image image

Risk/Benefit of Medical Radiation Exposures

Kelly Classic, Certified Medical Physicist

The question of risk from radiation exposure is a much-debated topic of discussion. The predominant risk from typical medical radiation exposure is the chance of getting cancer. The conservative approach is that there is a linear no-threshold relationship between exposure and risk—meaning that there is no amount of radiation exposure that is considered absent of risk and the greater the exposure, the greater the increase in risk. It is important to note that radiation exposure does not create a unique cancer risk situation, nor is the risk directly measurable or distinguishable from the risk caused by other sources (environmental, chemical, biological, etc.). According to the American Cancer Society, the natural average fatal cancer rate in the United States is around 25 percent. Diagnostic medical radiation exposures may increase this risk slightly, on the order of zero to a percent.

Many agents of benefit to humanity have associated risks. For instance, aspirin is extremely effective in many indications but, taken in large quantities, can be very harmful. Likewise with radiation, the risk of harm from radiation is largely dependent upon factors such as the size of the dose, the rate at which it was delivered, the type of radiation, the part of the body exposed, and the age and health of the exposed individual. As discussed earlier, small doses of radiation may pose a risk of increased cancer. This increase is generally regarded as small when compared to the natural incidence of cancer and other everyday risks such as flying in an airplane and driving a car. Additionally, when compared to other lifestyle factors such as smoking, alcohol consumption, diet, and sun exposure; environmental and occupational factors such as asbestos and pesticides; and infectious agents such as viruses and bacteria, the risk associated with routine medical radiation exposure is insignificant.

People typically misperceive radiation risks due to the fact that the topic of radiation risk is rather technical in nature and requires a specialized language (with specific units, terms, etc.) and a knowledge base requiring some specific education. Many members of the general public have perspectives that are largely subjective rather than technical or scientific because of the media, which tends to sensationalize issues and fictional portrayals of exposure like the "Hulk" and "Spiderman" in which the exposure resulted in "supernatural" qualities. In most of these instances, radiation dose and risk information are inaccurately conveyed and tend to perpetuate misperceptions. These misperceptions create a fear of radiation. It is assumed that all radiation exposure is equally harmful or risky.

In any situation, the radiation risk must be clearly identified and explained. It is especially important that the risks, real or calculated, are not only explained, but also understood by the individual so that he/she can make an informed decision regarding whether or not to accept the benefit in light of any risks.

Radiation effects are broadly categorized as either stochastic (more radiation dose increases the probability of a biological effect) or nonstochastic (probability does not vary with dose; instead there is a threshold dose below which the effect will not occur). Because stochastic effects do not exhibit a threshold, there is a probability of stochastic effects occurring even at low radiation doses. The stochastic effect of importance to the individual exposed to radiation is cancer induction. Nonstochastic effects (such as cell death) exhibit threshold doses typically at least 10- to 100-fold higher than doses encountered in medical procedures. Thus, there is little or no possibility of an adverse nonstochastic effect from exposures in medical procedures. The doses from an x ray, or even multiple x rays, are far too low. The concern that remains, then, is that of cancer induction.

The tissues of the body that are actually irradiated, and the amount of radiation they receive (and therefore the risk of a radiation effect), depends on the type of exam and the parameters used to take the picture (for an x ray, this includes kVp, mAs, distance, etc.; for a nuclear medicine study, this includes the radionuclide, activity administered, etc.). The latent period for solid cancers averages about 20 years, so the risk today from x rays taken within the past year or so is negligible. Unfortunately, scientific data is lacking to determine a precise risk of cancer in the future from radiation exposure today. According to the National Research Council and its report on health effects of exposure to radiation, it takes significant doses (>50 rad) to increase the risk (NRC 1990). This is well above doses that should be received in medical procedures that involve radiation.

It is also important to realize that equating risk for all x-ray procedures is like comparing apples to oranges. The radiation dose received by the head during dental x rays is not the same as x rays to the wrist, and neither is the same as x rays of the abdomen or chest. Risk from radiation dose is typically based on calculations of actual dose detriment to the whole body. This means that if the same dose is administered to an area that is more radiosensitive (such as the reproductive organs) and to a less-sensitive area (such as a hand), the detriment is considered to be greater for the dose to the more radiosensitive area. Determining this detriment is accomplished by multiplying the dose to an area or organ by a "weighting factor" for that area or organ. The more radiosensitive the organ, the higher the weighting factor. The goal is to come up with a sum of dose detriment that would be equivalent to irradiating the whole body with a given amount of radiation.

A problem that arises is that not all cancers are caused by radiation. There are many other chemicals and biological agents present in minute quantities along with radiation received from medical procedures. About one-fourth of all Americans will develop some form of cancer in their lifetime and in most cases it is not possible to determine whether or not a specific item triggered that cancer occurrence.

Most data that exists today regarding the risk of cancer (or other effects) from radiation is based on very high doses of radiation (atomic bomb survivors, radium dial painters, etc.). We can say with some certainty that high doses of radiation do lead to increased risks of cancer. The source of this information is from studies of populations that have received large doses of radiation. The classical example is the victim of the atomic bombings of Hiroshima and Nagasaki, as seen in the link taken from "Cancer risks among the atomic-bomb survivors" at the Radiation Effects Research Foundation website. From this and other numerous studies that have been made and analyzed over the years, estimates of cancer and other effects to various groups of exposed people can be made at various dose levels. Many scientists believe that if this information is carried down to low doses (that is, doses similar to diagnostic x ray or background radiation), there is still a small chance of causing an effect. The risk associated from diagnostic medical procedures is low and is considered justified because of the necessity of the procedure.

When we discuss radiation risks, the effects are probabilistic in nature. We say that the increase in the cancer incidence rate is about 0.05 percent per rem of radiation dose based on projections from high doses. However, it is impossible to say that any single person will get the radiation-induced cancer. Second, it may be impossible to even demonstrate that additional cancers have occurred as the normal incident rate of cancer deaths is about 20 percent plus or minus some natural variation in this rate. That is, for an exposure of 4 rem to a population of 10,000 people, the fatal cancer incidence changes from 2,000 to 2,020 as the risk changes from 20 percent to 20.2 percent. This is an increase of only 1 percent, which is probably within the normal variations of the cancer rate. In fact it has not been shown that there is an increase in cancers below 10 rem (HPS 2004). With regard to genetic abnormalities, in the BEIR V report it is stated that from studies on mice the exposure required to double the mutation rate in humans is about 100 rem. Genetic mutations have never been observed in human populations exposed to ionizing radiation.

Unlike the doses received by the atomic bomb survivors, doses people receive from medical exams involving radiation are low (<10 rem). In addition, it has not been shown that there is an increase in cancer rates at doses below 0.1 Sv (10 rem)(HPS 2004).

A medical procedure involving radiation should be done only when there is a question to be answered—is something broken, why the pounding headaches, could there be cancer? This is what we call justification; that is, there should be appropriate medical reason for the x ray to be performed. The issue of medical radiation exposure is not a matter of safety; it's a matter of benefit compared with risk. For properly performed common medical radiation procedures that are necessary in light of the patient's medical condition, safety is not an issue. The question should be rephrased as, "In regard to the medical condition of this person, are these x rays necessary for proper medical care?" That decision can only be made by someone who is familiar with the medical condition and the care that is necessary to properly manage it.

The x ray should be only of the affected area and using the most dose-effective settings on the equipment. This assures the best picture with the lowest radiation dose to you—what we call optimization. There are several reasons why we don't want needless x rays performed: it is unnecessary radiation exposure (no justification), it increases cost to the patient and the institution, and some procedures have inherent risks other than the radiation exposure (like a catheter being placed in the heart).

The decision to have a medical test that involves radiation performed must be made collectively between the patient and his/her physician. Any diagnostic test should be justified by the risk of not having the test performed; for example, there must be some benefit. This should be the basis for decisions made by physicians. Patients can refuse tests at any time or get a second opinion. You should feel free to express to your physician any concerns you might have, particularly regarding radiation exposure, and to seek assurance from him or her that the procedure is necessary and that the procedure will be performed in such a way as to reasonably minimize your radiation dose—without compromising the diagnostic information that is being sought. You may also ask if there are alternative diagnostic procedures. However, remember to ask about the limitations and potential risks of any alternative procedures as well.

The interested reader is referred to the following sources for additional information on the topic:

References

  • Health Physics Society. Position statement on radiation risk. McLean, VA: HPS; 2004. Available at http://www.hps.org/documents/risk_ps010-1.pdf.
  • Kereiakes JG, Rosenstein M. Handbook of radiation doses in nuclear medicine and diagnostic x-ray. Boca Raton, FL: CRC Press; 212-213; 1980.
  • National Research Council. Report of the Committee on the Biological Effects of Ionizing Radiations: Health effects of exposure to low levels of ionizing radiation. BEIR V. Washington, DC: National Academy Press; 175; 1990.