What unit of measure is used to express ionizing radiation dose to biologic material

Absorbed dose quantities and units for quantifying biologic risks

Absorbed dose refers to the energy that is deposited locally in an absorbing medium from ionizing radiation. For the energies typically used in diagnostic X-ray procedures, absorbed dose and kerma are equivalent. By contrast, for high-energy photon interactions, the more highly charged particles may deposit energy at sites distant from the initial interaction site, with a corresponding loss of electronic equilibrium. In this case, absorbed dose and kerma are not equivalent. The unit of absorbed dose is the gray, which is the same unit as for kerma, and 1 Gy of absorbed dose is equivalent to 1 J of energy absorbed by the medium per kilogram of absorbing medium. One gray of absorbed dose is equivalent to 100 rad, the traditional unit of absorbed dose. Doses, including skin dose, include the contributions from scattered radiation in addition to the primary radiation. Organ doses consist of the total energy absorbed by an organ per unit mass of the organ.1

Practitioners are often concerned about the absorbed dose to the eye, inducing cataracts.13,17,18 This biologic effect appears to have a threshold in that about 6 Gy of diagnostic X-irradiation over several weeks is necessary to produce cataracts in humans. It may be that absorbed doses of about 15 Gy are necessary to induce cataracts in the diagnostic fluoroscopy setting (Tables 21.2-21.4; Fig. 21.3)

Organ doses consist of the total energy absorbed by an organ per unit mass of the organ.

Absorbed Dose

Absorbed dose is usually defined as the average absorbed dose over an organ or tissue. Of course, this represents a simplification of the actual situation. Normally when an organ or individual is irradiated, the dose is not uniform throughout the volume of the organ but is rather inhomogeneous. A simplification used is the assumption that the detriment will be the same whether the organ is uniformly or nonuniformly irradiated. In extreme circumstances, this is obviously not the case; however, if the nonuniformity is less than 50% or so across the organ or individual of interest, the mean organ dose probably can be used effectively.

Absorbed Dose

The absorbed dose is the radiation energy absorbed per unit mass of an organ or tissue. The absorbed dose describes the intensity of the energy deposited in any small amount of tissue located anywhere in the body, and is used to assess the potential for damage to a particular organ or tissue. The unit is joule per kilogram (J/kg), which is assigned the special name “Gray” (Gy), thus replacing the units of “rad” (short for “radiation absorbed dose”). One hundred rad equals 1 joule/kilogram (J/kg), which also equals 1 Gy.

The absorbed dose is a measure of the actual energy deposited in the irradiated tissue and can be used to compare against deterministic effects, but it is not satisfactory for comparing the stochastic radiological risk. To estimate the stochastic risk, the type of radiation and the sensitivity of the irradiated tissues must be considered. The dose quantities equivalent dose and effective dose were devised to calculate the biological effect of an absorbed dose.

Dose Tables

Absorbed dose, measured in grays (Gy), quantifies the energy deposited per unit mass. The energy deposition of

1 J/kg of tissue is the equivalent of 1 Gy. Because not all types of radiation produce the same biological effect, the dose equivalent is often used instead of the absorbed dose. The dose equivalent is the product of the absorbed dose and a radiation weighting factor and is expressed in sieverts (Sv). Because the radiation weighting factor for X-rays and gamma rays is 1.0, 1 Gy is equivalent to 1 Sv in medical imaging. Effective dose is the sum of the products of the equivalent dose for each organ or tissue. Effective dose is expressed in Sv and is a single-dose parameter that reflects the risk of nonuniform exposure in terms of whole body exposure. Effective dose is age and sex averaged. Radiation doses in medical imaging are typically expressed as millisieverts (mSv). For reference, the average yearly background radiation dose (primarily from radon gas in the home) is around 3 mSv. In the United States, the yearly allowable radiation dose for X-ray technologists or physicians is 50 mSv. Note that radiation exposure doses are frequently quoted in an older unit of “rem” (roentgen equivalent in man). The “rem” unit is 10 times smaller than “mSv,” so that the yearly radiation exposure in radiation workers may be stated as 5 rem. The adult effective dose of various procedures is summarized in Table 41-2.

TABLE 41-2. Adult effective dose of various medical radiation procedures

The National Council on Radiation Protection and Measurements (NCRP) is an advisory body to the U.S. government that routinely publishes reports on various topics related to radiation measurements and protection. NCRP Report No. 160, Ionizing Radiation Exposure of the Population of the United States, was published in March 2009. This report describes the relative dose contributions to individuals and the population from a variety of sources. According to the report, the largest increase in radiation exposure to the U.S. population, compared with the previous report in the 1980s, has been found to come from patient exposure to medical procedures. Since the early 1980s, the magnitude and distribution of radiation exposure to the U.S. population have changed, primarily because of increased utilization of ionizing radiation in diagnostic, nuclear, and interventional medical procedures. Reported radiation exposure from medical procedures was 0.54 mSv in the NCRP Report No. 93 (1987); it was 3.0 mSv in the 2009 report. The 2009 report examined the medical exposure of patients in four main categories of procedures that involve ionizing radiation: (1) CT imaging, (2) nuclear medicine, (3) conventional radiography and fluoroscopy, and (4) interventional fluoroscopy. Among the four medical procedures, the largest contributor is CT. The number of CT examinations performed in the United States has been growing at a rapid rate, with nearly 67 million scans performed in 2006 alone. Overall, the collective effective dose from CT translates to an effective dose per capita of nearly 1.5 mSv. The contribution from CT alone is nearly three times that of the collective dose for the entire medical exposure category in the previous NCRP report. The second largest contributor is nuclear medicine, with nuclear cardiac procedures accounting for 85% of the effective dose in this category. The effective dose per capita from nuclear medicine is 0.8 mSv. The effective dose per capita from all categories of medical exposure of patients is provided in Table 41-3.

TABLE 41-3. Estimated number and effective dose from various medical imaging procedures

Absorbed Dose

The absorbed dose is the quantity of radiation energy absorbed per unit mass of tissue (i.e., concentration of radiation energy actually absorbed at a specific point in the tissue). It is directly related to the biologic tissues and is expressed in gray (Gy). Computed tomography dose index (CTDI) is the absorbed dose in a phantom model, determined at the center of the slice. It is a good indicator of tissue dose. Dose length product (DLP) is the total radiation to the patient during that CT procedure. DLP is a product of CTDI and length of the body area scanned. It is expressed in Gy-cm.

Nuclear and Radiological Disaster Definitions

Absorbed dose

The amount of energy deposited by ionizing radiation in a mass of tissue expressed in units of joule per kilogram (J/kg) and called “gray” (Gy).

Exposure to radiation that occurs in a matter of minutes.

A positively charged particle consisting of two neutrons and two protons. It is the least penetrating but most ionizing of the three common forms of radiation. Alpha radiation can be stopped by a sheet of paper but can cause significant long-term damage if inhaled or ingested. It carries more energy than beta or gamma radiation.

A serious, often fatal illness resulting from exposure to a high dose of penetrating radiation to the body in a short time (usually minutes). Also called “radiation sickness.”

Radiation from the natural environment originating primarily from the natural elements in rock or soil and from the cosmic rays.

The amount of a radioactive material that undergoes one decay (disintegration) per second.

An electron of either positive or negative charge that has been ejected by an atom in the process of a transformation. Beta particles are more penetrating than alpha radiation but less than gamma. They can cause serious skin burns with high exposures.

The total dose that accumulates from repeated or continuous exposures of the same part of the body, or of the whole body, to ionizing radiation.

The traditional measure of radiation based on the observed decay rate of 1 g of radium.

A complex syndrome resulting from excessive ionizing radiation exposure to the skin. The immediate effects can be reddening and painful swelling of the exposed area. Large doses can result in permanent hair loss, scarring, altered skin color, deterioration of the affected body part, and death of the affected tissue (requiring surgery).

The change of one radioactive nuclide into a different nuclide through the spontaneous emission of alpha, beta, or gamma rays or by electron capture. The end product is a less-energetic, more-stable nucleus.

The removal of radioactive contaminants by cleaning and washing.

A device designed to spread radioactive material by the explosion of a conventional device. A dirty bomb is relatively simple to make and kills or injures people through the initial blast of the conventional explosive while also spreading radioactive contamination. Also referred to as a radiological dispersion device (RDD).

An atom or element able to achieve nuclear fission.

The splitting of a heavy nucleus into two roughly equal parts accompanied by the release of a relatively large amount of energy in the form of neutrons and gamma rays. The three primary fissile materials are uranium-233, uranium-235, and plutonium-239.

A highly penetrating type of nuclear radiation, similar to X-radiation except it comes from the nucleus of an atom. Gamma rays penetrate tissue farther than beta or alpha particles but leave a low concentration of ions in their path to damage cells.

A Geiger–Müller detector and radiation measuring instrument containing a gas-filled tube that discharges electrically when ionizing radiation passes through it and a device that records the events. They are the most commonly used as portable radiation detection instruments.

A unit of measurement for the absorbed dose of radiation. The unit Gy can be used for any type of radiation, but it does not describe the biological effects of the different types of radiation.

A nuclear weapon built by a rogue nation or terrorist organization with illegally acquired fissile materials; A nuclear weapon bought, stolen, or provided by a nation with a nuclear program to a rogue nation or terrorist organization.

An atom with more (or fewer) electrons than protons, resulting in an electrical charge that makes it chemically reactive.

The process of adding one or more electrons to, or removing them from, atoms or molecules, thereby creating ions.

Radiation capable of displacing electrons from atoms, thereby producing ions. High doses of ionizing radiation can produce severe skin or tissue damage.

An element with the same atomic number but different atomic weights (different number of neutrons in their nuclei). Uranium-238 and uranium-235 are isotopes of uranium.

The time between exposure to a dangerous material and the appearance of a consequential health effect.

Radiation with lower energy levels and longer wavelengths than ionizing radiation. This type of radiation poses danger through producing heat in tissue but does not affect the structure of atoms. (eg, radio waves, microwaves, visible light, and infrared from a heat lamp).

Heat energy produced by nuclear fission inside a nuclear reactor or by radioactive decay.

A guide informing responders and authorities at what projected dose they should take action to protect people from accidental or intentional radiation releases in the environment.

Energy moving in the form of particles or waves. Familiar radiations include radio waves, microwaves, heat, and light. Ionizing radiation is a high-energy form of electromagnetic radiation.

The basic unit of absorbed radiation dose. It is a measure of the amount of energy absorbed by the body. The rad is the traditional unit of absorbed dose. However, it has been replaced by the unit Gy, which is equivalent to 100 rad.

The spontaneous disintegration of an atom’s nucleus.

The process of spontaneous transformation of the nucleus, generally with the emission of alpha or beta particles, often accompanied by gamma rays. This process is referred to as decay or as the disintegration of an atom.

A device designed to spread radioactive material by the explosion of a conventional device. It is relatively simple to make and kills or injures people through the initial blast of the conventional explosive while also spreading radioactive contamination. Also refers to a dirty bomb.

A unit of radiation exposure defined as the amount of X- or gamma-radiation, which produces 1 electrostatic unit of charge in 1 cubic centimeter of dry air under normal conditions.

A unit of equivalent dose. Not all radiations have the same biological effect, even for the same amount of absorbed dose. Rem relates the absorbed dose in human tissue to the effective biological damage of the radiation. Although it is the traditional unit of equivalent dose, it has been replaced by the sievert (Sv), which is equal to 100 rem.

Material used as a barrier between a radiation source and a potentially exposed person to reduce exposure.

The international standard unit for the amount of ionizing radiation required to produce the same biological effect as 1 rad of high-penetration X-rays, equivalent to a gray for X-rays. (100 rem or 8.38 R). This relates to the absorbed dose in human tissue, which varies by the type of radiation. Doses are usually expressed in millionths of a sievert or microsieverts.

Birth defects resulting from chemical or radiation exposures of a fetus.

Total Dose and Dose Rate

The term absorbed dose (total ionizing dose) describes the amount of radiation absorbed by an object or person. The units for absorbed dose are gray (Gy) and rad. Absorbed dose is a function of the mass and density of the media. Sometimes absorbed dose is called kerma (kinetic energy released in matter). Because exposure and dose are often used interchangeably, dose is often confused with exposure level. The absorbed dose depends not only on the radiation incident, but also on the absorbing material; a soft x-ray beam may deposit a dose four times greater in bone than in air, and none at all in a vacuum.

If the dose is delivered quickly in a single exposure at a high dose rate, there is almost no time to repair radiation damage, and the damage/unit of dose absorbed is high. Alternatively, if the total dose is delivered slowly, at a lower dose rate, or in divided dose fractions, over the life span of the subject, there is more time to repair radiation damage and the damage/unit of dose is much lower. For example, if multiple fractionated doses are delivered per hour, the damage/unit of total dose decreases significantly. In this situation, the radiation therapist can deliver higher doses to the area, but less total rem/h from the source or x-radiation machine.

14.10.2.2 Dose and Dose Rate

The absorbed doseD, measured in gray (Gy), is the amount of energy deposited per unit mass of material, for example, living tissue. One gray equals 1 J of radiation energy absorbed per kilogram of tissue (1 J kg−1 = 6.25 × 1018 eV kg−1) and the centigray (0.01 Gy) equals a rad. However, the types of radiation are diverse in how they deposit energy; therefore the absorbed dose is a poor descriptor of biological effects (Durante and Cucinotta 2008). A dose of energetic particles normally causes more damage than the same dose of energetic photons (X- or γ-rays). Subsequently, if the same biological event is induced by a dose of a standard radiation, for example, X-rays, and by a dose of a test radiation, for example, HZE ions, then the ratio of the standard dose to that of a test radiation is defined as the relative biological effectiveness, (RBE) of the test radiation. The RBE depends on several parameters, including LET, particles velocity and charge, dose and dose rate, biological end point, and oxygen concentration. To estimate biological effects it is customary to scale the absorbed dose by a quality factor Q(LET), which is estimated from the measured RBE values for late effects (Table 2). Current values for Q range from 1 at low LET (<10 keV μm−1) to 30 at high LET (around 100 keV μm−1) and then decrease at very high LET values because of what is called overkill or wasted energy (Durante and Cucinotta 2008). The dose equivalent or biologically dose equivalent, H = D × Q, represents the absorbed dose adjusted for the biological effectiveness of a particular type of radiation. H is measured in sievert (Sv) and the centisievert (0.01 Sv) equals a rem (rad equivalent in humans). Thus, the dose equivalent is intended to encompass all aspects of a certain radiation exposure influencing a biological effect. Finally, another important factor needs to be introduced to understand the influence of dose rate on the biological effect. The dose rate effectiveness factor (DREF) measures the difference between acute exposures (a single large exposure) and chronic exposures (an exposure fractionated over time) of the same type of radiation at the same dose. Similar to RBE, the DREF is expressed as a ratio and thereby is an important scaling factor for the physician whereby meaningful comparisons can be made between acute exposure events for which there is historical evidence linking outcome and dose, for example, atomic bomb survivor, and exposures involving low dose rates, for example, long duration human spaceflight (Jones and Karouia 2008).

Table 2. Quality factors associated with various types of radiation

Modified from Jones, J.; Karouia, F. In Principles of Clinical Medicine for Space Flight, 1st ed.; Barratt, M.; Pool, S., Eds.; Springer, 2008; pp 475–519.

RADIATION PARAMETERS

Principles of radiobiology

Radiation therapy employs ionizing radiations derived from the acceleration of electrons or other charged particles such as protons. In a linear accelerator, electrons are accelerated to high energy to create photons and are allowed to exit the machine as an electron beam. This beam of radiation with a source outside the patient is called external beam. Photons represent the most commonly type of energy used in linear accelerators in radiotherapy departments. Typical energies of the photons ranges between 4 MV and 25 MV (Joiner and Van Kogel, 2009).