As you get your chest X-ray, the technician scurries off to safety. Imagine getting this procedure done again. And again, 20, 50, 100 times in a row. You keep getting it – at least 290 times.
The point of this seemingly absurd vignette is that a computed tomography scan, or CT, of the chest is the equivalent of 290 to 400 chest X-rays, depending on the source.
But not to worry. It’s all still below a limit for annual occupational exposure set by the International Commission of Radiological Protection, an advisory board. Some experts even say that “low level” radiation is good for you.
The importance of X-rays to medicine from plain films to CT scans goes without saying. The U.S. Food and Drug Administration, however, has declared medical radiation a carcinogen. The question is, “How much is too much?”
– Dr. Howard Forman,
Yale University School of Medicine
What are patients to think, presuming they are fortunate enough to know anything about this in advance?
The leading theory of radiation safety is called “linear non-threshold,” which states in a nutshell that there is no safe dose. “In practice,” however, noted researchers in the British Journal of Radiology, there is a dose below which the risks are considered negligible. There are two competing theories, a “liberal” one and an “ultra-conservative” one.
Based on homeopathic medicine, the theory of hormesis says that although radiation is dangerous at high levels, it can be beneficial at low levels because there is an adaptive response to it. One of it proponents, Bernard Cohen of the University of Pittsburgh, published a well-known study on the safety of household radon that supports the theory. The implications, of course, are that current dosage protection standards can be safely relaxed. Despite some evidence for hormesis, it remains a minority position in physics and medicine.
Physician and medical physicist John Gofman leads the rival camp. Golfman isolated the first milligrams of plutonium. He counters with the assertion that regardless of whatever effects the adaptive response may produce, the radiogenic cancers eventually appear anyway.
Then, there is the bystander-effect model, which states that low-dose radiation may be even more harmful than proponents of linear non-threshold say. Although numerous recent studies support it, the bystander model also is in the minority.
In the May 26 issue of The Lancet medical journal, Richard Semelka, a radiologist at University of North Carolina at Chapel Hill School of Medicine, wrote that “referring physicians in the emergency department are largely unaware that there are potential harmful effects from CT radiation exposure, with only 9 percent aware of increased cancer risk.” Nearly half of radiologists in the study were unaware of the nature of the risk, Semelka added.
Another physician survey, this one led by Howard Forman, vice chair of diagnostic radiology at Yale University, showed that only one in five of emergency room doctors could correctly estimate a CT dose. Almost half thought the dose was less than 10 times that of a standard chest X-ray.
“Providers’ ignorance of radiation dose may compromise their ability to assess CT’s risks and benefits and render them unable to communicate this information to their patients,” Forma said.
The concern is most acute in pediatrics. Five years ago, medical physicist David J. Brenner of Columbia University published findings that made the front page of USA Today. Unlike regular X-ray films, CT does not register overdosage on the image. Thus, wrote Lee F. Rogers, editor in chief of the American Journal of Roentgenology, “if a child has been overexposed when undergoing CT, it was most likely not done knowingly or intentionally; radiologists and radiologic technicians may simply be unaware of the potential for danger.”
These articles came to the attention of the FDA, which issued a “warning” to radiologists about the hazard. The journalism was hyperbolic, but two eminent pediatric radiologists commented that “although much of this initial fallout from the press coverage was negative. the public attention has had a positive effect” of reducing the dosage to children undergoing tests.
Since then, most scanner manufacturers have incorporated automatic procedures for reducing dose in smaller patients. Many older scanners, however, may still be in use, in which case a technician needs to know how to manually reduce the dosage.
But whether a child or an adult is the patient, carelessness in ordering the test remains a problem. S.C. Bushong’s textbook Radiologic Science for Technologists noted, “many X-ray examinations are knowingly requested when the yield of helpful information may be extremely low or non-existent. When such an examination is performed, the benefit in the patient in no way compensates for the radiation dose.”
In general, doses have increased with the newest technology, spiral or helical CT, which continuously moves the patients through a scanner that rotates around them. They result “in doses up to four times higher than that for a conventional CT scan,” notes the Web site for the Health Physics Society. “To achieve such a detailed and expansive feat, it is necessary to expose the entire thickness of the patient to a greater quantity of X-ray photons.”
The dosage from CT scans overlaps with the amount of radiation that increased cancer in atomic-bomb survivors, Brenner wrote. Other writers, though, have suggested that some of these survivors have outlasted peers who were not exposed. In an interview several years ago, the late Alice Stewart, a prominent radiation researcher, criticized these reports, saying, “Our paper also shows that the A-bomb survivors were not a normal, homogeneous population. They were the best athletes-the top 10 percent-and did not include the young and the old. This means that we cannot base standards of radiation safety on such an elite cohort.”
“There is a problem with the way CT is being practiced,” wrote Dr. Donald Frush of Duke University in the October 2003 Radiology.
“I do not think that we, as radiologists, have reached a sufficient consensus on what the risks and benefits are. How long do we believe the patient is at risk after CT? What constitutes low-level radiation imaging? There are, to date, very few answers.”
A recent study by Dr. Mannudeep Kalra of Massachusetts General Hospital showed that CT doses can, in many cases, be cut in half without sacrificing the diagnostic quality of the images.
“The greater the radiation exposure to the subjects, the more the likelihood of radiation-induced risk, such as cancer. If we can reduce radiation dose in even a quarter of these exams, we will benefit society immensely,” Kalra said.
The FDA also recommends reducing the number of multiple scans with contrast material and eliminating inappropriate referrals for CT.
“There is no mystery about how to reduce doses, technically. What is lacking is leadership,” Gofman wrote in a patient guide to X-rays. “If just a few thousand American radiologists would openly endorse the goal, their leadership.could reduce the average dose-level administered by other radiologists overnight.”
In The Journal of Korean Medical Science, Kalra wrote that “the aim of CT scanning is to obtain diagnostic-quality images with the lowest possible radiation exposure and not ‘pretty pictures’ at the cost of greater radiation than actually needed for the study.”
But in a telephone interview, a radiation safety officer at a university hospital stated radiologists want “nice pictures.” Just as one can take a photo from twenty or more feet away and recognize the person’s face, he said, one can take it from a foot or two away and get a more pleasing image. A radiologist can take a picture with less radiation and visualize a tumor, for example, but it is not as pleasing as the shot of it that requires more radiation. When asked if he thought this might be ethically questionable, he replied, “It may be, but who’s going to do anything about it?”
Minneapolis scientist J. Thomas Payne said we are at another crossroads.
“Radiologists, referring physicians, medical physicists, CT technologists, CT equipment manufacturers and regulators collectively need to evaluate the appropriateness of the radiation dose for different CT studies and get the word out to all facilities,” he wrote in Radiologic Clinics of North America.
What, then, are patients to conclude from all of this ignorance, apathy and controversy? On a practical level, they could do worse than to take Dr. Semelka’s advice regarding CT scans: “Think twice about getting two. Think three times about getting three.”