the contribution of bystander effects to the risks posed by low radiation doses blyth
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The contribution of bystander effects to the The contribution of bystander effects to the risks posed by low radiation dosesrisks posed by low radiation doses
Benjamin J. Blyth & Pamela J Sykes
Haematology & Genetic Pathology School of Medicine
Flinders University and Medical Centre
Atomic Bomb Survivors - LSSPreston 2007, Radiation Research
( 0 – 100 mSv )
Natural background, inc. radonRadiographyOccupational exposuresAirline travel, inc. security screening
Atomic Bomb Survivor Data
Radiation Dose (mGy)
0 – 5 mGy Control Group
X and γ‐rays
3.7 MeV α‐particles
100 mGy, 100 kVp X‐raysor
5 Gy, 3.5 MeV α‐particles
0.1 mGy, 100 kVp X‐raysor
50 mGy, 3.5 MeV α‐particles
Security Backscatter X‐Ray Screening
Effective dose 0.05 μSv / scanHealth Physics Society (2009)
50 – 125 kVp photons, dose concentrated near the
skin’s surface
1 μGy to skin surface / scan0.1 – 1% cells traversed by an electron track
All exposed cells receive their dose within a 10 second period.
Domestic Radon Exposure
222Rn: 100 Bq m‐3
346 lung basal cell nuclei traversed by
a single alpha particle per day
(out of 1.5 × 109
cells)
Harley et al.
2008
1 cell in the lung hit on average, every 4 minutes
Basal bronchial cells are not hit more than once during their ‘lifetime’
Long‐Haul Airline Flights
Effective dose rate = 5 μSv h‐1
2.75 μSv h‐1 low‐LET + 2.25 μSv h‐1 high‐LET
Bottollier‐Depois
et al.
2000
0.5 – 2% of cells receiving energy deposition for a 10 h flight
LET range from 0.1 – 1000 keV μm‐1
Cellular dose per hit <1 mGy
to >100 mGy
Biophysical argument for linear extrapolation
Even the smallest radiation energy deposition to a cell will carry a small, but non‐zero risk of causing a carcinogenic
mutation. The net risk will therefore decrease in proportion with the number of cells receiving energy deposition.
10 x lower dose = 10 x fewer hit cells = 10 x lower risk
Radiation‐Induced Bystander EffectsRadiation‐induced effects in unirradiated cells remaining
within an irradiated tissue.
Cell death
DNA damage
Mutation
Proliferation
Genomic instability
In Vitro
Bystander Effect Methods
Proof‐of‐Principle: Unirradiated cells can, under the right conditions, respond to signals from irradiated cells.
Hypothetical Bystander Impact on Dose‐Risk Curve
What if all cells were at increased cancer risk even if
only a small fraction actually received energy deposition?
Bystander Effects*
•
Many experimental systems have used cellular doses and/or irradiated cell numbers
far above the relevant range
•
Bystander effects are not universally observed nor are they always reproducible
•
Contradictory bystander responses
•
Bystander effects remain to be demonstrated under relevant conditions in vivo
A better understanding of Bystander mechanisms
and the extent to which they are active in vivo
‘are
needed
before
they
can
be
confirmed
as
factors
to
be
included
in
the estimation of potential risk’
International Commission for Radiological Protection 2006 Report
Our Aim
To create an in vivo
experimental system that could be used to re‐create realistic bystander scenarios.
In vivo:
MouseTissue of interest:
Spleen
Relevant radiation exposure:
low‐LETSingle energy depositions:
3H, 1 d h‐1, 2.61 mGy hit‐1
Rare, known irradiated cells:
< 1 in 1000
Adoptive Transfer Model
StimulateCells to Divide
Incorporate 3H‐Thymidineor Non‐Radiolabelled
Thymidine
Label with Fluorescent Dye
Recipient MouseDonor Mouse
Isolate SpleenIsolate LymphocytesCulture cells
Inject Radiolabelled
or
Non‐Radiolabelled Cells
via Tail Vein
Collect, Freeze and Analyse
Recipient Mouse Spleen
Apoptosis: TUNELProliferation: Ki-6750 μm
3H +-
Bystander Effects In Vivo?
No induction of apoptosis or proliferation in unirradiated bystander
cells
–
After 22 h or 72 h
–
1 ×
3H disintegration h‐1
–
100 ×
3H disintegrations h‐1
–
0.1 or 1 Gy X‐rays
–
Irradiated cells lodged @ 2 – 20 × 10‐4
Blyth et al.
2010, Radiation Research
Local area around
donor cells Whole spleen
P
= 0.69
P
= 0.58
More tissues, endpoints, times, genetic background, doses etc.
Lodged Cells
Radioactive
Lodged Cells Non-
Radioactive
Lodged Cells
Radioactive
Lodged Cells Non-
Radioactive
Bystander effects are a concern because they could theoretically
increase the carcinogenic risk of very low dose exposures.
However, bystander effects are yet to be demonstrated in vivo
under the relevant exposure conditions.
Bystander effects have been characterised mostly in terms of ‘negative’ endpoints, however, it is unclear whether the net effect of these biological
changes would be pro‐
or anti‐carcinogenic.
Bystander effects, if relevant in vivo, will likely be a dynamic and complex response to radiation exposure, dependent on dose, dose‐rate, radiation‐ quality, stress, cell/tissue type, endogenous signalling and health of the individual.
The adoptive transfer model can now be used to explore many of these parameters in an in vivo
experimental system.
US Department of Energy
Low Dose Radiation Research Program, Grant # DE‐FG02‐05ER64104
Rebecca Ormsby
(Flinders University)
Alex Staudacher
(Flinders University)
Edouard I Azzam
(UMDNJ)
Roger W Howell
(UMDNJ)
Flinders University and Medical Centre