radiobiology chapter 5

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