John R. Cameron , Ph.D

                                                                                                                                                                                                         "John Cameron"

Explaining radiation dose to a patient using the BERT concept

Answering your patient's question about the amount of radiation would be easy if you knew the effective dose. However, it is unlikely the patient would be satisfied if your answer was "the mammogram will give you an effective dose of about 1 millisievert (mSv)." She probably would understand if you converted the effective dose into the amount of time it would take her to accumulate the same effective dose from background radiation. Since the average background rate in the U.S. is about 3 mSv per year, the answer in this case would be about four months. It is likely that she would understand and be satisfied with your answer.

This method of explaining radiation is called Background Equivalent Radiation Time or BERT.2,3 The idea is to convert the effective dose from the exposure to the time in days, weeks, months or years to obtain the same effective dose from background. This method has also been recommended by the U.S. National Council for Radiation Protection and Measurement (NCRP).4 To calculate BERT, I recommend using the average background in the U.S. including contributions to the lung from radon progeny. This is assumed to be 3 mSv/y (300 mrem/y). The background in different parts of the U.S. varies about ± 50% from this value. This uncertainty is unimportant for explaining radiation to patients. The effective dose from common diagnostic x-ray procedures are typically less than the amount of radiation you receive from nature in two years. (See Table 1.) Giving the answer in terms of background radiation has three advantages:

1. it does not imply any risk - it is just a comparison
2. it emphasizes that radiation is natural
3. the answer is understandable to the patient

It is natural that some patients will confuse x-rays with radiation from radioactivity. They may mistakenly think that man-made radiation is more dangerous than an equal amount of natural radiation. Most patients are unaware that most of their background radiation comes from radioactivity in their own body. Radiologists should explain to them that we are all radioactive. A typical adult has over 9,000 radioactive disintegrations in their body each second - over a half million per minute. The resulting radiation strikes billions of our cells each hour. The idea that radiation to one cell can initiate cancer is illogical - it assumes that the body has no defense or repair mechanisms. The body has several defense mechanisms to protect itself from doses up to about 200 mGy.1

   Most patients never get to see the radiologist. Questions about radiation are often asked of the radiographer. Radiographers are generally not prepared to answer a patient's question about radiation dose. However, if tables of effective dose and BERT are available at each x-ray unit, any radiographer can answer the patient's question about radiation dose. If the patient desires further information the radiographer should recommend a basic book, such as Understanding Radiation.6

Scientific quantities for radiation protection

The effective dose cannot be measured and it is difficult to calculate.9 Physicists use computer simulation programs to estimate the organ doses in a standard patient from typical exposure conditions for various projections. 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. Typical effective doses and BERT values for some common x-ray projections are given in Table I.
 

Entrance skin dose (ESD) is not a good indicator
of the dose to the patient

     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 to the patient. For example, the ESD for a dental intra-oral x-ray (e.g., a bitewing) is about fifty times greater than the ESD for a chest radiograph, yet the effective dose from the dental exposure is usually lower than from a chest radiograph.

Fluoroscopic radiation should be measured
with a dose-area product (DAP) meter

     During fluoroscopy the beam size, the organs exposed and the dose rate change. This makes it impractical to determine the effective dose. However, the fluoroscopic dose is very easy to measure with a transmission ion chamber covering the exit of the collimator. All of the radiation striking the patient must pass through the ion chamber. The ion current collected is a measure of the exposure-area product (EAP). The reading can easily be converted to the dose-area product (DAP). A meter for this purpose has been available for more than 30 years. Fluoroscopic procedures typically give larger doses to the patient than a roentgenogram. The reading from a DAP-meter is approximately proportional to the energy deposited in the patient-the imparted energy. If the kVp and HVL are known the DAP meter reading in Gy m2 can be converted to the imparted energy in joules (J) deposited in the patient.5 DAP meters, or their predecessor, exposure-area product meters, are little known or used in the U.S. In the UK and Germany they are required on all medical fluoroscopes. I think the NCRP should recommend that all medical fluoroscopes should include such an instrument and that fluoroscopes used for interventional radiology must have such a meter.

There is no risk from normal diagnostic x-ray doses

A-bomb survivors are living longer on the average
than unexposed Japanese

   A-bomb survivors who had large doses - greater than the equivalent of 150 years of background - 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 less than the equivalent of 60 years of background 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) a decade ago.10 This DOE sponsored study found that 29,000 nuclear shipyard workers with the highest cumulative doses had slightly less cancer than 33,000 job matched and age matched controls. The decreased cancer among nuclear workers was not statistically significant. However, the low death rate from all causes for the nuclear workers was statistically very significant. Nuclear workers had a death rate 24% (16 standard deviations) lower than the unexposed control group. If the nuclear workers had a death rate 24% higher than the controls, it would have made the world news in 1988.

Areas with high natural background 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 various 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 your 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 15% lower cancer death rate than the average for the country.11
Radon in mines increases lung cancer; radon in
homes reduces lung cancer

     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. However, a study of lung cancer death rates in 1600 U.S. counties representing over 90% of the U.S. population shows that counties with the highest radon levels (> 5 pCi/l) have 40% lower lung cancer death rates than the counties with lowest radon levels (< 0.05 pCi/l).12 It appears that radiation from radon progeny actually prevents some cancers caused by smoking!

Summary and recommendations

     Radiologists contribute most of the man-made radiation to the public. The benefits of this radiation are tremendous. There are no data to suggest a risk from such low doses. Radiologists have a responsibility to help educate their patients and others who ask them about radiation. You have a choice. You can increase the patient's fear of radiation by explaining the "official" policy of the NCRP and the American College of Radiology that even the smallest amount of radiation may cause cancer. Based on this assumption, a recent ACR publication13 shows that the risk of inducing a fatal cancer from a chest x-ray is ten times greater than the risk of dying in a commercial airline flight. The same table shows that a CT scan of the kidneys has a greater risk of inducing a fatal cancer than a cigarette smoker has of dying from lung cancer.

I strongly recommend that each clinical x-ray unit have a table of the effective dose for various projections and patient size. A separate column should give the BERT - the time to obtain the same effective dose from background. The radiographers should be taught how to answer the patient's questions using the BERT method. The BERT concept does not suggest any risk and is understandable to the patient.
 

References

1. Feinendegen LE, Bond VP, Sonhaus CA: Low level radiation may protect against cancer. Physics and Society News (In press) 1998
2. Cameron JR: A radiation unit for the public. Physics and Society News 20:2, 1991.
3. Cameron JR: How to explain x-ray exposure to your patient (30 min. video). Medical Physics Publishing, Madison, WI, 1993.
4. NCRP Report 117: Research needs for radiation protection, p. 51. National Council on Radiation Protection and Measurement, Bethesda, MD, 1993.
5. IPSM Report No. 53: Patient dosimetry techniques in diagnostic radiology, p. 53 and Table A7, p. 117. Institute of Physics and Engineering in Medicine, York, UK, 1988.
6. Wahlstrom B: Understanding radiation. Medical Physics Publishing, Madison, WI. 1996.
7. ICRP Publication 60 Recommendations of the International Commission of Radiological Protection, 1991.
8. NCRP Report 116: Limitation of exposure to ionizing radiation. National Council on Radiation Protection and Measurement, Bethesda, MD, 1993.
9. NCRP Report 100: Exposure of the U.S. population from diagnostic radiation, pp. 73-74. National Council on Radiation Protection and Measurement, Bethesda, MD, 1989.
10. Matanoski GM: Health effects of low-level radiation in shipyard workers final report. Baltimore, MD, DOE DE-AC02-79 EV10095, 1991.
11. Fremlin JH: Power production: What are the risks? 2nd ed. Bristol, UK: Adam Hilger, pg. 58, 1989.
12. Cohen BL: Test of the linear no-threshold theory of radiation carcinogenesis in the low dose, low dose rate region. Health Physics 68:157-217, 1995.
13. ACR Radiation Risk: A Primer. American College of Radiology, Reston, VA, p. 6, 1996.

 

 
 
 

E-mail Address of the Author:
John R. Cameron, Ph.D.