[ RadSafe ] Another example of risk vs. benefits in CT

Wed Aug 10 07:55:55 CDT 2005

Thought this is another example that people should
use.  And, no, there is no analysis of hormesis.  Just
saving lives.

Article is at
http://www.auntminnie.com/index.asp?Sec=sup&Sub=wom&Pag=dis&ItemId=67349&wf=239
But you may have to register.
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Even in pregnancy, CT rules for ruling out PE
8/8/2005
By: Eric Barnes

There's no cause for misgivings when CT pulmonary
angiography is needed to rule out pulmonary embolism
in a pregnant woman. CTPA not only provides the most
definitive diagnosis available in such patients, it
does so with less radiation than scintigraphy.

The Reston, VA-based American College of Radiology
(ACR) apparently agrees, and is expected to issue new
CTPA guidelines in the fall that specifically include
the possibility of using CT to rule out PE in pregnant
women.

At the 2005 Symposium on Multidetector-Row CT in San
Francisco, Dr. Phillip Boiselle, who is director of
thoracic imaging at Boston's Beth Israel Deaconess
Medical Center and associate professor of radiology at
Harvard Medical School, said that CT has a number of
advantages over ventilation perfusion scintigraphy
(VQ).

"(CTPA) has been shown time and time again to have
high specificity and sensitivity, and I think one of
the things that has helped it gain acceptance as well
is the fact that when it's negative for PE, an
alternative pulmonary or pleural diagnosis is often
established," Boiselle said.

(Picture) CT pulmonary angiography shows bilateral
acute pulmonary emboli (arrows). Image courtesy of Dr.
Phillip Boiselle.  

In contrast, the VQ scan, the only feasible
replacement, often yields indeterminate results -- and
a higher fetal radiation dose, he said.

A 2002 study in Radiology underpins this premise. It
revealed that CT may in fact be the best choice.
Twenty-three pregnant women underwent CTA to rule out
suspected PE. The CT protocol was 100 mAs, 120 kVp,
2.5-mm collimation, and 11-cm scan length, whereas
scintigraphy was performed with 37-74 MBq of
radiolabeled technetium-99m.

Dr. Helen Winer-Muram and colleagues from the Indiana
University School of Medicine in Indianapolis found
that the mean fetal CT radiation dose (3.3-20.2 µGy
during the first trimester, 7.9-76.7 µGy in the second
trimester, and 51.3-130.8 µGy in the third trimester)
was less than 6 mrad for CT compared to 10-37 mrad for
the VQ scan (Radiology, August 2002, Vol. 224:2, pp.
487-492).

"Even when they increased the mAs to 200, CT still
came out lower," Boiselle said. "CTA definitely comes
out a winner in this paradigm. The fetal dose for both
techniques (CT and scintigraphy) is well below the 5
rad limit that is considered safe for fetal exposure"
(Gibbs SJ, "Basic Mechanisms of Radiation Injury,
Somatic and Genetic," Radiation Risk: A Primer, ACR,
1996, pp. 5-13).

Perhaps more important, radiologists are getting the
message, Boiselle said.

"What are experts in thoracic imaging doing? We asked
that question a couple of years ago with a survey to
the Society of Thoracic Radiology membership, and we
found at that time roughly three-fourths of responding
departments were performing CTA in pregnant patients,"
he said.

Slightly more than half of respondents were performing
CTA rather than VQ as the initial imaging modality,
and nearly half had modified their standard protocol
to reduce the radiation dose, he said. Back in 2002,
the most common way of reducing dose was to decrease
the scan length in the z-axis. But many more
dose-reduction tools are available today, Boiselle
noted, "and it would be very interesting to see things
like how people are using automatic exposure control,
reduced kVp," and other techniques, he said.

No published study reflects the use of newer
dose-reduction techniques, nor are there official
guidelines to steer the imaging protocols for such
patients. But help is on the way, Boiselle said. At
the 2005 ACR meeting earlier this year, a resolution
was passed to update the CT practice guidelines for
CTPA.

"They directly mentioned that although the performance
of CT for PE involves ionizing radiation, the
estimated mean fetal dose may be lower than for
perfusion scintigraphy if a reduced-dose technique is
employed, referencing Winer-Muram," Boiselle said.

This language was included in a draft of the proposed
guideline obtained by AuntMinnie.com, although the
document does not include specific information about
dose-reduction techniques for pregnant patients.

The new CTPA practice guidelines will take effect
October 1, and will be available online shortly before
that date, said Margaret Wyatt, the ACR's manager of
guidelines and standards development, in a telephone
interview. In the meantime, the public can access the
existing practice guidelines in a PDF file on the
ACR's Web site. The final guidelines will be posted in
the same place in the ACR site's Guidelines and
Standards section, she said.

Boiselle said that CTA's advantages over perfusion
scintigraphy include a lower fetal radiation dose, its
ability to directly identify the thrombus, fewer
indeterminate results -- and not inconsequentially,
the capability of identifying other causes of symptoms
when there is no evidence of pulmonary embolism.

Still, doctors must remember that there is always a
small risk of contrast reaction anytime a CTA study is
performed, Boiselle said. He also listed three
recommendations by radiology legal expert Dr. Leonard
Berlin of Rush North Shore Medical Center, Skokie, IL,
who enumerated a number of steps for reducing
medicolegal risk in the imaging of pregnant women in
general (AJR, December 1996, Vol. 167:6, pp.
1377-1379):

Departments should have a process for evaluating
patients who are pregnant. 
Radiologists should be knowledgeable about the effects
of radiation, and accessible to patients and to the
referring physician. 
All discussions with patients pertaining to risks
should be documented in the radiology report. 
By Eric Barnes
AuntMinnie.com staff writer
August 8, 2005

Copyright © 2005 AuntMinnie.com

+++++++++++++++++++
"Every now and then a man's mind is stretched by a new idea and never shrinks back to its original proportion." -- Oliver Wendell Holmes, Jr.

-- John
John Jacobus, MS
Certified Health Physicist
e-mail:  crispy_bird at yahoo.com

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