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Repair of Radiation Damage
and the Dose-Rate Effect
Chapter 5
1
Overview
Classification of radiation damage
“Lethal”, “potentially lethal” and
“sublethal” radiation damage
Repair and radiation quality
Dose-rate effect & inverse dose-rate
effect
Brachytherapy or endocurietherapy
Intracavity and interstitial therapy
Radiolabeled immunoglobulin
therapy
2
Classification of Radiation
Damage (Mammalian Cells)
Lethal damage (LD)
leads to cell death
irreversible
irreparable
Sublethal damage (SLD)
can be repaired in hours unless additional
damage added
second dose interacts
evident by increased survival with
fractionated doses
3
Classification of Radiation
Damage
Potentially lethal damage (PLD)
damage which can be modified by post-
irradiation environment
evident by increased survival w/ delayed
assay
All terms are simply “operational”
mechanisms of cell repair & radio-
resistance not completely understood
at molecular level and in mammals
4
Potentially Lethal Damage
Examples of survival influence
varying environmental conditions: in vitro
incubated in balanced salt solution (not growth
medium) post exposure
increased survival (PLD repair)
not a good model of physiologic system however
better in vitro model (mimic tumor cells in vivo)
density-inhibited, stationary-phase cell cultures
cells remained density-inhibited for 6 - 12 hours
results in enhanced cell survival
5
X-ray Survival Curve for Density-
Inhibited, Stationary-Phase Cells
6
PLD - Relevance to
Radiotherapy
Repair also occurs in vivo
in experimental tumors
similar magnitude and kinetics to in
vitro
increased survival when allowing
several hours to elapse between in situ
irradiation and removal for
reproductive integrity
7
Repair of PLD in Mouse
Fibrosarcomas
8
Summary of Experimental
Data on PLD
PLD repaired and cell survival enhanced
if:
post-irradiation conditions are sub-optimal for
growth
i.e, so that cells do not undergo mitosis with
damaged chromosomes
damaged DNA can repair by delaying mitosis
Relevance to clinical radiotherapy still
debatable
9
Potentially Lethal Damage
and High-LET Radiations
No PLD repair
follows
exposure
Figure (lung
carcinoma)
line: immediate
explant
closed: 4 to 8 hr wait
open: 18 to 24 hr
wait
10
Sublethal Damage Repair
Operational term
describes increase in
cell survival observed
when radiation dose is
fractionated
Figure
Chinese Hamster cells
(cell cycling prevented)
no increased effect
when time between
doses is > 2 hrs
due to repair of SLD
11
Sublethal Damage Repair
What about cycling? survivors of 1st dose ~6 hrs later
These cells at 37 °C
Hamster cells (Tc ~ 10
hrs)
Initial dose
kills cells in sensitive
phases
resulting in “synchronized”
cells w/ majority in late S
Second dose
after 6 hrs, synchronized
cells now in very sensitive
G2 or M phase
12
Factors Involved in Repair
13
Repair and Radiation Quality
SLD repair (shoulder)
varies with type of
radiation
Dose fractionated for
x rays and neutrons
Results
x-rays: increased
survival
neutrons: little repair
of SLD
Chinese Hamster
Cells
14
Sublethal Repair and
Oxygenation
1.0
Surviving Fraction
Recovery
Factor
0.1 Next slide shows
this in a different
Hypoxic context.
0.0
1 Aerated
0 5 10 15 20 25 30
Dose (Gy)
single dose
fractionated dose
15
Ratio of SF (2 doses)
to SF (1 dose)
Sublethal Repair
in Other Systems
aerated
Observed in most
biological systems hypoxic
Aerated cells show
greater recovery
(repair) than do
hypoxic cells
Repair is an active
process requiring
oxygen
nutrients
Total dose (2 F) to leave
1 surviving cell per mm2
16
Dose-Rate Effect
17
The Dose-Rate Effect for Photons
Dose rate
determines
biological impact
Reducing dose
rate generally
reduces damage
Low dose ideal
curve is line F
18
Example of the Dose-Rate Effect
HeLa cells (photons)
broadened shoulder
as dose rate reduced
modest dose-rate
effect
HeLa cells have little
shoulder in acute
response curve
infers limited ability to
repair SLD
noticeable between
~1 and 100 rads/min
19
Examples of the Dose-Rate Effect
Chinese hamster
cells and x rays
broad shoulder
dramatic dose-rate
effect
significant
differences in
biological effect
survival decreases
as dose rate
increases
20
Human Cells In Vitro
40 different
human cell lines
irradiated at a
HDR and then a
LDR
LDR survival curves fan out due to variation in
radiosensitivity plus range of repair times of
SLD. 21
Survival Curves - Mouse Jejunum
Note dramatic
change in survival
below dose rate of
0.92 rad/min
Because cell division
begins to dominate
exposure time longer
than cell cycle
cell repopulation
occurs during long
exposures
22
The Inverse-Dose Rate Effect
In some cases,
decreasing dose rate
enhances cell killing
Small range of
‘optimal’ dose rates
~ 30 rad/hr = acute
response in the case
of HeLa cells
Why?
23
The Inverse-Dose Rate Effect
It’s the result of cell
cycling
at the ‘optimal’ dose
rate (~30 rad/hr in this
case) cells progress to a
G2 block
at higher dose rates
cells are frozen in place
during irradiation
at lower dose rates cells
continue to cycle
24
Dose-Rate Effect Summarized
During irradiation,
cells may pile up in the
sensitive G2 phase
25
Continuous, Very Low Dose Rates
Observing renewal of tissues of small
animals
examining dose rate necessary to maintain
steady-state cell population (births = deaths)
this dose rate varies with species/tissue:
rat small intestine: 400 rad/d
rat red blood cell: 50 rad/d
male rats, 10 generations: 0.2 rad/d
3 factors determine response
26
Response Determination
Sensitivity of stem cells
little or no shoulder (single dose) means lower
susceptibility to dose-rate effect
because shoulder is reconstructed during protracted
exposures
Duration of cell-cycle
accumulated dose over cell cycle is best measure of
cell lethality (more so than dose rate)
cells with long cycle receive more accumulated dose,
thus are generally more damaged, for a given dose
rate
Adaptability of cells
example, red blood cells at 0.45 Gy/d
initial adaptation, then blood cell production resumed
cell killing compensated by shortening of the cell cycle
27
Therapy
28
Brachytherapy or Endocurietherapy
Implanting sources directly into the body
Called:
Brachytherapy (brachy - short range)
Endocurietherapy (endo - within)
Two distinct forms of irradiation:
“intracavity”: placed in body cavity near tumor
“interstitial”: radioactive seeds placed in tumor
Radium used initially
encapsulated, yet tended to leak
29
Intracavity Therapy
Low dose-rate therapy
1 - 4 days, continuous irradiation
~ 50 rad/hr
most common use: cervical cancer
use of radium replaced by 137Cs, then 192Ir
74 d half-life and ~ 400 keV gamma
High dose-rate therapy
used in certain instances; several dose fractions
used to limit doses to normal tissues
healthy organs physically displaced during
irradiation
30
192Ir Therapy
short half-life (74 d)
dose rate changes during procedure
low photon energy
380 keV (average)
easier to handle than 137Cs
can be used in computer-controlled after-
loader
catheter implanted into tumor
sources transferred by remote control
31
Interstitial Therapy
Permanent or temporary implants
use of radium replaced with 192Ir
treatment of choice for accessible
human cancers (only ~5% of total)
range of dose-rates used:
range where biological effect varies rapidly
with DR
isoeffect curves relate dose rate, total dose
& tolerance dose (to health tissue)
32
Isoeffect Curves
33
Interstitial Therapy
Local tumor control and
necrosis rate at 5 years as a
function of dose in patients
treated for carcinomas of
the tongue/mouth. Tumor
control did not depend on
dose rate, if total dose was
sufficiently high.
Lower dose rates can be
used as long as total dose
exceeds ~65 Gy, resulting in
less necrosis and equal tumor
control.
34
Interstitial Therapy
Breast cancer patients
treated with 192Ir and
external beam therapy.
Shows impact of dose-rate
on tumor control (same total
dose). No information given
on late effects (e.g.,
necrosis).
35
Permanent Interstitial
Implants
Encapsulated sources
short half-life and low-E sources (125I)
permanently implanted (no 2nd
operation)
high dose-rate, initially
total dose ~ 160 Gy at tumor periphery
80 Gy delivered in first 60 d
cell killing
depends on cell cycle
optimal for slow growing tumors
36
Radiolabeled Immunoglobulin
Therapy
Delivers radioactive isotope to tumor
example: antiferritin (antibody) labeled with
iodine or yttrium
iron storage protein
synthesized preferentially by some tumors
Selective tumor targeting demonstrated
Nuclides for label:
131I, 90Y, 188Re, 186Re, 32P
target visualization may require g additive
targeting needs improvement
37
Summary
Lethal, sublethal, and potentially lethal
damage operational definitions
PLD - inhibiting cell cycle results in
increased survival
enhanced survival in sub-optimal growth
conditions
cell survival also shown to be enhanced in
vivo
SLD - dose fractionation results in
increased survival
repair, reassortment, repopulation
increased repair in smaller tumors, thus
repair apparently requires oxygen and
nutrients
38
Summary
Potentially lethal damage
modified by manipulating post-
irradiation conditions
prevent PLD from becoming lethal by
preventing cell division
significant for low LET, not high LET
resistant human tumors thought to
repair PLD
39
Summary
Sublethal damage
increase in survival by fractionating
doses
SLD half-time is ~ 1 hour in mammalian
cells
in tumors and normal tissues
demonstrated in vivo and in vitro
DNA repairs before aberrations are
created
40
Summary
Dose-rate effect
reduction in dose rate causes reduced cell
killing, due to repair of SLD
reduction in dose rate generally reduces
survival-curve slope (D0 increases)
inverse dose-rate effect occurs in some cell
lines at ‘optimal’ dose rate due to
accumulation of cells in G2
Brachytherapy/Endocurietherapy
implant sources in (interstitial) or near
(intracavity) tumor
Radiolabeled immunoglobulin therapy
41
