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Radiation Protection Terms

Absorbed Dose of Radiation: The rad (see Rad) was a unit of absorbed radiation dose in terms of the energy actually deposited in the tissue. The rad is defined as an absorbed dose of 0.01 joules of energy per kilogram of tissue. The more recent SI unit is the gray, (see Gray) which is defined as 1 joule of deposited energy per kilogram of tissue. To assess the risk of radiation, the absorbed dose is multiplied by the relative biological effectiveness (see Relative Biological Effectiveness) of the radiation to get the biological dose equivalent in rems (old unit) or sieverts (new unit).
Background radiation: The worldwide average annual effective dose from all natural sources, excluding radon, quoted in the UNSCEAR (2000) report is 1.2 mSv with a range of 0.8 to 2.4 mSv. This has been rounded by the Commission to 1 mSv/yr.
Backscatter: there is always a certain scattering of photons. This is comparable to light striking a glass surface; a certain portion of the light is always reflected. When an x-ray beam is incident on a patient, a proportion is scattered back. This is what makes makes incident dose measurements lower than entrance surface dose measurements.
Comforters and carers: these can have a higher dose constraint than the general public of around 20mSv a year. This definition includes the exposures of families and friends of patients discharged from hospital after diagnostic or therapeutic nuclear medicine procedures. Their exposure is different from that for public exposure, since the constraints on their exposures are not restricted by the dose limits. In Publication 73 the Commission specified that dose in the region of a few millisieverts per episode is likely to be reasonable. This constraint is not to be used rigidly. For example, higher doses may well be appropriate for the parents of very sick children, if they are properly informed of the risks.
Controlled areas: Workers in ?controlled areas? of workplaces are not strictly volunteers, but they are well informed and are specially trained, thereby forming a separate group of informed individuals.
Coulomb: One coulomb corresponds to the charge on 6.24 x 10^18 ion pairs. The average energy required to produce 1 ion pair in air is 33.5 eV. So the energy needed to produce this is 209,000,000,000,000,000,000 eV or 53,922,000,000,000,000 Rontgen (rather a lot! see exposure)
Deterministic effects: At high doses, associated mainly with accident situations, tissue reactions (including acute effects, and late effects) such as cataracts of the lens of the eye, necrotic and fibrotic reactions in many tissues and organs, may occur if exposures exceed a threshold dose. This threshold varies with the dose rate. in the radiation dose range of a few mGy up to a few tens mGy, no tissues are judged to show radiosensitivity that is sufficient to allow the dose threshold for clinically relevant functional impairment to be exceeded.
Diagnostic Reference Levels (DRLs): These are used in medical diagnosis to indicate whether, in routine conditions, the levels of patient dose or administered activity from a specified imaging procedure are unusually high for that procedure. If so, a local review should be initiated to determine whether protection has been adequately optimized or whether corrective action is required (Publication 73 ; ICRP, 1996a ). The derived reference level should be expressed as a readily measurable patient-related quantity for the specified procedure.
Diamentor: a particular make of transmission ionisation Chamber that attaches to the x-ray tube light beam diaphragm. All the x-ray beam passes through this Perspex box and the ionization of the air inside is measured (see Dose Area ProductIt is very important to calibrate the chamber and the electronic display together before relying on the indicated reading, as errors can be very large if not. Diamentors should be a minimum of 20cm away from any object being irradiated. These two points should be checked for in any paper reporting Dose Area Product (DAP).
Dose Area Product (DAP): The dose-area product is a measurement of the amount of radiation that the patient absorbs. It is usually measured behind the multi-leaf collimator, that is, on the side of the patient where the radiation enters the body, by attaching a measuring device in front of the X-ray tube and passing a beam through it. The dose-area product is independent of the distance between the X-ray tube and the measuring device because the further away from the X-ray tube this measurement is taken, the more the size of the device increases, and the dose itself decreases. The dose to the patient can be calculated from the dose-area product (if it is correctly calibrated see diamentor). Dose-area product = dose * surface area of the measuring device. The SI unit used to measure the dose-area product is the Gray * centimeter2 (Gy*cm2)
Dose Equivalent (DE): (see equivalent dose) DE = Absorbed Dose x Quality Factor (Q). Units are SIEVERT (Sv) - S.I. Unit (1 Sv = 100 rems). DE=Absorbed dose in x-ray
Dose limits and Dose constraints: These do not apply to patients. This is because by reducing the effectiveness of the patient?s diagnosis or treatment, one may do more harm than good. The emphasis is on the justification of the medical procedures.
Dose: the concept of dose is tricky. The term is used as a measure of the amount of energy at a point in an x-ray beam (see absorbed dose, Energy imparted, system dose, receptor dose etc.) But It is also used as a measure of risk (see Effective Dose) to get a measure of risk from a measure of energy required assumptions based on epidemiological data, and tissue weighting factors (wT) . These are updated by studying the few cases where known levels of radiation have been incident on large populations, e.g. the Japanese survivors of the atom bomb. These assumptions are published by the ICRP. IT IS NOT appropriate to think of measured ?dose? as a measure of risk. (see http://www.gehealthcare.com/inen/rad/xr/education/dose.html#5 as a useful introduction)
Doseage rate: Exposure occurs over time, of course. The more Sieverts absorbed in a unit of time, the more intense the exposure. And so we express actual exposure as an amount over a specific time period, such as 5 millisieverts per year.
Early tissue reactions: (see deterministic effects) these occur in days to weeks after radiation after the threshold dose has been exceeded may be of the inflammatory type resulting from the release of cellular factors or they be reactions resulting from cell loss (Publication 59 ; ICRP, 1991b). Inflammatory type (Erythematous skin reaction) or Cell loss type (Mucositis, epidermal desquamation)
Effective Dose (E): If the absorbed dose in different organs or tissue is not uniform, as is the case with radiography, the concept of effective dose is used. The basic idea is to express the risk from an exposure of a single organ or tissue in terms of the equivalent risk from an exposure of the whole body, in principle as well as in practice effective dose is a non-measurable quantity. The effective dose is calculated from the expression E=wT x HT where wT is the tissue weighting factor (see tissue weighting factor) and HT is the equivalent dose in the tissue. The increased risk of cancer mortality from an effective dose, E, of 0.09 mSv is approximately 4.5 in 1 million over one's lifetime
energy imparted: the mean total energy imparted to the patient, this may be derived by calculation and assumption from Dose Area Product readings.
Entrance surface dose (ESD): measured with an ionization chamber in contact with a tissue-equivalent phantom. Backscattered radiation contributes 27-45% to the measurement and is affected by collimation field size and chamber position.
Equivalent Dose: (see dose equivalent) The equivalent dose was introduced to take into account the dependence of the harmful biological effects on the type of radiation being absorbed. The equivalent dose is therefore a measure of the risk associated with an exposure to ionising radiation. The unit of equivalent dose is the sievert (Sv) and is the absorbed dose x the quality factor (Q). Photons have a quality factor of 1.
Exit dose: The exit dose serves in the evaluation of the X-ray image. It is measured in the radiation field in immediate proximity to the surface of the body where the beams exit from the body. On the basis of the exit dose and the surface dose, we can calculate how much energy imparted to the patient's body (making assumptions about scatter, of course). Radiation in the body = surface dose - exit dose. The SI unit used to measure the exit dose is the Gray (Gy)
exposure-area product (R cm2): an early unit measured by a diamentor (see dose area product)
Exposure: The concept of exposure is now seldom used in radiation protection but is usually reserved for expressing the output of radioactive sources of radiation. Radiographers use the term loosely for initiating the radiation. Defined in 1928 by the ICRP as a quantity that expresses the ability of radiation to ionize air and thereby create electric charges which can be collected and measured. Unit is coulomb per kilogram (c/kg of air). Old unit is the Roentgen (R), 1 R = 2.58 x 10-4 c/kg of air. This is not a unit of Dose. (see Coulomb)
General public and hospital staff: dose constraint of around 1mSv a year
Gray: is the current SI unit of absorbed radiation dose in terms of the energy actually deposited in the tissue. It is defined as 1 joule of deposited energy per kilogram of tissue.
Hands and feet dose limits: workers (500 mSv)
ICRP: International Commission for Radiation Protetion, The Commission?s 1990 system of protection, set out in Publication 60, was the result of developments over some 30 years. and has recently been updated. Most of the definitions here are referenced from this document.
Incident dose: Often confused with Entrance surface dose. Measured using a copper phantom. Calculation of Entrance surface dose can be achieved by using published empirical relationships (Martin, C.J., 1995) This allows measurements of incident dose rate made using copper to be linked to corresponding thicknesses of tissue-equivalent material. Incident dose can be related to dose-area product and entrance surface dose derived using backscatter factors. WITHOUT SUCH CORRECTION incident dose is not appropriate in recommending options to reduce patient dose.
Intensity of Radiation: The roentgen (R) (see rontgen) is an old measure of radiation intensity of xrays. It is formally defined as the radiation intensity required to produce and ionization charge of 0.000258 coulombs per kilogram of air. It was one of the standard units for radiation dosimetry, but it does NOT accurately predict the tissue effects of gamma rays of extremely high energies. The roentgen was mainly used for calibration of xray machines
Inter uterine exposure: In respect of the induction of malformations, the data strengthen the view that there are gestation age-dependent patterns of in utero radiosensitivity with maximum sensitivity being expressed during the period of major organogenesis. On the basis of animal data it is judged that there is a true dose-threshold of around 100 mGy for the induction of malformations; therefore, for practical purposes, the Commission ju dges that risks of malformation after in utero exposure to doses in the range up to a few tens of mGy may be discounted. The review of A-bomb data on the induction of severe mental retardation after irradiation in the most sensitive pre-natal period (8-15 weeks post-conception) now supports a true dose -threshold of at least 300 mGy for this effect and therefore the absence of risk at low doses.
Ionizing Radiation: The practical threshold for radiation risk is that of ionization of tissue. Since the ionization energy of a hydrogen atom is 13.6 eV, the level around 10 eV is an approximate threshold
Justification (general): a specified procedure with a specified objective is defined and justified, e.g. chest radiographs for patients showing relevant symptoms or a group at risk to a condition that can be detected and treated. The aim of this generic justification is to judge whether the radiological procedure will usually improve the diagnosis or treatment or will provide necessary information about the exposed individuals. This should be the decision of national bodies of professionals and should be regularly reviewed.
Justification (individual): An application of the procedure to an individual patient should be justified, i.e. the particular application should be judged to do more good than harm to the individual patient. This should take account of all the available information. This includes the details of the proposed procedure and of alternative procedures, the characteristics of the individual patient, the expected dose to the patient, and the availability of information on previous or expected examinations or treatment. It will usually be possible to speed up the procedure significantly by defining criteria and patient categories in advance.
Late tissue reactions|: (see deterministic effects) these occur in months to years after radiation. They can be of the generic type if they arise as a direct result of damage to that tissue. Generic type (Vascular occlusion leading to tissue necrosis) or Consequential type (Mucosal ulceration leading to intestinal stricture)
Lens of the eye annual dose limits: workers (150 mSv) public (15 mSv)
Linear No Threshold (LNT) Hypothesis: risks due to very low levels of radiation are estimated by extrapolating a line on a graph from the data existing from subjects who had much higher doses. The ICRP support this view in their latest publications the weight of evidence on fundamental cellular processes supports the view that in the low dose range up to a few tens of mSv, it is scientifically reasonable to assume that in general and for practical purposes cancer risk will rise in direct proportion to absorbed dose in organs and tissues. This view accords with that given by UNSCEAR (2000).However this method is being increasingly seen as overly conservative. there is insufficient evidence in using the LNT hypothesis in the projection of the health effects of low-level radiation.ANS Position Statement. 2001 and The Health Physics Society recommends that assessments of radiogenic health risks be limited to dose estimates near and above 0.1 Sv. Below this level, only [actual] dose is credible and statements of associated risk are more speculative than credible.Health Physics Society Position Statement on Risk Assessment. 1995
microgray (µGy): is one millionth of a gray (1/1,000,000)
microSievert (µSv): is one millionth of a Sievert (1/1,000,000)
microSievert (µSv): is one millionth of a Sievert (1/1,000,000)
milligray (mGy): is one thousandth of a gray (1/1,000)
Occupational dose limits: ?A limit on effective dose of 20 mSv per year, averaged over 5 years (100 mSv in 5 years), with the further provision that the effective dose should not exceed 50 mSv in any single year? (paragraph 166, Publication 60).
Optimisation: a process that is an important component of a successful radiological protection programme. In application, it involves evaluating and, where practical to do so, incorporating measures that tend to lower radiation doses to members of the public and to workers. But conceptually it is broader, in that it entails consideration of the avoidance of accidents and other potential exposures. It incorporates a range of qualitative and quantitative approaches. An important role of the concept of optimisation of protection is to foster a ?safety culture ? and thereby to engender a state of thinking in everyone responsible for control of radiation exposures, such that they are continuously asking themselves the question, ?Have I done all that I reasonably can to reduce these doses?? (see ALARA, ALARP)
Pregnant Patients: Prenatal doses from most properly done diagnostic procedures present no measurably increased risk of prenatal death, developmental damage including malformation, or impairment of mental development over the background incidence of these entities. Higher doses such as those involved in therapeutic procedures can result in developmental harm. The pregnant patient has a right to know the magnitude and type of potential radiation effects that might result from in utero exposure. Almost always, if a diagnostic radiology examination is medically indicated, the risk to the mother of not doing the procedure is greater than the risk of potential harm to the fetus.
Pregnant Radiographers: methods of protection at work for women who may be pregnant should provide a level of protection for any conceptus broadly comparable to that provided for members of the general public (1mSv a year dose constraint, not 20mSv a year). This is reasonable since while the mother may have chosen to be a radiation worker, the unborn child has not made such a decision. The restriction of the dose to the conceptus does not mean that it is necessary for pregnant women to avoid work with radiation or radioactive materials completely, or that they must be prevented from entering or working in designated radiation areas. It does, however, imply that the employer should carefully review the exposure conditions of pregnant women.
Public Dose limits: ?The limit should be expressed as an effective dose of 1 mSv in a year. However, in special circumstances a higher value of effective dose could be allowed in a single year, provided that the average over 5 years does not exceed 1 mSv per year? (paragraph 192, Publication 60).
Q: (see Radiation weighting factor (wT), Relative Biological Effectiveness) Q depends on the type of radiation. Q = 1 for gamma, x-ray and beta, Q = 20 for alpha. Q is used to compare the biological damage producing potential of various types of radiation, given equal absorbed doses. The relative biological effectiveness (see RBE) of radiation in producing damage is related to the energy loss of the radiation per unit path length. The term used to express this is linear energy transfer (LET). Generally, the greater the LET in tissue, the more effective the radiation is in producing damage. Units are REMS (old) and SIEVERT (Sv) - S.I. Unit. 1 Sv = 100 rems.
Rad: was a unit of absorbed radiation dose in terms of the energy actually deposited in the tissue. The rad is defined as an absorbed dose of 0.01 joules of energy per kilogram of tissue. It has been replaced by the SI unit Gray (see Gray) where 1 Gy=100 Rads
radiation weighted dose: new name (ICRP 2006) for dose in a tissue or organ. The unit of radiation weighted dose is the joule per kilogram with the special name sievert (Sv). The Commission is considering a new special name for radiation weighted dose so as to avoid the use of the name ?sievert? for both radiation weighted dose and effective dose.
Radiation weighting factor (wR): is a "quality factor" (see Q) which is an assessment of the effectiveness of that particular type and energy of radiation. For alpha particles the relative biological effectiveness (rbe) may be as high as 20, so that one Gray is equivalent to 20 Sieverts. However, for x-rays and gamma rays, the RBE is taken as one so that the Gray and Sievert are equivalent for those radiation sources.
radiation weighting factor (WR): see Relative Biological Effectiveness
Receptor dose: The image Receptor dose is measured at the film cassette, X-ray system's image intensifier assembly or Digital Detector. The image receptor dose is generally smaller than the exit dose, because the radiation weakens before it reaches the image receptor, for example by inverse square law, encountering objects behind the patient's body such as the radiation protection grid, anti-scatter grid or the table. Image receptor dose >= exit dose. The SI unit used to measure the image receptor dose is the Gray (Gy)
Relative Biological Effectiveness (RBE): ( see radiation weighting factor (wT))
Rem: The old special unit of any of the quantities expressed as dose equivalent. The dose equivalent in rems is equal to the absorbed dose in rads multiplied by the quality factor (see Q). The new unit is the Sievert (1 rem=0.01 sievert).
Rontgen: an old measure of radiation intensity of xrays. It is formally defined as the radiation intensity required to produce and ionization charge of 0.000258 coulombs per kilogram of air. It was one of the standard units for radiation dosimetry, but it does NOT accurately predict the tissue effects of gamma rays of extremely high energies.
Sievert: The SI unit of any of the quantities expressed as dose equivalent. The dose equivalent in sieverts is equal to the absorbed dose in grays multiplied by the quality factor (see Q) (1 Sv=100 rems).
Skin dose limits: workers (500 mSv) public (50 mSv)
stochastic effects: Radiological protection in the low dose range is primarily concerned with protection against radiation-induced cancer and hereditary disease. These diseases are are probabilistic in nature and are believed to have their origins in damage in single cells. For protection purposes, it is assumed that these effects increase with increasing radiation dose, with no threshold, and that any increment of exposure above the natural background produces a linear increment of risk (see LNT hypothesis).
System dose: (See Receptor Dose)
Tissue weighting factor (wT): each tissue has an estimated propensity to radiation damage that leads to a specific weighting factor being applied to it. To calculate risk to the person from a radiographic examination, one must arrive at an effective dose by using tissue weighting factors, the same absorbed dose in different parts of the body will lead to different risks. High tissue weighted organs (from high to low) are, breast, red bone marrow, colon, lung, stomach, bladder, gonads, liver, oesophagus & thyroid. The Health Physics Society suggests that in a few cases where the gender or age-related risks of radiation differ significantly, that the Commission include separate weighting factors. Two examples using BEIR VII data are thyroid cancer (an ERR for children >10 years old of 9.5/Sv and an ERR for adults of approximately 1/Sv) and female breast cancer (an ERR for women >35 years old of 13/Sv and an ERR for other men/women of 1-2/Sv).
Training: The practitioner involved in the processes that irradiate patients should always be trained in the principles of radiological protection. This is because the exposures of patients are deliberate. It is not the aim to deliver a dose of radiation, but rather to use the radiation to provide diagnostic information or to conduct interventional radiology. That exposure is not limited by any regulatory process, but is controlled by the practitioner, who therefore should be aware of the risks and benefits of the procedures involved. The need for training is accentuated by several recent cases of radiation injury to patients, the root cause of which appears to be insufficient training.
Volunteers and experimental subjects: medical exposures are incurred by those volunteering for research involving exposures to radiation and insurance companies may require individuals to receive medical exposures. In these cases again, the public constraints are not appropriate
body dose: The body dose is the comprehensive concept for the organ or partial-body dose equivalent and the effective dose. In the practical application of radiation protection, however, local and individual doses are monitored, because body doses cannot be measured directly. The Radiation Protection Regulations therefore use the concept of effective dose, in which all the individual doses to the irradiated organs or parts of the body are multiplied by a factor and then added together. The resulting value may not exceed the dose limit for the effective dose that a patient is allowed to receive. Body dose = sum of all organ doses x tissue weighting factors. (see effective dose). The SI unit used to measure the body dose and the effective dose is the sievert, where 1 sievert = 1 Sv = 1 Joule/kilogram = 1 Gray