phantom used to measure dose from CT
David J. Brenner
Essential Science IndicatorsSM from
Reuters, the paper, "Current concepts—computed
tomography—an increasing source of radiation
exposure" (Brenner DJ, Hall EJ, N. Engl. J. Med.
357: 2277-84, 29 November 2007), is ranked at #5 among
Clinical Medicine Hot Papers and at #12 among Hot Papers
overall, with 416 cites up to October 31, 2009. In the
Web of Science®,
this paper currently has 504 cites.
Lead author Dr. David Brenner is the Higgins Professor
of Radiation Biophysics and the Director of the Center for
Radiological Research at the Columbia University Medical
Center in New York. His citation record in our database
includes 96 papers, classified under Clinical Medicine and
Biology & Biochemistry, cited a total of 4,060 times
between January 1, 1999 and October 31, 2009.
In the interview below, Dr. Brenner talks with
ScienceWatch.com about this paper and its impact on the medical
What factors prompted you and your coauthor
to undertake this study?
In the US, the average radiation dose to which we are exposed has doubled
in the past 30 years. The average dose from natural background sources has
not changed, but what has changed is a more than six-fold increase in the
average radiation dose from medical imaging. The biggest contributor to
this increase in the radiation dose from medical imaging is from CT: In
1980, about 3 million CT scans were done each year in the US, whereas the
number now is over 70 million. Radiation doses from CT are typically more
100 times those from conventional x-ray exams, such as a chest x-rays or
mammograms, and there is now direct epidemiological evidence of a small but
statistically significant increased lifetime cancer risk at CT doses.
Would you sum up the major conclusions of the
First, there is compelling, though not definitive, epidemiological evidence
that radiation doses typical of those given in a CT series are associated
with a small but statistically significant increase in lifetime cancer
risk, particularly for children. Individual risks from CT are small, but
the increasing population dose resulting from greatly increased CT usage
leads to concerns about future public health problems. We estimated that if
the current practices continue, up to 2% of all cancers in the US might, in
the future, be associated with the radiation from diagnostic imaging (we
are frequently misquoted as suggesting that this 2% refers to current
cancer rates—rather it is a projection some decades into the future).
Of course when a radiological imaging procedure is clinically appropriate,
the benefit-risk balance is almost always overwhelming. But the key here is
"clinically appropriate." In particular, of the more than 70 million CT
scans that are being performed in the US this year, it is appropriate to
ask how many of these scans are actually clinically justified. Based purely
on medical considerations, perhaps one third of all CT scans could be
avoided all together, or replaced with a different diagnostic tool. We
suggested that it is possible, though hard, to reduce this large number of
CT scans which are clinically unwarranted.
What was the reaction of the medical
Sources of average
radiation dose in the
Frequency of CT scans
per year in the US and
The reaction of the medical community, understandably, has been mixed.
While there is general agreement that more CT scans are being performed
than might be considered clinically necessary, critiques have generally
fallen into four categories:
Critique 1: "Cancer risks at very low doses are very uncertain,
and depend on extrapolating risks from A-bomb survivors who were exposed to
high doses." At extremely low radiation doses, cancer risks are indeed
highly uncertain. However, at the doses corresponding to a few CT scans
there are direct epidemiological data from about 30,000 A-bomb survivors
who were on the peripheries of Hiroshima and Nagasaki, and who were exposed
in this low-dose range. This low-dose subpopulation has been followed for
more than 50 years, and shows a small but statistically-significant
increased cancer risk. Thus, in the context of CT doses, we do not need to
extrapolate cancer risks from higher doses, with all the attendant
uncertainties which that involves.
Critique 2: "No studies of individuals having CT scans have
shown an increased cancer risk." This is certainly true. Because CT is
a relatively recent modality, only now has it become practical to start
such studies. So, to date, the risk estimates have come from studies of
atomic bomb survivors (see above) who were a considerable distance from the
explosions, and so were exposed to doses similar to those from typical CT
scan series. This is not an unreasonable approach, but it is pleasing that
direct epidemiological studies of CT risks in children have now been
initiated in the UK, Ontario, Israel, and Australia. It is disappointing
that such studies are not being undertaken in the US, but tracking
health-care records is much harder here.
Critique 3: "Many who need CT scans will refuse them because of
the publicity associated with cancer risk estimates." The evidence
does not support this: for example, in a published study, when parents were
informed about CT risks and benefits, their willingness to have their child
undergo a CT did not significantly change, although they became more
willing to consider other imaging options, if equally effective. No CTs
were cancelled or deferred after receiving this information.
Critique 4: "It will be very difficult to target the medically
unwarranted CT scans." This is certainly true. Physicians are subject
to significant pressures, from throughput, to medico-legal, to economic, to
patient pressure. Our goal was to promote already ongoing dialogues among
radiologists, emergency department and other physicians, and indeed the
public, as to practical ways to minimize medically unwarranted CT scans,
without compromising patient care.
How concerned should ordinary patients be about
having CTs done?
The individual risks of a CT scan are small, and if the CT scan is
medically warranted, the benefits will greatly outweigh these small risks.
It is perfectly appropriate, however, for a patient/parent to talk to their
physician about the CT scan, and ask if it is indeed warranted based on
Do you envision these findings having a
significant effect on clinical practice, or is a seemingly prevalent
mentality of "order more tests" becoming too firmly
Reducing the number of CT scans that are not clinically justified is a hard
task, because there are a variety of very real factors pushing in the other
direction, ranging from throughput, to economic, to patient preference, to
We see the solution in terms of the application of clinical decision
guidelines (decision rules, appropriateness criteria, etc.). Based on a mix
of clinical data and expert judgment, decision guidelines provide scenarios
as to when a given imaging procedure is medically justified. Many decision
guidelines are now available for the appropriate use of CT in different
settings. When used, decision guidelines have the potential to "trump"
some, though not all, of these factors that result in CT scans being
over-prescribed, and so represent a potentially powerful tool for
optimizing CT usage. Of course decision guidelines are not useful if they
are not applied, and this is too often the case. A successful approach to
increasing utilization of CT decision guidelines has been to incorporate
them into computerized imaging ordering systems.
It is pertinent here to mention the financial implications. If one could
eliminate the perhaps 1/3 of CT scans that are clinically unnecessary,
there would of course be a corresponding reduction in imaging costs.
To sum up, it is impossible to imagine the current practice of medicine
without modern-day imaging and, in the final analysis, the clinician is in
the best position to assess the imaging needs of his/her patient. But,
along with all the high-tech imaging tools that are now available,
optimization of imaging usage with the aid of clinical decision guidelines
is essential. This is not easy to implement, but it can be done, and it
should be done.
David J. Brenner, Ph.D., D.Sc.
Center for Radiological Research
Columbia University Medical Center
New York, NY, USA
David J. Brenner's current most-cited paper in Essential
Science Indicators, with 557 cites:
Brenner DJ, et al., "Estimated risks of radiation-induced fatal
cancer from pediatric CT," Amer. J. Roentgenol. 176(2): 289-96,
Note: This interview pertains to the paper, Brenner DJ, Hall EJ, "Current
concepts—computed tomography—an increasing source of radiation
exposure," N. Engl. J. Med. 357(22): 2277-84, 29 November 2007,
with 416 cites.
Essential Science Indicators from
KEYWORDS: COMPUTED TOMOGRAPHY, CT, RADIATION EXPOSURE, CANCER RISK, AVERAGE
DOSE, CHILDREN, BENFIT-RISK BALANCE, CLINICALLY APPROPRIATE, A-BOMB
SURVIVORS, CLINICAL DECISION GUIDELINES, IMAGING COSTS.