Environmental Health PerspectivesVol. 91, pp. 45-48, 1991

Radiation Dosimetryby John Cameron*

This article summarizes the basic facts about the measurement of ionizing radiation, usuallyreferred to as radiation dosimetry. The article defines the common radiation quantities and units;gives typical levels of natural radiation and medical exposures; and describes the most importantbiological effects ofradiation and the methods used to measure radiation. Finally, a proposal is madefor a new radiation risk unit to make radiation risks more understandable to nonspecialists.

Definition of Ionizing RadiationIonizing radiation includes any electromagnetic or

particle radiation with sufficient energy to ionize com-mon molecules. In this article I will consider only theelectromagnetic component, specifically X-rays andgamma rays, encountered in the medical area.

Radiation Quantities and UnitsThe evolution of terminology in radiation dosimetry

finds us in a state of transition. We are going from oldunits, which have been used for many decades, to newunits based on SI (International System) units. Sinceboth sets of units are encountered in the literature, itis necessary to have an understanding ofboth sets. Thebasic radiation quantities are exposure, dose (or ab-sorbed dose), and dose equivalent (and its related quan-tity "effective dose equivalent").

Exposure = Ionization in Air

The old unit to measure exposure is roentgen (R),which is defined in terms of the amount of ionizationproduced in air. The unit for exposure is based oncharge/mass of air (C/kg), where 1 R = 2.58 x 10'C/kg. The new unit for exposure has no name and isgiven as C/kg. Exposure is only measured in air anddoes not apply to ionization by charged particles or byphotons with energies above 3 million electron volts(MeV). The concept of exposure is gradually being re-

placed by "air kerma," which is not yet in common useand will not be defined.

Dose (or Absorbed Dose) = Energy/MassThe old unit for dose or absorbed dose is the rad,

where 1 rad = 100 ergs/g. It was convenient that 1 R

*Department of Medical Physics, University of Wisconsin-Madi-son, Madison, WI 53706.

of exposure would give a dose of about 1 rad in wateror human soft tissue. The new unit of dose is the grey(Gy), where Gy = 1 J/kg, thus 1 Gy = 100 rad. You willsometimes see doses given in centigray (cGy) which, ofcourse, is a way to beat the switch to the SI system andstill think in rad. Dose is based on energy/mass, butthe very small energies encountered in measurementsare made using the ionization of air or other sub-stances. The results are converted to energy/mass bycalculation.

Dose Equivalent Includes Bioeffects ofRadiationThe third important radiation quantity is the dose

equivalent (H). H is not a measured quantity. It is de-fined as the dose times a quality factor (QF). QF takesinto account the relative biological effectiveness (RBE)of the type of radiation being used. For photons andelectrons (beta rays), QF is defined as 1.0. For a denselyionizing particle, such as an alpha particle, QF is 20.The RBE depends on the biological system studied, sothat QF is at best an approximation. In the old units,H = rad x QF rem. In SI units, H = Gy x QF sieverts(Sv). Since 1 Gy = 100 rad, 1 Sv = 100 rem. Note thatsince QF is a numerical factor, the basic units of doseequivalent are the same as for dose, energy/mass. Alsonote that since QF is 1.0 for photons and electrons, thedose equivalent is numerically equal to the dose fornearly all medical radiations.

How Partial Body Doses Are Added:The Effective Dose EquivalentGenerally, the radiation dose to the body is not uni-

form. For example, the dose to the lungs from alphaparticles originating from radon and its daughterproducts ismuch greater than the dose to the rest ofthebody from natural radiation. Also, medical X-rays are

J CAMERON

limited to a small part ofthe body. To take this nonuni-formity into account, we use the concept of "effectivedose equivalent." That is, the effective dose equivalentis the amount of radiation that would result in thesame radiation risk if it had been given to the wholebody.

Natural or Background RadiationVersus Medical ExposuresSince the beginning of the Earth, natural radiation

from cosmic rays and natural radioactivity have beenpresent. This is still the major source ofradiation to thepublic. No measurable harm has been demonstratedbecause ofthis radiation. However, based on biologicaleffects at much higher doses, it is possible to extra-polate to these low doses and predict a certain numberof cancers from this cause. An alternate explanation isdiscussed in the next section.The recent inclusion of the large natural dose to the

lungs from radon and its daughter products has causedthe average annual dose from background to increaseby a factor of about three in recent years. It used to begiven as about 1 mSv; now it is about 3.0 mSv. Theaverage American receives about 0.3 mSv effectivedose equivalent from medical exposures. The doseequivalent for common X-ray studies range from 0.03mSv for dental X-ray to about 7 mSv for a barium ene-ma (lower gastrointestinal) X-ray study. See Table 1 fora new way of looking at these values.

The Hormesis Effect: Is a SmallAmount of Radiation Healthy?Studies in nuclear workers often show that they have

less cancer than other members of the population andeven of other workers with similar jobs. This is usuallyexplained as the healthy worker effect. That is, for rea-sons not understood, radiation work attracts healthyworkers. An alternate explanation which is rarely

Table 1. Typical values of radiation risk.Study BERTaX-RaysDental 1 weekChest 2 weeksSkull 1 monthThoracic spine 4 monthsLumbar spine 1 yearBarium meal 3 yearsBarium enema 6 years

Nuclear medicineVitamin B,2 absorption 2 monthsRed cell volume 4 monthsThyroid scan (99mTc) 8 monthsThyroid scan (131i) 20 yearsKidney scan (99mTc) 1 yearBone scan (9mTc) 2 yearsBrain scan (9mTc) 3 yearsThyroid uptake (1311) 30 yearsa BERT, background equivalent radiation time.

mentioned is the possibility that a small amount ofradiation is good for you. This is referred to as the"hormesis" effect. Since humans and all of our ances-tors evolved in a sea of natural radiation, it is possiblethat mutations have occurred that produce the hor-mesis effect. Animal experiments have demonstratedthe hormesis effect. Rats exposed to increased radia-tion have a longer survival than their controls.

Biological Effects of Radiation:Cancer, Mutations, and Birth DefectsThe biological effects of ionizing radiation were not

recognized until man-made X-rays were produced (1).The primary risks are carcinogenesis, mutagenesis,and teratogenesis; in other words, the possibility of in-ducing cancer, mutations, and birth defects. For diag-nostic uses ofradiation, the risk from carcinogenesis isthe major concern. The probability ofinducing a cancerdepends on the amount of radiation energy absorbedby the body and the tissues that absorb the radiation.The energy absorbed by the tissues is usually of theorder of 5 to 500 mJ. The carcinogenesis risk is greaterfor some tissues. For example, the blood-forming cellsin the bone marrow are most sensitive for the inductionof leukemia. Cancer is a very common disease, affect-ing about 25% of the population during their lifetime.The amount that is induced by ionizing radiation is notmeasurable but is generally believed to be very small.For example, studies on the survivors of the atomicweapons dropped on Hiroshima and Nagasaki foundno increase in cancer in survivors with 0.1 Gy (10 rad)of whole body dose. All of our predictions of radiationrisks at low levels are based on extrapolations of muchlarger doses. It is possible that the effects of radiationgiven at low dose rates are much less than from radia-tion given at the high dose rates used for most radia-tion research.

The Genetic Doubling DoseThe genetic effect of radiation has been known for

about 50 years. Mutations occur for other reasons. Itis estimated that it would require a dose to the gonadsofabout 2 Gy (200 rad) to double the natural occurrenceof mutations. Of course, this risk is limited to indi-viduals who are still capable of producing offspring.The concept of "genetically significant dose" has beendeveloped to take into account that radiation exposureto older men and women is less likely to producemutations.

The Greatest Radiation Risk to theFetus: TeratogenesisA serious radiation risk from diagnostic X-rays in-

volves the possibility ofsevere mental retardation ofanindividual who received a large amount of radiationduring the eighth to fifteenth week of gestation. Thisis the time when important cellular specialization is

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taking place in the brain of the fetus. For example, abarium enema study to a woman at this stage of preg-nancy produces a probability of about 1:200 of severemental retardation in the child. Of course, this effectcan be caused by other physical, chemical, or geneticfactors. Fortunately, this type of radiation exposure isrelatively rare. No radiologist would intentionally do aradiation study of a woman at this stage of pregnancythat involved significant radiation to the fetus. If thewoman did not inform the doctor of her pregnancy, theproblem could occur.

How Is Radiation Measured?Radiation is very easy to measure but difficult to

measure accurately. Fortunately, in radiation protec-tion, an accurate measurement is not needed. However,in the treatment of cancer with radiation the accuracyof delivered dose to the tumor should be better than5%. For radiation workers or patients in diagnosticradiology, dose accuracy is seldom better than 20%,and an accuracy of 50% is generally acceptable. Thedose to patients in diagnostic radiology is seldom mea-sured. When they are measured, a large variation isfound in doses for the same X-ray study.A related problem is that conventional radiation

quantities and units, as discussed earlier, are notadapted for easy communication with the patient. Inthe last section I suggest a new radiation unit that mayhelp solve this problem.

Measuring Radiation by IonizationMethodsThe oldest accurate technique for measuring radia-

tion involves measuring the charge produced by the ra-diation (2). This can be done in two different ways. Ifthe radiation is more or less constant, it is possible tomeasure the ionization current. This is a dose ratemeter. The results will be given in R/hour or a similarunit. If the exposure is short, as in the case of an X-rayexposure, all of the ionization charge is collected andmeasured. This is called an "integrating dosimeter." Asimple dosimeter of this type is a pocket or pen dosim-eter. A capacitor is charged to about 400 volts. As theair in the chamber is ionized by the radiation, the ionsproduced are collected and discharge the capacitor.The charge loss on the capacitor during a given time isa measure of the radiation exposure. Most pen dosim-eters include a simple electroscope to measure the re-maining charge. They include a scale which indicateszero when fully charged. As it discharges, the scaleshows the remaining voltage. The scale is calibrated toread directly in milliroentgen (mR).

Thermoluminescent Dosimetry: Casting aNew Light on Radiation DosimetryThermoluminescent dosimetry (TLD) was invented

in 1954 by Professor Farrington Daniels of the Univer-

sity of Wisconsin-Madison. It was not brought to com-mercial applicability until the early 1960s. I waspleased to play a small role in this process. TLD is ba-sically a simple technique that involves some compli-cated solid-state physics. I will not try to describe thedetails of the physics. TLD is based on the observationthat many insulating crystals, when exposed to ioniz-ing radiation, store some of absorbed energy, which islater released as light when the crystals are heated toa few hundred degrees celsius (well below the level ofincandescence). The amount of light emitted can bemeasured and used to determine the amount of radia-tion that was absorbed. This phenomenon of emittinglight when heated is called thermoluminescense. It isclosely related to phosphorescence where light is emit-ted slowly at room temperature. Heating acceleratesthe emission of the stored energy. TLD crystals canstore the energy for many years or even for centuries.TLD is the most widely used technique to monitor

workers at nuclear power plants. It is gradually replac-ing the older technique of film dosimetry to monitorworkers in hospitals. Film dosimeters have advantageswhich will not be discussed here. TLD is in generalmore reliable and more accurate than film dosimetry.In addition, TLD has a much larger useful range. It canmeasure radiation from background levels to muchgreater than the lethal dose (5 to 10 Gy).

Did a Radiation Exposure Years AgoCause Cancer? Use of PC TablesThere are two basic reasons for measuring radiation.

First, radiation is measured to determine if the radia-tion exposure is in the "safe level" for the situation andif not, to correct the situation. A second reason is to es-tablish evidence for the amount of radiation receivedin case a claim is later made that the individual's can-cer was caused by unnecessary radiation. Since canceris very common (about one out of four Americans willhave cancer sometime during their lives), it is possiblethat some former radiation workers may feel that theircancer was caused by their occupational exposure.There are numerous law cases in the court system deal-ing with such situations. If the employer has good rec-ords establishing a low radiation exposure, the plain-tiff often does not win the case.In evaluating such cases, it is convenient to use PC

tables. PC stands for "probability of causation," whichin turn is an abbreviation of the phrase "the probabili-ty that a known radiation dose delivered at a particu-lar age a known number ofyears ago will induce a can-cer." That is, if a worker received 1 Sv of whole bodyradiation at age 20 and at age 30 developed cancer,what is the probability that this cancer was caused bythe 1 Sv dose equivalent 10 years earlier? PC tables arebased on experimental data from various sources, in-cluding the incidence of cancer in the approximately80,000 survivors of the atomic bombs in Hiroshimaand Nagasaki. There are no data for calculating risksat the low levels usually encountered in medical ex-

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posures. The PC for these doses are extrapolated frommuch higher exposures.

A New Unit of Radiation Risk forPatients and Workers

I wish to propose a new and improved patient radia-tion quantity called radiation risk and to define itsbasic unit to be year. Fractions ofa year would natural-ly be expressed in days, weeks, or months as appro-priate rather than as decimals. This unit is calledbackground equivalent radiation time or BERT. (H. T.Richards, University ofWisconsin-Madison, suggestedthe name for the unit.)Most patients are primarily concerned with the car-

cinogenic effect of X-rays. For medical X-rays this effectis roughly proportional to the energy imparted to thepatient in mJ. Although the energy imparted is impos-sible to measure directly, for medical X-rays a goodestimate of the energy imparted can be obtained fromphysical parameters measured during the exposure(3-6).The quantity radiation risk is related to the carcino-

genic risk from radiation exposure. This risk for a diag-nostic X-ray is typically 10- to 10- per X-ray study.Patients generally have difficulty understanding suchsmall risks. Thus I define radiation risk in terms oftheequivalent risk from annual natural radiation in theUnited States. Let us assume that the probability of

inducing cancer from a given X-ray study is Y, then thepatient's radiation risk would be y/x year. Some typicalvalues of radiation risk are given in Table 1.This unit does not cover the relatively rare case of ir-

radiation to the fetus during the eighth to fifteenthweek. This risk will need a separate unit, perhaps de-fined in terms ofthe normal incidence of severe mentalretardation in an unexposed population.

REFERENCES

1. Hall, E. J. Radiation and Life, 2nd ed. Pergamon Press, NewYork, 1984.

2. Attix, F. H. Introduction to Radiological Physics and RadiationDosimetry. John Wiley and Sons, New York, 1986.

3. Hall, B. F., Harris, R. M., and Spiers, F. W. Patient DosimetryTechniques in Diagnostic Radiology. IPSM Report No. 53. TheInstitute of Physical Sciences in Medicine, York, UK, 1988.

4. Alm-Carlsson, G., Carlsson, C. A., and Persliden, J. Energy im-parted to the patient in diagnostic radiology: calculation of con-version factors for determining the energy imparted from mea-surements of the air collision kerma integrated over beam area.Phys. Med. Biol. 29: 1329-1341 (1984).

5. Shrimpton, P. C., Wall, B. F., Jones, D. G., and Fisher, E. S. Themeasurement of energy imparted to patients during diagnosticX-ray examinations using the Diamentor exposure-area productmeter. Phys. Med. Biol. 29: 1199-1208 (1984).

6. Shrimpton, P. C., and Wall, B. F. Comparison ofmethods for esti-mating the energy imparted to patients during diagnostic radio-logical examinations. Phys. Med. Biol. 28: 1160-1162 (1983).

7. Upton, A. C. Radiation Carcinogenesis. Elsevier, New York,1986.