Pediatric Trauma - Hennepin County Medical Center

Technology in Critical Care
Figure Two (a-left):
Normal dose technique
demonstrates a stone in
lower pole of right
kidney. Low dose
technique used on
follow-up scan (b-right)
demonstrated the same
stone with about 20%
the radiation dose from
standard technique.
1. Significant errors in measurement of low-level
radiation doses.
2. Significant errors in the estimation of effective
dose, which is calculated from a regularly
updated conversion factor.
3. Biological plausibility. The biological effects of
radiation decreases as dose decreases, and
DNA repair and elimination of defective cells by
death is a dynamic process.
4. Uncertainty regarding linear-no-threshold model.
5. Non-transferability of the risks and mortality from
radiation. Most medical doses are delivered to a
smaller and more elderly segment of the US
population, but the risks are extrapolated to the
general population.
Radiation risks therefore must be taken in the
appropriate context. If the estimated lifetime number
of deaths from a single CT scan with a dose of 10
mCi is indeed 0.5 per 1000, the estimated lifetime
number of deaths in the same group of individuals
from a lightning strike is 0.0 13 , and from drowning is
0.9. Similarly, the death rate from motor vehicle
accident is 11.9 15 . This does not dissuade most
Americans from driving! Similarly, while the dose
from a head CT is the same as about 20 chest
x-rays, this is still significantly less than a year of
background radiation.
Primum non Nocere
Measurement and quantification of risks from CT
radiation form a fascinating and often controversial
topic, with strong feelings on all sides. Above all, it is
our responsibility as physicians to make sure our
patients are not harmed. With this in mind, any
radiation dose should be assumed to be potentially
harmful, and careful risk-benefit analysis should be
performed. This is particularly important with children,
who are more vulnerable because of longer expected
life spans and more radiation-sensitive body tissues.
At the same time, it would be unconscionable to
deny a patient the benefits from a CT scan that is
appropriately indicated and optimally performed.
What can be done?
Justification: A CT scan should be performed only
when there is the potential of clear benefit. In other
words, the risk of not performing the examination
must exceed the potential risks. A classic situation is
following high-speed trauma, where the risk of not
performing at CT scan includes missing life-threatening
internal organ injury or aortic injury. This clearly exceeds
potential risk of inducing neoplasm later in life.
In most situations, the risk /benefit ratio is more
difficult to evaluate, and individual physician
judgment is paramount. There are many resources
available to assist physicians in making this
judgment. For example, the American College of
Radiology has provided evidence-based guidelines
for appropriate imaging tests 16 . Discussion between
radiologists and physicians, especially physicians-intraining,
clearly helps in the decision-making process
and in tailoring the exam to answer the clinical
question. Decision support systems integrated into
electronic medical systems may also be helpful. It
has been reported that 30% or more of CT scans
currently performed may be unnecessary. This
number must be reduced 17 .
Optimization: Careful attention to technique can
significantly reduce radiation dose. This includes
standardizing protocols, reducing multiple series
within each examination (avoiding "protocol creep"),
implementing dose reduction strategies, and limiting
scanning to part of interest (avoiding "scan creep").
The Society for Pediatric Radiology offers very useful
online information regarding tailoring exams in
children, as part of its "image gently" program 18 .
Of late, all CT vendors have introduced numerous
dose reduction techniques. These differ among
vendors, usually involve some form of dose
modulation tailored to the patient body habitus, and
are quite effective in reducing radiation dose while
keeping image quality adequate. In addition, there
are several situations where a low-dose exam with
noisier (grainier, less pleasing) images is appropriate.
In patients with kidney stones, the low-dose
technique takes advantage of inherent contrast
between the stone and surrounding tissues and
produces images of diagnostic quality (Figure Two).
Similarly, low-dose technique may be utilized in
patients with lung nodules for follow-up.
14 | Approaches in Critical Care | June 2011