The four R's of radiobiology are repair, repopulation, redistribution, and reoxygenation. Repair refers to cells' ability to fix radiation-induced DNA damage through pathways like base excision repair. Redistribution occurs as cells in different phases of the cell cycle have differing radiosensitivities. Repopulation is the regrowth of cells after irradiation. Repopulation can reduce the effectiveness of fractionated radiotherapy if it accelerates in tumors. Reoxygenation describes how hypoxic tumor cells may reoxygenate between fractions, increasing their sensitivity to subsequent doses. Fractionating radiation therapy exploits these four R's to better kill tumor cells while allowing normal tissues to recover.

1. 4 R’s of Radiobiology
By Dr. Deepa Gautam
1st yr resident, Radiotherapy
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2. • The four R’s of radiobiology are the concepts to
explain the rationale behind the fractionation of
radiotherapy.
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3. • Repair (few hrs)
• Reassortment/Redistribution (few hrs)
• Repopulation (5-7 wks)
• Reoxygenation (hrs to few days)
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4. Repair
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5. Direct and Indirect Actions of Radiation
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6. • Types of DNA damage:
– Single strand break
– Double strand break
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7. Types of radiation induced damage
• Lethal damage
• Potentially lethal damage
• Sublethal damage
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8. Lethal damage:
• irreversible and irreparable
damage that leads to cell
death.
Eg.
• Dicentric chromosome
• Ring chromosome
• Anaphase bridge
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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
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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
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11. Repair
• Base Excision Repair
• Nucleotide Excision Repair
• DNA DSB Repair:
– Homologous Recombination Repair
– Nonhomologous End Joining
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12. Base Excision Repair
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13. Nucleotide excision repair
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14. Homologous Recombination Repair
• Occurs in late S/G2 phase
• Undamaged sister
chromatid acts as template
• slow process
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15. Nonhomologous End Joining
• Occurs in G1 phase of
cell cycle
• Fast but error prone and
thus potentially
mutagenic
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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
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17. Reassortment/Redistribution
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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
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20. Repopulation
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21. • Repopulation is the process of increase in cell
division seen in normal and malignant cells
after irradiation
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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.
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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.
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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
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26. Reoxygenation
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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.
1. Acute Hypoxia
2. Chronic Hypoxia
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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.
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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
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31. Reoxygenation
• The phenomenon by which hypoxic cells become
oxygenated after a dose of radiation.
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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.
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33. Time Sequence of Rexygenation
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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
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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
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36. Radiosensitivity, the newer
member of the R’s
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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.
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39. Thank You
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