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UNITS OF RADIATION DOSE — The amount of radiation (ie, radiation dose) absorbed by the patient's tissues is highly predictive of its biological effects. Such doses are defined as the amount of energy of ionizing radiation deposited per unit of tissue mass at a specific point .
It is important to distinguish ionizing radiation (eg, x-rays, gamma rays, proton beams used for radiation therapy) from non-ionizing radiation (eg, microwaves, radio waves, infrared light). Nonionizing radiation generally causes damage through direct or indirect transfer of thermal (heat) energy; sunburn and microwave heating are classic examples of such exposures. On the other hand, ionizing radiation acts at the cellular level and has the potential to cause structural and chemical damage to vital targets such as nucleic acids and proteins.
The terms most often used for quantifying radioactivity, radiation dose, and radiation injury are described in the table (table 1). Those that will be employed in this review are defined below.
Absorbed Dose — The rad (radiation absorbed dose) is the traditional unit of absorbed dose, and is defined as the transfer of 100 ergs per gram of tissue. The rad has been superseded in the SI (Système International) by the Gray (Gy). One Gy, the unit most commonly used to measure radiation therapy dose, is equivalent to 100 rad (1 joule/kilogram), while one cGy is equivalent to 1 rad or 1000 mrad.
Dose Equivalent — The rem (Roentgen equivalent in man) is a unit for the dose equivalent and represents the product of the absorbed dose (in rads) and weighting factors that take into account the differential sensitivity among tissues as well as the biological effectiveness ("quality factor") of various sources of ionizing radiation . The rem has been superseded in the SI by the Sievert (Sv). One Sv is equivalent to 100 rem.
For most therapeutic radiation exposures (eg, x-rays, gamma rays) the Sievert and Gray are approximately equal. However, when there is exposure to highly ionizing particles (eg, neutrons, alpha particles) the radiation dose equivalent reflects resulting tissue damage better than the absorbed dose. As an example, the quality factor for x-rays, gamma rays, and beta particles is 1, while that for alpha particles is 20, and can range from 4 to 22 for neutrons, depending on neutron energy.
Dose rate — The "dose rate" refers to the amount of radiation delivered per unit of time and is most often measured in rads/hour or Gy/hour. Geiger counters typically provide an estimate of dose rate that may, in turn, be used to estimate the degree of hazard in a particular environment (eg, accident scene, patient clothing or bodily wastes). Common, hand-held Geiger counters are suitable for monitoring radioactive sources emitting gamma rays, but more specialized equipment is required for sources emitting certain low energy beta particles, neutrons, and alpha particles.
For cases involving exposure to x-rays, gamma rays and beta particles, a reduction in the radiation dose rate results in a decreased radiation response. For example, it is believed that the carcinogenic effect of these radiations delivered at a lower dose rate is less than that of the same total dose delivered at a higher dose rate. Similarly, a dose of 1 Gy delivered over 1 minute might cause signs and symptoms of acute radiation injury, whereas the same dose delivered over 100 days would not .
Examples of possible radiation exposures — The average annual dose to persons residing in the United States is approximately 3.6 mSv (360 mrem) . The majority of this dose (55 percent) is due to exposure to radon daughter products from the earth and construction materials, with man-made sources of radiation (eg, medical imaging studies), cosmic radiation, and natural radiation from endogenous sources (eg, the naturally occurring radioactive isotope of potassium, potassium-40) contributing the majority of the rest.
Examples of the ranges of exposures that might be seen following medical imaging procedures include the following :
- A standard chest x-ray delivers a dose of 6 to 11 mrem (0.06 to 0.11 mSv, 0.06 to 0.11 mGy).
- Interventional cardiologists working in a high-volume catheterization laboratory may have collar badge exposures exceeding 600 mrem (6 mSV) per year .
- A barium enema with 10 spot images delivers a dose of approximately 0.7 rem (700 mrem, 7 mSv, 7 mGy). Similar doses (7 to 8 mSv) are delivered from a CT scan of the chest or a PET scan, while a combined PET/CT scan is estimated to deliver a dose of 25 mSv .
The biologic effect of radiation doses higher than those achieved after routine medical imaging procedures are outlined below:
- The lowest radiation dose resulting in an observable effect on bone marrow depression in man, with a resultant decrease in blood cell counts, is in the range of 10 to 50 rem (100 to 500 mSv, 0.1 to 0.5 Gy).
- The lowest total body dose at which the first deaths may be seen following exposure to ionizing radiation is in the range of 1.0 to 2.0 Gy. Depending upon the type of support given, 50 percent of people exposed to a dose of 3 to 4 Gy will be expected to die of radiation-induced injury.
- There is virtually no chance of survival following a total body exposure in excess of 10 to 12 Gy.
The risk of radiation-induced carcinogenesis and other adverse health effects from medical imaging is controversial and is discussed separately.
Terms employed for measurement of ionizing radiation
|The amount of material whose atoms are undergoing a certain number of radiaoactive decays per unit time. One Curie, which was originally set as the number of atomic disintegrations per second in 1 gram of radium, is equal to the amount of radiactive material undergoing 3.7 x 10(10) disintegrations per second. This unit has been superceded by the Becquerel (Bq). One Becquerel is equal to the amount of material which is undergoing 1 disintegration/second.|
|The amount of x-ray or gamma ray radiation which produces a certain number ofionizations (ion pairs) in a fixed quantity of air. One Roentgen is the quantity of x- or gamma-radiation which produces, in 0.001293 grams of air, ions carrying one electrostatic unit of electrical charge.|
|Roentgen equivalent in man (rem)|
|One rem is the amount of ionizing radiation of any type which produces in man the same biologic effect as one Roentgen of x- or gamma-rays. It is equal to the absorbed dose, measured in rads, multiplied by the relative biologic effectiveness (rbe) of the radiation in question. As examples, the rbe of x- and gamma rays is 1; the rbe for alpha particles and neutrons is 20. This unit has been superceded by the Sievert (Sv). One Sievert is equal to 100 rem.|
|Radiation absorbed dose (rad)|
|One rad is equal to the absorption of 100 ergs of ionizing radiation per gram of tissue. This unit has been superceded by the Grey (gy). One Grey is equal to 100 rad.|
Lethal dose of radiation — Estimation of the dose associated with death in 50 percent of those similarly exposed (ie, the LD 50) have been made in various scenarios. As an example, virtually all survivors of the explosion of a nuclear device at Hiroshima had estimated exposures of less than 3 Gy . Depending on the incident, estimates for the LD 50 have ranged from 1.4 Gy among atomic bomb survivors in Japan to 4.5 Gy following uniform total-body exposure to external photons . Several factors determine the lethality of ionizing radiation. These include:
- Dose rate — doses received over a shorter period of time cause more damage.
- Distance from the source — For point sources of radiation, the dose rate decreases as the square of the distance from the source (inverse square law).
- Shielding — Shielding can reduce exposure, depending upon the type of radiation and the material used . As examples, alpha particles can be stopped by a sheet of paper or a layer of skin, beta particles by a layer of clothing or less than one inch of a substance such as plastic, and gamma rays by inches to feet of concrete or less than one inch of lead.
- Available medical therapy — The availability of supportive therapy (eg, antibiotics, transfusion, use of cytokines, hematopoietic cell transplantation) is critical for those exposed to moderately high doses of radiation. These subjects are discussed separately.
Based upon an analysis of all available data, the LD 50 at 60 days (LD 50/60) for humans has been estimated to be approximately 3.5 to 4.0 Gy in persons managed without supportive care, 4.5 to 7 Gy when antibiotics and transfusion support are provided, and potentially as high as 7 to 9 Gy in patients with rapid access to intensive care units, reverse isolation, and hematopoietic cell transplantation