1. Radiology Resident Physics Course
Dose Issues in Computed Tomography
George David MS, FAAPM, FACR
Associate Professor of Radiology Augusta University

2. Good Old Days of CT
First axial images of live person
Dose? Who cares?

3. Things Changed
Are we killing people with CT?

4. Worse still…
Are we killing children with CT? ?

5. 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 …”
Approximately 1 in 1000

6. 1 of 1,000 …

7. Are We Killing Children?
Really Slow Bullet
30-40 Years
CT Gun

8. It Gets Worse
2007
On the basis of data from , ~ 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 %.
Computed Tomography — An Increasing Source of Radiation Exposure
David J. Brenner, Ph.D., D.Sc., and Eric J. Hall, D.Phil., D.Sc.

9. 2007 It Gets Worse Based upon calculations.
On the basis of data from , ~ 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 %.
Based upon calculations.
Not based upon observations!

10. 100 People Diagnosed with Cancer in 2035
Caused by CT

11. 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
Coronary artery calcification CT
Coronary CT angiogram 16
From the FDA

12. Typical Effective Dose (mSv)1
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
Coronary artery calcification CT
Coronary CT angiogram 16

13. Linear No-threshold Model
No safe level of radiation
Low Doses are Harmful
Model used for Regulations
Line
Biological Effects
Science
Radiation Dose

14. Biological Effect from Radiation
High dose effects well known
What happens at low doses?
Biological Effects
Known effects
Our
Patients ? Us
Radiation Dose

15. Linear Model
If a 1,000 pound lion can kill 100 Romans in an hour

16. Suffering succotash… I mean ROAR
Linear Model *
A 10 pound puddy tat can kill 1 Roman in an hour.
Suffering succotash… I mean ROAR
Oh no!

17. Water Can Kill You!!!
H2O

18. Things that are dangerous in excess are not dangerous in moderation
Threshold Model
Low Doses are NOT Harmful
Things that are dangerous in excess are not dangerous in moderation
Biological Effects
Radiation Dose

19. Radiation Hormesis Model
Low Doses are Beneficial
Radiation is good for you!
Biological Effects
(bad)
(good)
Radiation Dose

20. Expert Panel Clarification
Linear No-Threshold
Threshold
Hormeses
Low Doses Are Beneficial, Nyuk, Nyuk, Nyuk
Low Doses Are Meaningless
Low Doses Can Kill You

21. 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.

22. 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

23. 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

24. 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

25. 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.”

26. Why All the Attention? USA Today Brenner / Hall Article CT “accidents”
Increase in CT usage

27. 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!!!

29. Any wonder our patients/parents are concerned?

30. 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

31. 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