Fractionated radiation and the dose-rate effect can impact cell survival through mechanisms like sublethal damage (SLD) repair and cell cycle redistribution. SLD repair occurs as double-strand breaks are rejoined after radiation exposure. At lower dose rates, more SLD can be repaired between fractions, improving cell survival. However, very low dose rates may instead reduce survival if cells become frozen in radiosensitive phases of the cell cycle. The dose-rate effect varies between cell types and depends on factors like oxygenation and ability to progress through the cell cycle after radiation.

1. Fractionated Radiation
AndThe Dose-rate
Effect
PARAG ROY
DEPT OF RADIOTHARAPY
LOK NAYAK HOSPITAL

2. • Classifications of Radiation Damage
• Potentially Lethal Damage Repair
• Sublethal Damage Repair
• Mechanism of Sublethal Damage Repair
• Repair and Radiation Quality
• The Dose-Rate Effect
• Examples of Dose-Rate Effect InVitro and InVivo
• The Inverse Dose-Rate Effect
Overview

3. Classifications of Radiation Damage
• Radiation damage to mammalian cells can
operationally be divided.
• (1) Lethal damage (LD)
• Irreversible and irreparable
• Leads irrevocably to cell death
• (2) Potentially lethal damage (PLD)
• Can be modified by post-irradiation environmental
conditions
• (3) Sublethal damage (SLD)
• Can be repaired in hours unless additional SLD is added.

4. Potentially Lethal Damage Repair
• Potentially lethal : under ordinary circumstances, it causes cell
death.
• Manipulation of post-RT environment – PLD can be repaired
• Repaired if cells are incubated in a balanced salt solution.
• But this treatment does not mimic a physiologic condition
• If mitosis is delayed by suboptimal growth conditions, DNA
damage can be repaired.

5. PLD Repair……
• X-ray survival curves for
densityinhibited stationary-
phase cells, subcultured
(trypsinized and plated) either
immediately or 6 or 12 hours
after irradiation.
• Cell survival is enhanced if cells
are left in the stationary phase
after irradiation, allowing time
for the repair of potentially
lethal damage

6. PLD repair …..

7. PLD & Radioresistance
• Radioresistance of certain types of human tumors is
linked to their ability to repair PLD
• Radiosensitive tumors repair PLD inefficiently
• Radioresistant tumors have efficient mechanisms to
repair PLD.
• This is an attractive hypothesis, but never been proven.

8. Sub Lethal Damage Repair
• SLD is the operational term
• Increase in cell survival that is
observed if a given radiation dose
is split into two fractions separated
by a time interval.
• The increase in survival in a split-
dose experiment results from the
repair of sublethal radiation
damage.

9. • In first few hours, prompt repair of
SLD evident, but at longer
intervals between the two split
doses, the surviving fraction of
cells decreases, reaching a
minimum with about a 5-hour
separation.
• “Age response function”-
asynchronous population of cells
exposed to a large dose of
radiation, more cells are killed in
the sensitive than in resistant
phases. The surviving population
of cells to be partly synchronized.
SLD Repair….

10. • Fig is a combination of 3 processes occurring
simultaneously.
• 1. the prompt repair of SLD.
• 2. Reassortment
• Progression of cells through the cell cycle.
• 3. Repopulation
• Increase of surviving fraction resulting from cell division.
SLD Repair……

11. Four R of Radiobiology
• “Four Rs” of radiobiology
• Repair
• Reassortment
• Repopulation
• Reoxygenation
• The dramatic dip in the split-dose curve at 6 hrs caused
by reassortment.
• The increase in survival by 12 hrs because of repopulation
are seen only for rapidly growing cells.

12. • Dramatic dip in the curve at 6 hrs
caused by reassortment.
• More repair in small 1-day tumors than
in large hypoxic 6-day tumors.
• Repair is an active process requiring
oxygen and nutrients.

13. • SLD repair in split dose experiment.
• A: dose is delivered in two fractions there is an increase in cell survival
• B: The fraction of cells surviving a split dose increases as time interval between
the two dose fractions increases. interval increases from 0 to 2 hours, increase
in survival results from SLD repair. In cells with a long cell cycle or that are out
of cycle, there is no further increase in cell survival by separating the dose by
more than 2 or 3 hours. In a rapidly dividing cell population, there is a dip in cell
survival caused by reassortment.

14. Mechanism of SLD Repair
• Te repair of SLD is simply the repair of double-strand
breaks.
• Rejoin and repair of double-strand breaks.
• The component of cell killing that results from single-
track damage is the same whether the dose is given in a
single exposure of fractionated.
• The same is not true of multiple-track damage.

15. Repair and Radiation Quality
• The shoulder on the acute survival curve
and the amount of SLD repair indicated by
a split-dose experiment vary with the type
of radiation used.
• The effect of dose fractionation with x-rays
and neutrons is compared in Figure

16. The Dose-Rate Effect
• For x- or r-rays, dose rate is one of the principal
factors that determine biologic consequences of a
given absorbed dose.
• Lowered dose rate and extended exposure time
generally occur reduced biologic effect.
• The classic dose-rate effect results from the repair of
SLD that occurs during a long radiation exposure.

17. Dose-Rate Effect……
• Continuous low-dose-rate(LDR)
irradiation may be considered to be
an infinite number of infinitely
small fractions.
• No shoulder, shallower than for
single acute exposures.

18. DRE InVitro & InVivo
• Survival curves for HeLa cells
cultured in vitro and exposed to X-
rays at high and low dose rates.
• dose-rate effect from the repair of
SLD varies enormously among
different types of cells
• HeLa cells have small initial shoulder.
• As the dose rate is reduced, the
survival curve becomes shallower
and the shoulder tends to disappear

19. • Chinese hamster cells
• Broad shoulder, large dose-rate
effect.
• There is a clear-cut difference in
biologic effect, at least at high
doses, between dose rates of
1.07, 0.30, and 0.16 Gy/min.
• Differences between HeLa and
hamster cells reflect differences
in apoptosis.
DRE …….

20. • At LDR, the survival curves “fan out”.
• Show greater variation of slope
• Inherent radiosensitivity ( evident in HDR)
• And variant range of repair times of SLD.
Dose Survival Curves…

21. • Response of mouse jejunal
crypt cells irradiated with ɣ-
rays from cesium-137 over a
wide range of dose rates.
DRE……

22. Inverse Dose-Rate Effect
• Decreasing the dose rate
results in increased cell
killing.

23. • In HeLa cell, dose decreasing
1.54 to 0.37 Gy/h is almost as
damaging as an acute exposure.
• At low dose progress through
cell cycle and arrested in G2
(radiosen)
• At higher dose they frozen in
phase of cell cycle (at start of
radiation)
• At higher dose rates, they are
“frozen” in the phase of the
cycle they are in at the start of
the irradiation.

24. Dose-Rate Effect
• DRE resulting from repair of SLD,
redistribution, proliferation.
• Dose-response curve for acute exposures
has broad initial shoulder.
• As dose rate is reduced, survival curve
becomes progressively more shallow as
more SLD is repaired, but cells are
“frozen” in cell cycle
• As the dose rate is lowered further
survival curve steepens again because
cells progress through block in G2, but
still cannot divide.
• further lowering of dose rate below this
critical dose rate allows cells to escape
the G2 block and divide

25. Take Home……
• PLD repair can occur if cells are prevented from dividing for 6
hours or more after irradiation
• PLD repair is significant for x-rays but does not occur after
neutron irradiation.
• Half-time of SLD repair in mammalian cells is about 1 hour, but
it may be longer in late-responding normal tissues
• SLD repair is significant for x-rays, but almost nonexistent for
neutrons.
• dose rate is reduced, the slope of the survival curve becomes
shallower (D0 increases), and the shoulder tends to disappear.

26. Thank you