4R of Radiobiology

1. 4 R’s of Radiobiology
Dr. Lokendra Kunwar
MD Resident
Dept. of Clinical Oncology

2. Introduction
• The four R’s of radiobiology include:
 Repair of DNA damage
 Redistrubition/Reassortment of cells in cell cycle
 Repopulation
 Reoxygenation of hypoxic tumor cells
• 5th R: Radiosensitivity

3. • The four R’s of radiobiology are the concept to explain the rationale
behind the fractionation of radiotherapy.
• The multifraction regimens used in
conventional radiation therapy are the
consequences of different radiobiological
experiments done back in early 1900s.

4. Repair
• DNA Damage
• Etiology: Exogenous and Endogenous
• Mechanism: Direct and Indirect action of RT

5. Radiation induced DNA damage

6. Types of DNA Damage
• Single strand break
• Double strand break

8. Types of radiation induced damage
• Lethal Damage (LD): irreversible, irreparable and leads to cell death.
This usually results from direct effect of radiation causing double
strand DNA breaks.
• Potentially Lethal Damage (PLD): radiation damage that can be
modified by postirradiation environmental conditions.
• Sublethal Damage (SLD): repaired in hours under normal conditions
unless additional sublethal damage is addded. This usually results
from indirect effect of radiation causing single strand DNA breaks.

9. PLD Repair
• Normally causes cell death, but can be modified by manipulation of
the post irradiation conditions.
• PLD is repaired if cells are incubated in a balanced salt solution
instead of full growth medium for several hours after irradiation.
• PLD repair in clinical RT is still a matter of debate.

10. SLD Repair
• SLD repair is the operational term for increase in cell survival that is
observed if a given radiation dose is split into two fractions separated
by a time interval.
• If a dose is split into two fractions separated by a time interval, more
cells survive than for the same total dose given in a single fraction.
• The repair of the SLD reflects the repair and rejoining of double
strand breaks before they can interact to form lethal lesion.

11. Fig 1: Fractionated RT and cell survival
curve
Fig 2: Survival of Chinese hamster cells
exposed to two fractions of x-rays

12. • Repair half time (t ½), is the time required for cell repair after
radiation damage.
• The t ½ of SLD repair in mammalian cells is about 1 hour, but it may be
longer in late-responding normal tissues in vivo.

13. Clinical implications
• When the total radiation dose is given by dividing it into small
fractions, and if the interval between two fractions is >6 h, normal
tissues can protect themselves from radiation through SLD repair.
• The repair of SLD in the spinal cord is much slower than that in other
normal tissues. Thus, the interfraction interval should be at least 8 h
in spinal cord irradiation.

14. Redistribution
• Cells may be in different phases of cell cycle during irradiation.
• The radiosensitivity of cells vary with the phase of the cell cycle. The
most sensitive phases are M and G2, while the most resistant is the S
phase.
• Surviving cells continue the cycle and may reach sensitive phase when
second dose of radiation is given.

15. • If about 6hrs are allowed between the two doses of radiation, group
of cells progresses around the cell cycle at the time of 2nd dose of
radiation.
• The probability that tumor cells will be exposed to radiation during a
sensitive phase increases, and there will be maximum tumor cell kill.
• The process of self sensitization due to movement through cell cycle
is called redistribution.

16. Fig: Redistribution of cells in different phase of cell cycle

17. Repopulation
• This is the increase in cell division that is seen in normal and
malignant cells at some point after radiation is delivered.
• The time to onset of repopulation after radiation and the rate at
which it proceeds vary with the tissue.
• Repopulation is seen if the interval between the spilt dose is 10-12
hours, because this exceeds the length of cell cycle of rapidly growing
cells.

18. • Resting cells in the G0 phase enter the cell cycle in order to
compensate for the cells killed by radiotherapy, and they undergo
mitosis → repopulation.

19. Fig: Repair, Redistribution and Repopulation processes
occurring simultaneously

20. Accelerated Repopulation
• Treatment with any cytotoxic agent, including RT, can trigger surviving
cells (clonogens) in a tumor to divide faster than before, this is known
as Accelerated Repopulation.
• Repopulation of tumor cells is slow at the beginning of radiotherapy,
but it speeds up after the first doses of radiation therapy causing
accelerated repopulation.
• A study done by Wither and his colleagues in Head and Neck Cancer
suggested that accelerated repopulation begins after 28 days of
initiation of RT in head and neck Ca.

21. Clinical implications
• Prolonging treatment duration longer then necessary has
disadvantage.
• If the overall treatment time becomes longer than the period
required, the tumor enters the accelerated repopulation mode and its
response to radiation decrease due to tumor proliferation.
• If a break in treatment is necessary because of acute toxicity, it should
be kept as short as possible.

22. Reoxygenation
• With the proliferation of tumor cells, the vascularity of the tumor
tissue becomes insufficient to meet its requirements, and hypoxic-
necrotic regions begin to occur within the tumor tissue.
• Tumor less than 1 mm are fully oxic and over 1 mm develops region
of hypoxia.
• Hypoxic cells are 2-3 times more resistant to radiation as oxygen is
required for the indirect effect to occur.

23. • Hypoxia in tumor results from:
Acute hypoxia
Chronic hypoxia
• Acute hypoxia: occurs due to temporary closure or blockage of
particular blood vessels.
- It results from transient fluctuations in blood flow because of
malformed vasculature causing hypoxia in tumor cells.

24. • Chronic hypoxia: results from limited diffusion distance of oxygen in
respiring tissues.
- Distance that oxygen can diffuse in respiring tissue is about 70
micrometer.
- Cells that become hypoxic by chronic hypoxia remain hypoxic for
long time until they die and become necrotic (reason for treatment
failure).

25. Fig: Acute and Chronic hypoxia occurring in tumor cells

26. Process of Reoxygenation
• The phenomenon by which hypoxic cells become oxygenated after a
dose of radiation is called Reoxygenation.
• Tumor contain both aerated and hypoxic cells. Dose of radiation kills
greater proportion of aerated cells than hypoxic cells.
• Hypoxic cells gradually obtain much better vascularity and
oxygenation, and their radiosensitivity increase.

28. Mechanism of reoxygenation
• Some tumors take several days to reoxygenate and some other
tumors complete within 1hr.
• Contains 2 components: Fast and Slow reflecting types of hypoxia;
acute and chronic.
• Some tumor shows both the components.

29. Fast Component
• First component of reoxygenation, completes within hours.
• Caused by reoxygenation of acutely hypoxic cells .
• It occurs due to reopening of temporarily closed blood vessels.

30. Slow Component
• Takes place over a period of days in chronic hypoxic cells.

31. • Oxygenating a tumor from the hypoxic state:
 If hemoglobin is low, a blood transfusion may be given.
 High-pressure oxygen or carbogen may be applied during
radiotherapy.
 The patient may be prevented from using hypoxic materials like
cigarettes during radiotherapy.

32. Summary
• Fractionation
- allows maximum tumor cell kill with minimal damage to normal tissues
- allows the repair and regeneration of normal tissues
- sensitizes the tumor cells to radiation
• Repair in between the doses benefits the normal tissues but makes tumor
cells more resistant.
• Redistribution makes the tumor sensitive to RT.

33. • Repopulation benefits the normal tissues in between the doses but
makes the tumor cells resistant to RT.
• Reoxygenation makes the tumor sensitive to RT.
• Recovery in normal tissues after RT= repair + regeneration.
• Reassortment and Reoxygenation sensitize tumor cells to RT causing
max. tumor cell kill.

34. THANK YOU