Applied Radiology

Dr. Siegel is a Professor of Radiology and the Vice Chairman, Department of Diagnostic Radiology, University of Maryland Medical Center, and Chief of Imaging, Veterans Affairs Maryland Healthcare System, Baltimore, MD.He is also a member of the editorial board of this journal.

There is no question that high doses of X-rays are bad for you. The early pioneers in radiology used to check if the X-ray tube was working by putting an arm in front of the tube; if the arm turned red, it proved that the tube was working, and they could expose the patient. Many of those pioneers, including C. Lester Leanard, the first Chairman of Radiology at the Hospital of the University of Pennsylvania, died of sarcomas that originated in their arms. There is a memorial to the early dentists who died of sarcomas that originated in their thumbs-they used to hold the small plates in their patient's mouths before they realized it made more sense to ask the patients to hold their own plates. Many patients treated for an enlarged thymus by radiating their chests (a treatment of questionable efficacy to begin with) developed thyroid cancer before that practice was stopped. Of course, there was Hiroshima, where many individuals, too far from "ground zero" to be killed instantly, survived only to succumb to cancers due to the high X-ray dose created in the atom bomb blast.

From these lessons came the teachings that X-rays are bad, that they are, at best, a necessary evil to be avoided if at all possible. There is what might be called hysteria that has been created among patients and many of their doctors that the CT scan they have been asked to receive may kill them, that their annual screening mammogram will cause cancer rather than detect it early enough to save their lives, and that their child will surely die if they agree to an X-ray study. An article in the February 2001 issue of the American Journal of Roentgenology by Brenner and colleagues ¹ warned that 500 children receiving CT studies that year would ultimately die of cancer. That study precipitated a flurry of editorials and great angst among doctors and patients alike about the dangers of X-rays.

But are X-rays always bad? Is there a level below which X-rays cause no harm? Is it the total dose, accumulated over time, that is harmful, or is it the peak dose that is harmful? (I am reminded of a friend of mine who observed that the average power of a hand grenade, integrated over time, is almost zero…it's the peak power that kills you.) Could it be that very large X-ray doses that are absorbed over a very short time are bad, but that smaller doses absorbed over long times are not so bad (and maybe even good)? If so, what is a small dose and what constitutes a long time?

The underlying assumption in the article by Brenner, ¹ and indeed in most articles and warnings today, is that it is total dose, accumulated over time, that counts. Even more importantly, a second underlying assumption is that the effects of accumulated dose are linear and that there is no threshold-a lot of little doses accumulated over time is as bad as one large dose received all at once. Based on these assumptions, epidemiologists are able to predict the likelihood of fatal cancers as a function of dose. Again, in the article by Brenner et al, ¹ it is stated that analysis of mortality data based on Japanese atomic bomb survivors indicates there is "strong evidence of an increased cancer mortality risk at equivalent doses greater than 100 mSv, good evidence of an increased risk for doses between 50 and 100 mSv, and reasonable evidence for increased risk for doses between 10 and 50 mSv (10 mSv = 1 rem)." It is these calculations that have led to patient and physician concerns and to fear of radiologic studies.

But the evidence of everyday living refutes at least some of these underlying assumptions. ² We are constantly bombarded by X-radiation-from the skies above (cosmic radiation) and the rocks and air (radon, for example) around and below us. Every round-trip airplane flight from the United States to Europe exposes each passenger to an X-ray dose nearly equivalent to obtaining a chest radiograph. If the underlying assumptions of linearity and the lack of a threshold are true, then airline pilots, flight attendants, and business executives are at tremendous risk for cancer. Even worse, people who live at high altitudes--in Santa Fe, La Paz, Quito, Mexico City, or Denver--are, by these assumptions, at extremely high risk for developing cancer. A person living in Denver (the lowest-altitude city in this list) receives approximately 500 mrems per year, mostly from cosmic radiation but also some from radiation emanating from subsurface rocks. In 2 years, that person would receive 1 rem (equal to 10 mSv), which is enough, by the calculations above, to give him "reasonable evidence for an increased risk" of cancer. In 20 years, he would receive enough exposure to make the risk of cancer almost a certainty--even if he never had a diagnostic radiograph or CT study. By the way, the exposure to this X-ray dose includes the fetus in utero. By the time a baby is born, he has already received a considerable X-ray dose. One would think that cancer would be rampant in these "mile-high cities," that the cancer wards would be full, and that people would be dropping like flies. We should, perhaps, put warning labels on the signs at the city gates…maybe something like, "Welcome to Denver, the mile-high city: Living here is hazardous to your health."

But the cancer rate is not significantly higher in Denver than it is in Baltimore or Boston or New York. We have evolved in an environment of constant X-ray exposure. The human body has repair mechanisms that fix a bond in a DNA molecule broken by X-rays and that remove a cell when it is too damaged to be repaired-apoptosis, for example. Large X-ray doses, delivered quickly, overwhelm the repair mechanisms, and radiation sickness or cancer can result. Low doses, delivered slowly, are tolerated. The question is: Where is the threshold between toleration and destruction, between too little to cause harm and too much to overcome? It must be someplace between the dose of daily living and the dose unleashed at Hiroshima.

The amazing thing is that even now, more than 110 years since the discovery of X-rays and their introduction to medical practice, we know so little about the real risks. Rather than promoting fear, we should be promoting research so that we can educate ourselves and our patients about the proper use of, and concerns about, X-ray radiation. Until we gain this knowledge, of course, it does make sense to follow the concept of "as low as reasonably achievable" (ALARA)-to use as little radiation as possible to achieve a diagnosis or treatment--but it does not make sense to deny the benefits of modern diagnostic and therapeutic devices because of unrealistic, and largely unproven, concerns.