CT, radiation, and risk: two views

["John Jacobus" <crispy_bird@YAHOO.COM>]

I received this article from another list server, and

thought I would pass it along.  The original can be

found at

http://www.auntminnie.com/default.asp?Sec=sup&Sub=cto&Pag=dis&ItemId=62145



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CT, radiation, and risk: two views

6/24/04

By: Eric Barnes



SAN FRANCISCO - Several years after the publication of

Dr. David Brenner and colleagues' eye-opening report

on the risk of CT-induced cancer in children,

questions about the dangers of ionizing radiation

still hover uneasily in the minds of physicians and

parents around the world (AJR, February 2001, Vol.

176, pp. 289-296).



The concern is understandable, considering that CT use

continues to, well, spiral, replacing x-ray and even

clinical exams in an ever-growing list of

applications. As reported in 2000 by the United

Nations Scientific Committee on the Effects of

Radiation (UNSCEAR, 1991-1996 data), in level 1

countries CT imaging accounted for 6.1% of all imaging

exams, yet contributed 40.6% of the annual collective

dose. Per 1,000 in population, residents of the U.S.,

Germany, the Netherlands, and the U.K. underwent 91,

64, 32, and 22 CT scans, respectively. The U.S.

performs nearly 25% of the world's CT scans, not

including those performed in Japan, the runner-up.



More recently, the FDA's NEXT survey estimated some 58

million annual exams in 2000-2001, while the 2004 U.S.

census from market research firm IMV Medical

Information Division of Des Plaines, IL, estimated

that 45.4 million CT studies are conducted annually.

Meanwhile, CT utilization is expected to continue

growing by 4%-10% per year.



Are these exams dangerous? Are all of them necessary?

Yes and no, according to the results of a debate at

the International Symposium on Multidetector-Row CT,

sponsored by Stanford University of Stanford, CA. In

yesterday.s opening sessions, two radiologists from

the Netherlands dutifully spelled out the opposing

positions assigned to them by conference organizers.



CT and the case for real risk



"Linear non-threshold (LNT) dose relation is the best

approximation of individual risk and current use

patterns, and the steady increase in the use of CT

will result in relevant public health risks on the

population level," said Dr. Aart van der Molen from

Leiden University Medical Center in the Netherlands.

"Users are still unaware of dose levels and dose

risks, and optimization on a nationwide basis is quite

a slow process," he said. "There is a lack of

justification and of evidence of benefit/risk ratios

of CT in some places."



Radiation risks can be divided into deterministic

effects such as skin injuries, GI syndrome, cataracts,

and stochastic effects, including carcinogenesis and

genetic effects, he said. Most of the risk data come

from World War II Japanese atomic bomb survivors, of

course, as reported by the Radiation Effects Research

Foundation (RERF). The results show a small but

statistically significant risk in the 0-100 mSv

radiation range (Radiation Research, October 2003,

Vol. 160:4, pp. 381-407).



"However, true-positive evidence from epidemiological

studies is never measurable, because if you want to

measure something in the 10 mSv range you would

probably need a population of 10 million that you

would follow their entire lives," van der Molen said.

Nevertheless, the LNT dose relationship is the most

comprehensive model available.



A report from the National Council on Radiation

Protection and Measurements examined all of the

previous atomic bomb survivor studies (data 1950-1997)

and estimated that among 86,500 bomb survivors, 440

(4.9%) of the solid-cancer deaths and 250 (0.8%) of

the non-cancer deaths were associated with radiation

exposure (Health Physics, July 2003, Vol. 85:1, pp.

15-22). 



The report concluded that "...the weight of evidence

suggests that lesions that are precursors to cancer

(i.e., mutations and chromosome aberrations), and

certain types of cancer as well, may increase in

frequency linearly with the dose in the low-dose

domain. On this basis, the committee concluded that no

alternative dose-response model is more plausible than

the linear non-threshold model, although other

dose-response relationships cannot be excluded,

especially in view of growing evidence that the

dose-response relationship may be modified by adaptive

responses, bystander effects, and other variables." 



Another study found that despite the lack of evidence

of risk from low radiation levels, there is

reasonable-to-good epidemiological evidence for

increased cancer risk for acute exposures with doses

above 10-15 mSv.



For its part, the International Commission on

Radiological Protection calculated the excess risk of

cancer at 5% per Sievert for fatal cancers, and an

additional 1% per Sievert for non-fatal cancers

overall, though the risk was significantly higher in

children (e.g., 0.3% for female newborns) and lower

for older people (e.g., 0.01% for 60-year-old males).



"What happened with multidetector?" van der Molen

asked. "We got increased productivity per scanner, we

did more multiphasic exams, and we found newer

indications, not always the lowest-dose ones,

including coronary CTA, CT urography with up to four

phases per exam, (and) total-body trauma CT. Don't

look at the patient anymore, get him off the

helicopter and put him in the CT scanner. (There is)

kidney stone CT, and new indications like dynamic

perfusion CT."



In his institution, CT accounts for about 9% of all

radiologic exams, but 68% of the collective radiation

dose, van der Molen said. One study calculated deaths

from CT-induced cancers at 208 per year in the

Netherlands, and 5,695 annual deaths in the U.S.



Also troubling, a study of radiation dose awareness

found that 50% of emergency physicians and 60% of

radiologists underestimated the radiation dose that

they delivered in a given CT exam (Radiology, May

2004, Vol. 231, pp. 393-398).



And despite a plethora of recent articles on low-dose

pediatric CT techniques, a survey of Society of

Pediatric Radiology members found that only 4% used

less than 120 kVp, 11%-44% still used high mAs values,

especially in older children, and that a substantial

number of CT scans were done without major adjustments

for weight (American Journal of Roentgenology,

February 2003, Vol. 180:2, pp. 401-406). An ALARA (as

low as reasonably achievable) symposium found that

10%-30% of pediatric CT is not justified.



"LNT dose relation, in my view, is the best we have

today, and it shows a small individual risk," van der

Molen said. "However, on a societal level, a large

number of exams will turn a small risk into a

considerable health risk. CT use is high and steadily

increasing, and the increase in use is faster than the

decrease in dose per exam. The number of unjustified

CTs may be considerable. Users are still not enough

aware of doses and risk, and this all results in a

collective risk for the public that I think may be

increasing in the future."



Weighing CT risk 



Following van der Molen was Dr. Mathias Prokop from

the University Medical Center in Utrecht, the

Netherlands, who had the unenviable job of convincing

symposium attendees that "dose issues are not really

important," he said.



But with regard to radiation risks, a couple of very

important questions stand out, he said. First, is

there a real radiation risk for CT? Second, what about

individual risk relative to population risk? One has

to look at the effect of limited healthcare resources

as well.



"Malignancy increases over time, and malignancy is

strongly age-dependent," Prokop said. "Above 200 mSv

there is no (doubt) that there is a linear risk

dependency between radiation-induced cancer and

radiation. Below 5 mSv we'll probably never be able to

prove (risk), and if you look at the A-bomb survivors

(half of whom who received less than 5 mSv) you

actually had less cancer deaths than expected in a

(normal) population group. Somewhere between 5 mSv and

200 mSv we have a lack of good data, and we have to

form a curve which is based on radiological models."



But the task of filling in the curve accurately has

confounded generations of researchers; for one thing

there is massive standard deviation. Radiation risk is

strongly age- and sex-dependent. Brenner and

colleagues found that only above 125 mSv of exposure

was there a small but statistically significant

increase, Prokop said.



There are a number of problems with the LNT model as

well, he said. The theory asserts that as photons pass

through cells they cause mitochondrial damage, and

further that the damage increases in a linear fashion

with the increase in photons, or radiation dose.



Yet "the risk for small animals like mice, and the

risk for us is the same, and we have definitely more

cell nuclei, but we don't get more liver cancers or

colon cancers," Prokop said. "That means there must be

some type of repair mechanism that plays a role," he

said, and a few have been documented. 



Prokop said that a threshold model may actually work

better than the LNT for leukemia and sarcoma data,

according to the recent Health Physics study on atomic

bomb survivor data.



"...the fit of the A-bomb solid tumor and leukemia

incidence data are significantly improved by the

addition of a threshold term in comparison with the

purely linear or linear quadratic model," the authors

wrote. "The results from the mortality data suggest

that the leukemia data agree more with the threshold

model than the linear quadratic model, although the

linear quadratic model is statistically equivalent,

while the solid tumor data does not suggest any

improvement with a threshold" (Health Physics,

December 2003, Vol. 85:6, pp. 709-20).



Considering radiation hormesis



Then there is the radiation hormesis model, which

suggests that at very low levels, radiation may have a

protective effect. And while the evidence is

conflicting, some evidence in favor of that model can

be seen at the cellular level.



"There is, for example, the production of enzymes that

repair RNA damage that's upregulated (with radiation

exposure)," Prokop said. "There is stimulation of

apoptosis, basically the suicide of damaged cells.

Also for low radiation we find a stimulation of human

T killer lymphocytes; in other words, a stimulation of

the immune system. At high doses, it's inverse. And

the stimulation of scavenging processes is also

stimulated (with radiation)."



Finally, he said, no study has really proven a risk in

the range below 20 mSv, the range most radiologists

work in with their patients. Still, the LNT theory is

legally accepted, and remains the basis of most

regulation around the world, including in the European

Union, and soon in the U.S.



"We should be prudently using radiation, despite the

fact that there's no proven damage in our CT dose

range," Prokop said.



A recent study by Berrington de Gonzalez looked at the

risk of cancer from x-rays in 15 countries, with data

corrected for factors such as age, frequency of

radiologic exams, type of exposure, etc. Yet the group

found it could not correct for prognosis; a cancer

patient has a different prognosis than an age-matched

control, for example. The authors concluded that the

population effects of radiologic exams may well be

overestimated, Prokop said (The Lancet, January 31,

2004, Vol. 363:9406, pp. 345-351).



At the same time, one must be very careful with the

use of radiation in children, in anyone with benign

disease, and in the case of frequent follow-ups,

Prokop said. But overall, given a population risk of

about 33% of cancer death, the comparative risk of a

chest or abdominal CT of 0.05% (10 mSv) is

comparatively insignificant.



The risks of not scanning



Comparative risks are also highly relevant, Prokop

said. If a radiologist opts for a chest x-ray over a

chest CT in suspected thoracic trauma, he risks

missing 30% to 40% of clinically significant injuries,

some of which are potentially life-threatening.

Similarly, a third of patients with embolism will die

if it is not found and treated; a negative CT is a

good sign, as embolism recurs in less than 1% of

patients, he said.



"What we have to weigh is, on the one hand, the acute

and immediate risk for the patient, and on the other

side, a long-term very low risk," Prokop said. Without

CT, the immediate risks include wrong diagnosis, no

diagnosis, or delayed diagnosis.



Clinical diagnosis, ultrasound, and MR are possible

alternatives to CT, but each carries risks of its own.

Clinical diagnosis is often unreliable; MR is

substantially more expensive and may require sedation

in children. Relying on MR exclusively would mean

fewer exams on a limited budget, as most MR exams are

more than twice as expensive as CT. And ironically,

ultrasound exams often generate additional CT exams to

resolve questions that arise from the first look, and

most such findings turn out to be benign.



"CT poses a very low stochastic long-term risk, which

probably will not manifest tomorrow, but only in a

number of years," he said. "And it grows over time, so

the younger the patient, the more risk you will have.

CT is justified if the disease risks outweigh the

excess radiation risks. This is almost always the case

in life-threatening diseases."



Still, radiologists should avoid numerous repeat

follow-up scans in cancer patients with complete

remission, and in patients with reduced life

expectancy, for example. It is also important to adapt

dose to patient size, adapt the dose to the

indication, differentiate between high- and low-dose

indications, and lower the dose in kids and

contrast-enhanced chest CT, he said.



"Scan thin (slices) and reconstruct thick, which is

one of my favorite ideas," Prokop said. "I think that

CT does have a role, the risk is very limited, and

actually there is room for a lot of improvement on our

side to get that low risk even lower."



Both Prokop and van der Molen agreed that better dose

reduction and dose awareness are essential, as are

stricter justification for the use of CT exams, and

quantification of the modality.s benefits in a given

exam. This will require more detailed guidelines,

accreditation, and quality criteria in order to

maximize the benefits and minimize the risks of CT,

they said.



By Eric Barnes

AuntMinnie.com staff writer

June 24, 2004



 . .

Copyright © 2004 AuntMinnie.com













=====

+++++++++++++++++++

"To be persuasive, we must be believable,

To be believable, we must be credible,

To be credible, we must be truthful."

Edward R. Murrow



-- John

John Jacobus, MS

Certified Health Physicist

e-mail:  crispy_bird@yahoo.com





		

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