours are assumed to reoxygenate from the evidence that many tumours respond to the doses on the order of 60Gy in 30#s 35

36. Radiosensitivity , the newer member of the R’s 36

37. Apart from previous 4 R’s, there is an intrinsic radiosensitivity or radioresistance in different cell types. The radiosensitivity of the tumor cells is now thought to be the primary determinant of tumor response to radiation. 37

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39. Thank You 39