Radiology Resident Physics
Course
Dose Issues in
Computed
Tomography
George David MS, FAAPM, FACR
Associate Professor of Radiology
Augusta University
Good Old
Days of CT
 First axial images of live person
 Dose? Who cares?
Things Changed
 Are we killing people with CT?
Worse still…
 Are we killing children with CT?
6/19/2001
“Each year, about 1.6 million children in the USA
get CT scans to the head and abdomen — and
about 1,500 of those will die later in life of
radiation-induced cancer …”
1 of 1,000 …
Are We Killing
Children?
Really Slow Bullet
It Gets Worse
 On the basis of data from 1991- 1996, ~ 0.4% of
future cancers may be attributable to radiation
from CT.
 By adjusting this estimate for current CT use, this
estimate may go up to
1.5-2.0%.
Computed Tomography — An Increasing Source of
Radiation Exposure
David J. Brenner, Ph.D., D.Sc., and Eric J. Hall,
D.Phil., D.Sc.
It Gets Worse
 On the basis of data from 1991- 1996, ~ 0.4% of
future cancers may be attributable to radiation
from CT.
 By adjusting this estimate for current CT use, this
estimate may go up to
1.5-2.0%.
Based upon calculations.
Not based upon observations!
100 People Diagnosed with
Cancer in 2035
Caused by CT
How Much Radiation from CT?
Diagnostic Procedure
Typical Effective Dose (mSv)1
Chest x-ray (PA film)
0.02
Lumbar spine
1.5
I.V. urogram
3
Upper G.I. exam
6
Barium enema
8
CT head
2
CT chest
7
CT abdomen
8
Coronary artery calcification
CT
3
Coronary CT angiogram
16
From the FDA
Does CT Really Kill?
Diagnostic
Procedure
Typical Effective
Dose (mSv)1
Chest x-ray (PA
film)
0.02
Lumbar spine
1.5
I.V. urogram
3
Upper G.I. exam 6
Barium enema
8
CT head
2
CT chest
7
CT abdomen
8
Coronary artery 3
calcification CT
Coronary CT
angiogram
16
http://library.thinkquest.org/
Linear No-threshold Model
 No safe level of radiation
 Low Doses are Harmful
 Model used for Regulations
Line
Biological
Effects
Science
Radiation Dose
Biological Effect from
Radiation
 High dose effects well known
 What happens at low doses?
Biological
Effects
Known
effects
Our
Patients
Us
Radiation Dose
Linear Model
 If a 1,000 pound lion can kill 100 Romans in an hour
*
Linear Model
 A 10 pound puddy tat can kill 1 Roman in an hour.
Suffering
succotash
… I mean
ROAR
Oh no!
Water Can Kill You!!!
Threshold Model
 Low Doses are NOT Harmful
Biological
Effects
Radiation Dose
Things that are
dangerous in excess
are not dangerous in
moderation
Radiation Hormesis Model
 Low Doses are Beneficial
Biological
Effects
(bad)
(good)
Radiation Dose
Radiation is
good for
you!
Expert Panel Clarification
Linear NoThreshold
Low Doses Can
Kill You
Threshold
Hormeses
Low Doses Are
Meaningless
Low Doses Are
Beneficial,
Nyuk, Nyuk,
Nyuk
2011 CT Summit: “Admit we don't
know CT radiation risk”
 The linear model should be used for setting
radiation protection standards
 most conservative
 Senseless to use to predict cancer deaths.
We May Never Know
 It's difficult to model cancer risk from radiation at
small doses, such as produced by CT. To have enough
statistical power to detect such small effects, you need
to track > 10 million patients for years
 At the end of the day the controversy will never go
away.
Cynthia McCollough, Professor of
Radiologic Physics, Mayo Clinic
Linear No-Threshold
 Following slides from “Radiation Risks of Medical
Imaging: Separating Fact from Fantasy” (Hendee,
O’Connor, Radiology August 2012)
Major source of
knowledge for health
effects to individuals
from ionizing radiation
Hendee & O’Connor
 “Most population studies have revealed no or much
smaller demonstrable health effects of radiation
exposure”
 <100 mSv, it is not possible to identify increased
incidence of cancer with any confidence
 LNT model appears to conflict with current
understanding of biologic mechanisms
Hendee & O’Connor (cont.)
 “Studies of 500,000 occupationally exposed workers in
the nuclear industry … demonstrated reduced cancer
 BEIR VII largely excludes these studies from its
analyses”
 Claimed exposed population in better health than
general population
Beir VII: “Because of limitations in the data
…, risk estimates are uncertain, & estimates
… 2 -3 X larger or smaller cannot be
excluded.”
Why All the Attention?
 USA Today
 Brenner / Hall Article
 CT “accidents”
 Increase in CT usage
Cedars-Sinai CT Overexposures
 Brain perfusion studies
 Repeated exposure to same
anatomy
 Table didn’t move
 No equipment defects
 Protocols altered
 Decreased image noise
 Caused mA/dose to increase X 8
 Problem not identified for 18
months!!!
Any wonder our patients/parents are
concerned?
•Medical imaging procedures should be
appropriate & conducted at the lowest
radiation dose consistent with acquisition of
desired information
•Discussion of dose risks should be
accompanied by acknowledgement of
procedure benefits
•Risks of medical imaging at effective doses <
50 mSv (5 rad) for single procedures or 100
mSv(10 rad) for multiple procedures over
short time periods are too low to be detectable
& may be nonexistent.
• Predictions of hypothetical cancer incidence
and deaths in patients exposed to such low
doses are
• highly speculative
• should be discouraged