What Are X-Rays? Why Are They Used?
The use of X-rays allows physicians to look inside the body to diagnosis an injury or illness. When done for appropriate situations, X-rays are safe and beneficial. It is important that X-rays are not misused or overused because over a lifetime, a person may be exposed to a fairly large amount of cumulative radiation, and it is important than the benefit of each X-ray test be considered before it is done.
Radiologic technologists are trained to use the least amount of radiation possible to produce an image that will help with diagnosis. The technologist or the radiologist (the physician who supervises the testing and then interprets the X-ray images) is often able to tell the patient how much radiation is being used.
If you ask and are told a dose of radiation, you may not understand what a dose of 1 millisievert (mSv) might mean. But if this effective dose is converted into the amount of time it would take you to accumulate the same effective dose from background radiation, you could make a comparison. For example, the average background rate of radiation you are exposed to from the environment just by living in the United States is about 3 mSv per year. So a mammogram with a dosage of 1 mSv would translate into the amount of radiation you would get by just living in the U.S. for about four months.
This method of explaining radiation is called Background Equivalent Radiation Time or BERT. The idea is to convert the effective dose from the exposure to the time in days, weeks, months, or years it would take to obtain the same effective dose from background radiation. This method has also been recommended by the United States National Council for Radiation Protection and Measurement (NCRP).
However, radiation doses may accumulate quickly, depending upon the situation. A trauma victim who is critically injured may be exposed to 30 mSv during treatment. To put this in perspective, a Hiroshima survivor may have been exposed to 50-150 mSv of radiation.
Radiation vs. Radioactive X-Rays
It is natural that we might confuse X-rays with radiation from radioactivity. You may think that man-made radiation is more dangerous than an equal amount of natural radiation, but this is not necessarily the case.
Most background radiation comes from radioactivity in a person's body. We are all radioactive. A typical adult has over 9,000 radioactive disintegrations in his or her body each second. That's over one-half million per minute. The resulting radiation strikes billions of our cells each hour. There are two scientific quantities used in the discussion of radiation protection: equivalent dose and effective dose. Neither of these quantities can be directly measured.
Effective dose, E, is defined by the International Commission for Radiological Protection (ICRP) and was adopted by the US National Council for Radiation Protection and Measurement (NCRP). The concept of effective dose is appealing but unattainable. E is intended to equate the relative risk of inducing fatal cancer from a partial body dose (such as radon progeny in the lungs) to the whole body dose that would have the same risk of inducing fatal cancer.
The effective dose cannot be measured, and it is difficult to calculate. Physicists use computer simulation programs to estimate the organ doses in a standard patient from typical exposure conditions for various X-ray examinations. The results of these simulations can be used to estimate E for various patient exposures. Once a table of effective doses is constructed for a particular X-ray unit, it is a simple matter to calculate the BERT-the time to get the same effective dose from background radiation. Typical effective doses and BERT values for some common X-ray projections are listed here.
Typical effective doses and BERT values for some common X-ray studies in an adult (adapted from IPSM Report 53)
|Type of X-ray||Effective Dose (mSv)||BERT (same dose from nature)|
|Dental, intra-oral||0.06||1 week|
|Chest X-ray||0.08||10 days|
|Thoracic spine||1.5||6 months|
|Lumbar spine||3||1 year|
|Upper GI series||4.5||1.5 years|
|Lower GI series||6||2 years|
An effective dose should not be confused with the entrance skin dose (ESD), which was commonly used for describing patient radiation up until about 20 years ago. The ESD is easy to measure, but it is not a good measure for the amount of radiation a patient receives. For example, the ESD for a dental intra-oral X-ray (for example, a bitewing) is about 50 times greater than the ESD for a chest X-ray, yet the effective dose from the dental exposure is usually lower than the dose from a chest X-ray.
Diagnostic X-Rays Do Not Increase the Risk of Cancer
No studies of radiation in humans have demonstrated an increase in cancer at the doses used in diagnostic X-rays.
A-bomb survivors (from Hiroshima and Nagasaki) who had large doses-greater than the equivalent of 150 years of background radiation-had a slight increase in cancer. In the last 50 years, there was an average of fewer than 10 radiation-induced cancer deaths per year in about 100,000 A-bomb survivors. A-bomb survivors who received a dose of less than the equivalent of 60 years of background radiation showed no increase in the incidence of cancer. Survivors in that dose range tended to be healthier than the unexposed Japanese. That is, their death from all causes was lower than for the unexposed Japanese. The improved health of those with low doses more than compensated for the radiation-induced cancer deaths, so that A-bomb survivors as a group are living longer on the average than the unexposed Japanese controls.
Nuclear shipyard workers were much healthier than non-nuclear shipyard workers. Evidence for health benefits from low-dose-rate radiation comes from the nuclear shipyard workers' study (NSWS) over a decade ago. This DOE-sponsored study found that 28,000 nuclear shipyard workers with the highest cumulative doses had significantly less cancer than 32,500 job-matched and age-matched controls. The low death rate from all causes for the nuclear workers was statistically very significant. Nuclear workers had a death rate of 24% (16 standard deviations) lower than the unexposed control group.
People living in areas with high natural background radiation generally have less cancer. Humans receive ionizing radiation from several natural sources: radioactivity inside their body, radioactivity outside their body, and cosmic rays. The amount of radiation from these last two sources varies with the geographical location and the material used in the buildings where you work and live. In addition, the contribution from radon varies depending on the construction of a person's home and the amount of uranium in the soil beneath it. If ionizing radiation is a significant cause of cancer, we would expect the millions of people who live in areas with high natural levels of radiation to have more cancer. However, that is not the case. The seven western U.S. states with the highest background radiation-about twice the average for the country (excluding radon contributions)-have a 15% lower cancer death rate than the average for the country.
Radon in mines increases lung cancer. (Radon is a radioactive gas found naturally in soil.) Uranium miners had a higher incidence of lung cancer from the high concentrations of radon in underground mines. This was the basis for the Environmental Protection Agency (EPA) to estimate that high levels of radon in homes cause thousands of lung cancer deaths each year in the U.S.
Recommendations for X-Rays
Radiographs contribute most of the man-made radiation to the public-on average, approximately 15% of the amount a person gets from nature. The benefits of this radiation are tremendous in diagnosing disease. There are no data to suggest a risk from such low doses.
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Matanoski GM. Health effects of low-level radiation in shipyard workers final report. Baltimore, Md: 1991.
National Council on Radiation Protection and Measurement. Exposure of the US population from diagnostic radiation. 1989. NCRP Report.