Internal dosimetry

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Internal dosimetry is the science and art of internal ionising radiation dose assessment due to radionuclides incorporated inside the human body. [1]

Ionizing radiation radiation that carries enough energy to liberate electrons from atoms or molecules

Ionizing radiation is radiation that carries enough energy to detach electrons from atoms or molecules, thereby ionizing them. Ionizing radiation is made up of energetic subatomic particles, ions or atoms moving at high speeds, and electromagnetic waves on the high-energy end of the electromagnetic spectrum.

A radionuclide is an atom that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferred to one of its electrons to release it as a conversion electron; or used to create and emit a new particle from the nucleus. During those processes, the radionuclide is said to undergo radioactive decay. These emissions are considered ionizing radiation because they are powerful enough to liberate an electron from another atom. The radioactive decay can produce a stable nuclide or will sometimes produce a new unstable radionuclide which may undergo further decay. Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay. However, for a collection of atoms of a single element the decay rate, and thus the half-life (t1/2) for that collection can be calculated from their measured decay constants. The range of the half-lives of radioactive atoms have no known limits and span a time range of over 55 orders of magnitude.

Contents

Radionuclides deposited within a body will irradiate tissues and organs and give rise to committed dose until they are excreted from the body or the radionuclide is completely decayed.

The committed dose in radiological protection is a measure of the stochastic health risk due to an intake of radioactive material into the human body. Stochastic in this context is defined as the probability of cancer induction and genetic damage, due to low levels of radiation. The SI unit of measure is the sievert.

The internal doses for workers or members of the public exposed to the intake of radioactive particulates can be estimated using bioassay data such as lung and body counter measurements, urine or faecal radioisotope concentration, etc. The International Commission on Radiological Protection (ICRP) biokinetic models are applied to establish a relationship between the individual intake and the bioassay measurements, and then to infer the internal dose.

Bioassay analytical method to determine concentration or potency of a substance by its effect on living cells or tissues

A bioassay is an analytical method to determine concentration or potency of a substance by its effect on living cells or tissues. Bioassays were used to estimate the potency of agents by observing their effects on living animals or tissues.

The International Commission on Radiological Protection (ICRP) is an independent, international, non-governmental organization, with the mission to provide recommendations and guidance on radiological protection concerning ionising radiation.

Committed dose

The internal radiation dose due to ingested or inhaled radioactive substances is known as committed dose.

The ICRP defines Committed effective dose, E(t) as the sum of the products of the committed organ or tissue equivalent doses and the appropriate tissue weighting factors WT, where t is the integration time in years following the intake. The commitment period is taken to be 50 years for adults, and to age 70 years for children. [2]

The ICRP further states "For internal exposure, committed effective doses are generally determined from an assessment of the intakes of radionuclides from bioassay measurements or other quantities (e.g., activity retained in the body or in daily excreta). The radiation dose is determined from the intake using recommended dose coefficients". [3]

Routes of intake

There are a few routes of intake (of radionuclide) namely,

In an radioactive area, radionuclide particulate may be suspended in the air and can enter the body by inhalation. These particulates may be deposited in different parts of the respiratory tract depending upon their aerodynamic diameter. [4]

Monitoring techniques

In-vivo monitoring
Internal dose monitoring of the radionuclides which emit radiation which can penetrate out of the body. For example X-rays, gamma rays of sufficient energy. It can be measured by devices such as a whole body counter.

A whole body counter [5] has a low background arrangement with counting systems

HPGe detectors are replacing detectors for measuring the low energy and high energy photons with appropriate electronic systems.
Calibration of these systems is carried out with different type of physical and mathematical phantoms. Physical phantoms include BOMAB, LLNL, JAERI, thyroid and the knee phantoms. Some of the renowned mathematical phantoms are MIRD, CRISTY and nowadays voxel phantoms also known as Computational human phantoms.

In-vitro monitoring

Monitoring of the radionuclides present in the body using the bio-assay sample taken out of the body; this includes samples of urine, sweat, feces, etc.

Biokinetic modelling

The ICRP models are used to simulate the distribution of the isotopes inside the human being. All current ICRP models, compiled in the ICRP Database of Dose Coefficients (ICRP 2001), [6] can be represented by compartmental systems with constant coefficients. The conceptual model used by ICRP can be summarized as it follows.

The human body can be divided into three systems:

a) The human respiratory tract model (HRTM). This model is applied for modeling the intake of radioactive aerosols by inhalation. The detailed description is given in ICRP 66 (1994). If a person inhales instantaneously a quantity I, it is deposited directly in some compartments of the HRTM. The fraction deposited in each compartment is called Initial Deposition Fraction or IDF. It is a function of Activity Median Aerodynamic Diameter (AMAD), which includes size, shape, density, anatomical and physiological parameters as well as various conditions of exposure. The IDF values may be calculated either following the procedure described in ICRP 66 (1994) or obtaining it from the Annex F of ICRP 66 (1994). The general model of the HRTM is common to any element except the absorption rates {spt, sp, st} which are related to the chemical form of the element. ICRP gives default values of absorption rates according to types F, M or S.

b) The gastrointestinal tract (GI). This is applied for modeling the intake of particles in the GI tract following the model provided in ICRP 30 (ICRP 1979) and ICRP 105(ICRP 2005). Particles can be introduced in the GI Tract directly by ingestion, or from the RT. Deposition is in the stomach (ST). Part or all the flow is transferred, through SI, to the blood (B). The rate transfer from SI to B, is given by λB = f1 λSI/(1 – f1), where f1 is the fraction of the stable element reaching the blood (or body fluids). If f1 = 1 all flows from the stomach it goes to B. The value of f1 is associated to the element and their chemical form The GI tract model will be replaced by the called Human Alimentary Tract Model (HATM), but it is not published yet.

c) Systemic compartments. They are specific to an element or groups of elements (ICRP 2001). ICRP 78 (1997) establishes three generic groups: (i) hydrogen, cobalt, ruthenium, caesium, and californium, (ii) strontium, radium, and uranium and, (iii) thorium, neptunium, plutonium, americium, and curium. For other elements not included in ICRP78, the ICRP 30 model is applicable and they have the same generalized compartmental model as group (i). For the elements of each group the same model is applied although some parameters are specific to the element. From a mathematical point of view we can establish two groups: a) Elements whose biokinetic model does not involve recycling, this includes the group (i) and the elements where ICRP 30 is still applicable, and b) elements whose biokinetic models involve recycling, this includes group (ii) and (iii). A few computer codes have been developed to estimate intake and calculate internal dose using biassay data. [7]

Bioassay evaluations

Biokinetic modeling is widely used in internal dosimetry and to evaluate bioassay data. Computer programs can be used for bioassay evaluations. [8] The bioassay measurement values can be used to estimate unknown intake. [9]

See also

Related Research Articles

Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is not due to deliberate introduction of radiation sources.

A radiation dosimeter is a device that measures exposure to ionizing radiation. As a personal dosimeter it is normally worn by the person being monitored, and is a record of the radiation dose received. Older dosimeters, such as a film badge, require processing after use to reveal the cumulative dose received. Modern electronic personal dosimeters can give a continuous readout of cumulative dose and current dose rate, and can warn the person wearing it when a specified dose rate or a cumulative dose is exceeded.

Sievert SI derived unit of equivalent dose of ionizing radiation

The sievert is a derived unit of ionizing radiation dose in the International System of Units (SI) and is a measure of the health effect of low levels of ionizing radiation on the human body. The sievert is of importance in dosimetry and radiation protection, and is named after Rolf Maximilian Sievert, a Swedish medical physicist renowned for work on radiation dose measurement and research into the biological effects of radiation.

Radiation dosimetry in the fields of health physics and radiation protection is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both internally, due to ingested or inhaled radioactive substances, or externally due to irradiation by sources of radiation.

Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". The IAEA also states "The accepted understanding of the term radiation protection is restricted to protection of people. Suggestions to extend the definition to include the protection of non-human species or the protection of the environment are controversial". Exposure can be from a radiation source external to the human body or due to the bodily intake of a radioactive material.

Equivalent dose is a dose quantity H representing the stochastic health effects of low levels of ionizing radiation on the human body which represents the probability of radiation-induced cancer and genetic damage. It is derived from the physical quantity absorbed dose, but also takes into account the biological effectiveness of the radiation, which is dependent on the radiation type and energy. In the SI system of units, the unit of measure is the sievert (Sv).

Health physics

Health physics is the applied physics of radiation protection for health and health care purposes. It is the science concerned with the recognition, evaluation, and control of health hazards to permit the safe use and application of ionizing radiation. Health physics professionals promote excellence in the science and practice of radiation protection and safety. Health physicists principally work at facilities where radionuclides or other sources of ionizing radiation are used or produced; these include hospitals, government laboratories, academic and research institutions, nuclear power plants, regulatory agencies, and manufacturing plants.

Radioactive contamination Presence of radioactive substances where they are undesirable

The sources of radioactive pollution can be classified into two groups: natural and man made.

The film badge dosimeter or film badge is a personal dosimeter used for monitoring cumulative radiation dose due to ionizing radiation.

Hot particle

A hot particle is a microscopic piece of radioactive material that can become lodged in living tissue and deliver a concentrated dose of radiation to a small area. A controversial theory proposes that hot particles within the body are vastly more dangerous than external emitters delivering the same dose of radiation in a diffused manner. Other researchers claim that there is little or no difference in risk between internal and external emitters.

Naturally Occurring Radioactive Materials (NORM) and Technologically Enhanced Naturally Occurring Radioactive Materials (TENORM) consist of materials, usually industrial wastes or by-products enriched with radioactive elements found in the environment, such as uranium, thorium and potassium and any of their decay products, such as radium and radon.

The National Calibration Reference Centre for Bioassay and In Vivo Monitoring (NCRC) is administered by the Radiation Protection Bureau of the Canadian Federal Department of Health. It was created in 1982 through a Memorandum of Understanding (MOU) signed between the regulator of the nuclear industry, the Canadian Nuclear Safety Commission (CNSC), formerly the Atomic Energy Control Board, and the Department of Health with the specific mission of providing "practical reference standards" for measurements used for internal dosimetry. Two other Reference Centres were created at the same time. These were to have equivalent roles for (1) external dosimetry and (2) radon and radioactive atmospheres, and were administered, respectively, by the Canadian National Research Council and the federal Department of Energy, Mines and Resources. The choice of the three agencies to act as Reference Centres was based on their expertise acquired over years of work in their respective fields and the fact that they operate independently of both the CNSC and the nuclear industry.

In health physics, whole-body counting refers to the measurement of radioactivity within the human body. The technique is primarily applicable to radioactive material that emits gamma rays. Alpha particle decays can also be detected indirectly by their coincident gamma radiation. In certain circumstances, beta emitters can be measured, but with degraded sensitivity. The instrument used is normally referred to as a whole body counter.

Radiobiology is a field of clinical and basic medical sciences that involves the study of the action of ionizing radiation on living things, especially health effects of radiation. Ionizing radiation is generally harmful and potentially lethal to living things but can have health benefits in radiation therapy for the treatment of cancer and thyrotoxicosis. Its most common impact is the induction of cancer with a latent period of years or decades after exposure. High doses can cause visually dramatic radiation burns, and/or rapid fatality through acute radiation syndrome. Controlled doses are used for medical imaging and radiotherapy.

Committed dose equivalent and Committed effective dose equivalent are dose quantities used in the United States system of Radiological Protection for irradiation due to an internal source.

Radiation monitoring measurement of radiation dose or radionuclide contamination for reasons related to the assessment or control of exposure to radiation or radioactive substances, and the interpretation of the results

Radiation monitoring involves the measurement of radiation dose or radionuclide contamination for reasons related to the assessment or control of exposure to radiation or radioactive substances, and the interpretation of the results.

Banana equivalent dose informal measurement of ionizing radiation exposure; approximately 0.1 microsievert

Banana equivalent dose (BED) is an informal measurement of ionizing radiation exposure, intended as a general educational example to compare a dose of radioactivity to the dose one is exposed to by eating one average-sized banana. Bananas contain naturally occurring radioactive isotopes, particularly potassium-40 (40K), one of several naturally-occurring isotopes of potassium. One BED is often correlated to 10-7 Sievert (0.1 µSv); however, in practice, this dose is not cumulative, as the principal radioactive component is excreted to maintain metabolic equilibrium. The BED is only meant to inform the public about the existence of very low levels of natural radioactivity within a natural food and is not a formally adopted dose measurement.

Computational human phantoms are models of the human body used in computerized analysis. Since the 1960s, the radiological science community has developed and applied these models for ionizing radiation dosimetry studies. These models have become increasingly accurate with respect to the internal structure of the human body.

Effective dose is a dose quantity in the International Commission on Radiological Protection (ICRP) system of radiological protection.

References

  1. IRPA paper 54302 - Internal Dosimetry: The science and art of internal dose assessment
  2. ICRP publication 103 - Glossary.
  3. ICRP publication 103 - Paragraph 144.
  4. Aerodynamic diameter
  5. Whole Body Monitoring
  6. International Commission on Radiological Protection. ICRP Database of Dose Coefficients: Workers and Members of the Public. Oxford: Pergamon Press; (ICRP); 2001.
  7. G. Sanchez Health Phys. 92(1):64–72(2007)
  8. Bioassay evaluations with Biokmod
  9. Optimal design and mathematical model applied to establish bioassay programs