2. Introduction of fractionation
4 “ R “ s of radiobiology
5th R of radiobiology
Probably 6th R of radiobiology
3. THE INTRODUCTION OF FRACTIONATION
The multifraction regimens
commonly used in conventional
radiation therapy are a
consequence of radiobiologic
experiments performed in France
in the 1920s and 1930s.
It was found that a ram could not
be sterilized by exposing its testes
to a single dose of radiation
without extensive skin damage to
the scrotum
4. If the radiation was spread out over a period of weeks
in a series of daily fractions, sterilization was possible
without producing unacceptable skin damage .
It was postulated that the testes were a model of a
growing tumor, whereas the skin of the scrotum
represented a dose-limiting normal-tissue.
proved that fractionation of the radiation dose
produces, in most cases, better tumor control for a
given level of normal-tissue toxicity than a single
large dose.
5. THE FOUR Rs OF RADIOBIOLOGY
Now, more than 80 years later, we can account for
the efficacy of fractionation based on more
relevant radiobiologic experiments. We can appeal
to the
“four Rs” of Radiobiology:
Repair of sublethal damage
Reassortment of cells within the cell cycle
Repopulation
Reoxygenation
6. Radiation damage to mammalian cells can
operationally be divided into three categories:
(1) lethal damage,irreversible and irreparable and
causes cell death.
(2) potentially lethal damage (PLD), can be
modified by postirradiation environmental
conditions.
(3) sublethal damage (SLD), can be repaired in
hours unless additional sublethal damage is added
with which it can interact to form lethal damage
7. Potentially Lethal Damage (PLD) Repair
causes cell death
PLD is repaired if cells are incubated in a balanced
salt solution instead of full growth medium for
several hours after irradiation which does not
mimic a physiologic condition
Possible only in vitro
The importance of PLD repair to clinical
radiotherapy is a matter of debate.
8. 1.Sublethal Damage (SLD) Repair
operational term for the increase in cell
survival that is observed if a given radiation
dose is split into two fractions separated by a
time interval.
10. DNA REPAIR PATHWAYS
Nucleotide Excision Repair (NER)
Base Excision Repair (BER)
DNA Double-Strand Break Repair :
homologous recombination repair(HRR)
requires undamaged DNA strand as a participant in
the repair as a template.
occurs primarily in the late S/G2 phase of the cell
cycle, when an undamaged sister chromatid is
available to act as a template.
predominant pathway in lower eukaryotes like yeast.
11. nonhomologous end joining (NHEJ):
occurs in the G1 phase of the cell cycle, when no
such template exists.
error prone and probably accounts for many of the
premutagenic lesions induced in the DNA of
human cells by ionizing radiation.
12. MECHANISM OF SLD REPAIR
the repair of sublethal damage reflects the repair
and rejoining of double-strand breaks before they
can interact to form lethal lesion.
If a dose is split into two parts separated by a time
interval, some of the double-strand breaks
produced by the first dose are rejoined and
repaired before the second dose and more cells
survive.
13. cultured mammalian cells
maintained at room
temperature (24°C) between
the dose fractions to prevent
the cells from moving
through the cell cycle during
this interval. This rather
special experiment is
described first because it
illustrates repair of sublethal
radiation damage
uncomplicated by the
movement of cells through
the cell cycle.
14. 2.Reassortment
If an asynchronous population of cells present in
different phases of cell cycle is exposed to a large
dose of radiation, more cells are killed in the
sensitive than in the resistant phases of the cell
cycle. The surviving population of cells progresses
around cell cycle and said to be partly
synchronized.
15. In Chinese hamster cells, most of the survivors from
a first dose are located in the S phase of the cell
cycle.
If about 6 hours are allowed to elapse before a
second dose of radiation is given, this cohort of cells
progresses around the cell cycle and is in G2/M, a
sensitive period of the cell cycle, at the time of the
second dose.
If the increase in radiosensitivity in moving from
late S to the G2/M period exceeds the effect of repair
of sublethal damage, the surviving fraction falls.
16. 3.REPOPULATION
Repopulation is the increase in cell division that is
seen in normal and malignant cells at some point
after radiation is delivered.
In normal tissues Repopulation occurs in different
speeds depending on the tissue.
early responding tissues begin repopulation at
about 4 weeks.
By increasing treatment time over this amount, it
is possible to reduce early toxicity in that tissue.
17. Late responding tissues only begin repopulation
after a conventional course of radiation has been
completed, and therefore repopulation has
minimal effect on these tissues (the repair 'R' is
more important for late tissues).
18. Accelerated repopulation
Treatment with any cytotoxic agent, including
radiation, can trigger surviving cells (clonogens) in
a tumor to divide faster than before. This is known
as accelerated repopulation.
There is evidence for a similar phenomenon in
human tumors.
Withers and his colleagues surveyed the literature
on radiotherapy for head and neck cancer.
19. The analysis suggests that clonogen repopulation
in this human cancer accelerates at about 28 days
after the initiation of radiotherapy in a
fractionated regimen.
A dose increment of about 0.6 Gy (60 rad) per day
is required to compensate for this repopulation.
Such a dose increment is consistent with a 4-day
clonogen doubling rate, compared with a median
of about 60 days for unperturbed growth.
20. The conclusion to be drawn from this is that
radiotherapy, at least for head and neck cancer and
probably in other instances also, should be completed
as soon after it has begun as is practicable.
It may be better to delay initiation of treatment than
to introduce delays during treatment.
If overall treatment time is too long, the effectiveness
of later dose fractions is compromised because the
surviving clonogens in the tumor have been triggered
into rapid repopulation.
23. representation of the dependence of radiosensitivity on oxygen concentration. If the
radiosensitivity under extremely anoxic conditions is arbitrarily assigned unity the relative
radiosensitivity is about 3 under well-oxygenated conditions. Most of this change of sensitivity
occurs as the oxygen tension increases from 0 to 30 mm Hg. A further increase of oxygen content
to that characteristic of air or even pure oxygen at high pressure has little further effect. A relative
radiosensitivity halfway between anoxia and full oxygenation occurs for a pO2 of about 3 mm Hg,
which corresponds to a concentration of about 0.5% oxygen.
24.
25. Chronic hypoxia
(1) those that appear to be proliferating well and (2)
those that are dead or dying.
Between these two extremes, region in which cells
would be at an oxygen tension high enough for cells
to be clonogenic but low enough to render the cells
protected from the effect of ionizing radiation.
Cells in this region would be relatively protected
from a treatment with x-rays because of their low
oxygen tension and could provide a focus for the
subsequent regrowth of the tumor.
26. Acute hypoxia
Regions of acute hypoxia develop in tumors as a
result of the temporary closing or blockage of a
particular blood vessel. If this blockage were
permanent, the cells downstream, would
eventually die and be of no further consequence.
But tumor blood vessels open and close in a
random fashion, so that different regions of the
tumor become hypoxic intermittently.
27. In fact, acute hypoxia results from transient
fluctuations in blood flow due to the malformed
vasculature.
At the moment when a dose of radiation is
delivered, a proportion of tumor cells may be
hypoxic, but if the radiation is delayed until a later
time, a different group of cells may be hypoxic.
29. A modest dose of x-rays to a mixed population of aerated
and hypoxic cells results in significant killing of aerated
cells, but little killing of hypoxic cells.
Consequently, the viable cell population immediately after
irradiation is dominated by hypoxic cells.
If sufficient time is allowed before the next radiation dose,
the process of reoxygenation restores the proportion of
hypoxic cells to about 15%.
If this process is repeated many times, the tumor cell
population is depleted, despite the resistence of hypoxic
cells to killing by x-rays.
In other words, if reoxygenation is efficient between dose
fractions, the presence of hypoxic cells does not have a
significant effect on the outcome of a multifraction
regimen.
30. MECHANISM OF REOXYGENATION
Some tumors take days to reoxygenate ; and in
others process completes with in 1 hr
Contain two components fast and slow reflecting
two types of hypoxia acute versus chronic
Some tumors show both fast and slow components
31. SLOW COMPONENT
TAKES PLACE OVER A PERIOD OF DAYS IN CHRONICALLY HYPOXIC
CELLS
After a dose of radiation
Tumor cells killed and removed from population
tumor shrinks in size and restructuring or a revascularization
of the tumor occurs
surviving cells previously beyond the range of oxygen
diffusion become closer to a blood supply and so reoxygenate.
32. FAST COMPONENT
First component of reoxygenation
complete within hours
caused by the reoxygenation of acutely hypoxic cells.
Those cells that were hypoxic at the time of irradiation
because they were in regions in which a blood vessel
was temporarily closed quickly reoxygenate when that
vessel is reopened.
33. Importance of Reoxygenation in RT
If all the human tumors reoxygenate rapidly , use
of a multifraction course of radiotherapy,
extending over a period of time, can deal
effectively with any hypoxic cells in human
tumors.
34. - mouse mammary carcinoma
that reoxygenates rapidly and well .
- rat sarcoma that shows two
waves of reoxygenation .
- mouse osteosarcoma that does
not reoxygenate at all for several days
and then only slowly .
- mouse fibrosarcoma that
reoxygenates quickly but not as
completely as the mammary
carcinoma .
- mouse fibrosarcoma that
reoxygenates quickly and well .The proportion of hypoxic cells as a function
of time after irradiation with a large dose of
x-rays for five transplanted tumors in
experimental animals.
35. Making optimal choice of fractionation, demands a
detailed knowledge of the time course of
reoxygenation in the particular tumor to be irradiated.
Unfortunately, this information is available for only a
few animal tumors and no information at present for
human tumors. Indeed, in humans it is not known
with certainty whether any or all tumors reoxygenated
Evidence from radiotherapy clinics that many tumors
are eradicated with doses of 60 Gy in 30 fractions
argues strongly in favor of reoxygenation.
36. SUMMARY
Fractionation
allows the repair and regeneration of normal
tissues.
sensitizes the tumor cells to radiation.
Repair in between the fractions benefits the
normal tissues but makes the tumor cells more
resistant.
Reassortment makes the tumor sensitive to RT
37. Repopulation or regeneration benefits the normal
tissues in between fractions but makes the tumor
cells resistant to RT
Reoxygenation makes the tumor sensitive to RT
Recovery in normal tissues after RT = repair +
regeneration
REASSORTMENT & REOXYGENATION - FRIENDS
OF TUMOR RADIOTHERAPY
REPAIR & REPOPULATION – FOES OF TUMOR
RADIOTHERAPY (but friends of normal tissues)
38. 5th R of RADIOBIOLOGY
RADIOSENSITIVITY
Law of Bergonie & Tribondeau
Radiosensitivty of living tissues varies with
maturation & metabolism;
1. Stem cells are radiosensitive. More mature cells are
more resistant
2. Younger tissues are more radiosensitive
3. Tissues with high metabolic activity are highly
radiosensitive
4. High proliferation and growth rate, high
radiosensitivty
39. Response of tissue determined by amount ofenergy
deposited per unit mass (dose in Gy)
Two identical doses may not produce identical responses
due to other modifying factors
Physical Factors Biological Factors
- linear energy thansfer – Oxygen Effect
- relative biological effectiveness – Age
- fractionation – Recovery
– Chemical Agents
– Hormesis
41. 6th R of radiobiology
? REMOTE CELL KILL – BYSTANDER EFFECT
Phenomenon in which unirradiated cells exhibit
irradiated effects as a result of signals received from
nearby irradiated cells.
In November 1992, Hatsumi Nagasawa and John B.
Little first reported this radiobiological phenomenon
Might be due to activation cells of innate immune
system by ionizing radiation to produce pro-
inflammatory mediators of genomic instability and
cause suppression of cytokine production in the
surrounding non-irradiated cells via the bystander
effect.
42. ABSCOPAL effect(ab - away from ; scopal -
target)
Defined as a reaction of cells within an organism
that had not been directly exposed to irradiation,
but cause tumor regression of the non-irradiated
tumors (Postow et al., 2012).
Thought to be mediated by activation of the
immune system via cytokines.
