Committed dose, radionuclide

Adams, N., Dependence on Age at Intake of Committed Dose Equivalents from Radionuclides, Phys. Med. Biol. 26 1019-1034 (1981). 

Kendall, G.M., Kennedy, B.W., Greenhalgh, J.R., Adams, N. Fell, T.P. (1987) Committed doses to selected organs and committed effective doses from intake of radionuclides. Report GS7. National Radiological Protection Board. Chilton, Oxon. 

The ALI is the activity of a radionuclide that can be taken into the body in a year, by inhalation or ingestion, without exceeding a committed effective dose equivalent (CEDE) of 5 rem/year or a committed dose equivalent to any organ of 50 rem/year, whichever is more limiting. The total effective dose equivalent TEDE is the sum of the CEDE and any penetrating external dose (10 CFR 20). If any external dose is present the ALI must be reduced by a proportional amount to ensure that the dose limits are not exceeded. For example, if a worker received an external dose of 1 rem/year, the ALI would have to be reduced by 20% to ensure that the TEDE did not exceed 5 rem/year. 

The intake by inhalation, ingestion or through the skin of a given radionuclide in a year by the reference man which would result in a committed dose equal to the relevant dose limit. 

The Food and Drug Administration (FDA) develops standards for radioactive material concentrations in food (FDA 1998), and medical devices used in radiation therapy (FDA 1997). The FDA recently updated its guidance document that presents recommended action levels for radionuclides in foods, both domestic and imported (FDA 1998). These derived intervention levels (DILs) are estimated levels in food that could lead to individuals receiving a radiation equivalent dose equal to the FDA protection action guide (PAG) that is set as the more limiting of either 0.5 rem (5 mSv) for committed effective dose or 5 rem (50 mSv) committed dose equivalent to any individual tissue or organ. Table 8-2 presents the most restrictive DILs for strontium. 

Radiotoxicity depends on energy deposition in tissue or organs by the radionuclide, the specific tissue exposed to the radionuclide, and the tissue radiation sensitivity. Energy deposition by a radionuclide is a function of its emitted radiations and half-life. Biokinetic studies have identified for most radionuclides of interest the pattern of movement through the body and the effective turnover rate (the sum of the biological and radioactive turnover rates). Biokinetic information also identifies the appropriate type of sample to be collected among blood, urine, feces, saliva, breath, hair, teeth, nasal swipes, and tissue obtained incidental to unrelated operations, and collection frequency. The measured radionuclide concentrations are combined with biokinetic information to calculate the committed dose equivalent, the indicator of radiation impact on the subject (NCRP 1987b). 

Annual limit of intake (ALI) means the derived limit for the amount of radioactive material taken into the body of an adult worker by inhalation or ingestion in a year. It is the smaller of (1) the value of the intake of a given radionuclide in 1 year by the reference man that would result in a committed dose equivalent of 5 rems (0.05 Sv) (2) a committed dose equivalent of 50 rem (0.5 Sv) to any individual organ or tissue. 

In radiation protection practice, most of the dose limits correspond to an annual dose (IBSS 1996). The annual dose is the sum of the external exposure in a year and the committed dose (see Sect. 47.3.2.1) the sum of intakes of radionuclides in that year. 

For most radionuclides, the inhalation dose coefficient is higher for children than for adults, but the inhalation rate (I) is probably lower for children. Therefore, the inhalation dose rates for the same level of air contamination may be even less for children and infants than for adults. The inhalation rate (I), consumption habits, and committed dose conversion coefficients with respect to groups of different ages, species of contamination in air (ionic, aerosol bound, aerosol diameter, etc.) or foodstuffs, and radionuclides are listed in tables of dose assessments (Jacob et al. 1990 IBSS 1996). These values are derived from calculations assuming various types of body phantoms and measurements. 

In steady-state conditions, the committed dose from ingested radionuclides maybe assessed in a similar way as given for the inhalation pathway. Instead of the inhalation rate I, the... 

To assess the total committed (internal) dose, the results obtained from each pathway (inhalation, ingestion, different foodstuffs, and radionuclides, etc.) are to be summed. In radiation protection, the total committed dose due to a whole year s uptake is determined and monitored systematically. 

The set of data that are regularly obtained on radionuclide concentrations in locally produced agricultural foodstuffs can be used directly to assess the annual intake and the associated committed dose. In regions where the inhabitants normally consume substantial amounts of natural food products (e.g. game, freshwater fish, forest mushrooms and berries) with elevated radionuclide concentrations, available data from measurements should also be used for the estimation of intakes of radionuclides. 

The set of regularly obtained data on radionuclide concentrations in air can be directly used to assess the annual intake and the associated committed dose. If measurement data are unavailable or insufficient, radionuclide concentrations in air can be roughly estimated from soil deposition rates by using a resuspension model. 

The DAC values relate to one of two modes of exposure either external submersion or the internal committed dose equivalents resulting from inhalation of radioactive materials. Derived air concentrations based upon submersion are for immersion in a semi-infinite cloud of uniform concentration and apply to each radionuclide separately. 

Two appendices and an annex provide additional information. Appendix 1 provides suggested criteria to indicate whether individual monitoring is necessary. Appendix 11 defines procedmos for calculating detection limits for measurement methods. The Annex provides, for ease of reference, some basic data relevant to the assessment of occupational exposttre due to intakes of radionuclides, namely tissue weighting factors and dose coefficients (committed doses per unit intake) and derived air concentrations (DACs) for selected chemical forms of some common radionuclides. 

The fraction of an intake that remains in the body (for direct methods) or that is being excreted from the body (for indirect methods) at time t after an intake may be designated m t) [8, 9]. This fraction depends on the radionuclide, its chemical and physical form and the route of intake, as well as t. To estimate the intake for dose assessment, the measured body content or excretion rate must be divided by the appropriate value of m f) (see Section 7). The committed dose can be seriously underestimated if the dose coefficient e(g)j is applied directly to the measmed body content rather than to the inferred intake. 

Direct or indirect measurements provide information about the amount(s) of radionuclides present in the body, in parts of the body such as specific organs or tissues, in a biological sample or in a sample from the working enviromnent. The first use of these data is likely to be an estimation of the intake of the radionuclide by the worker. Biokinetic models which describe body and organ contents, and activity in excreta, as a function of time following intake, and exposure models which relate intake to workplace conditions, are used for this purpose. Alternatively, measurements of activity in the body can be used to estimate dose rates directly. The calculation of committed doses from direct measurements still involves the assumption of a biokinetic model if sufficient measurements are not available to determine retention functions. 

The Annual Limit of Intake (ALI) for any radionuclide is obtained by dividing the annual average effective dose limit (20 mSv) by the committed effective dose (E) resulting from the intake of 1 Bq of that radionuclide. ALI data for Individual radionuclides are given in ICRP (1991b). 

The dose of radiation delivered by an internally deposited radionuclide depends on the quantity of radioactive material residing in situ. This quantity decreases as a function of the physical half-life of the radionuclide and the rate at which the element is redistributed or excreted (i.e., its biological half-life). Because the physical half-life is known precisely and the biological half-life can be characterized within limits for most radionuclides, the dose to a tissue that will ultimately be delivered by a given concentration of a radionuclide deposited therein can be predicted to a first approximation. The collective dose to a population that will be delivered by the radionuclide—the so-called collective dose commitment—serves as the basis for assessing the relevant long-term health effects of the nuclide. 

For radionuclides with half-lives zero to 3 months the equivalent dose is equal to the armual dose of the year of intake (Harley, 2001). It is often convenient to estimate the fraction of the committed equivalent dose which remains. For a radionuchde with a half-life of 3 months, after 1 year of exposure, it had undergone the passage of four half-fives so tfiat fraction of dose delivered was (1 /2 ) 94% of... 

The upper bound for annual release can then be derived from the dose upper bound by using the overall transfer factors (. ., ) where j represents population group, k represents release mode and / represents the radionuclide. If the dose commitment to the critical group / per unit release of a radionuclide is given by fj,, then the release upper bound, Ri i, is given by... 

By considering all possible transfer routes, one can estimate what amount of a radionuclide released to the environment may end up in plants, animals, or man. When these figures are combined with the dose conversion factors ("committed effective dose equivalent per unit intake", according to ICRP) in Table 18.12, it is possible to calculate the dose received by man from intake of a radionuclide in the environment. The dose conversion factors dep d on the mode of intake (usually only inhalation or ingestion). Thus... 

Cosmogenic radionuclides due to inhalation and ingestion (internal, committed effective dose) 0.012 (0.008-0.02)... 

Radionuclides Effective dose commitment/ jiSv External Ingestion Inhalation Total exposure... 

Release radioactivity/PBq Radionuclides Reactor Reprocessing Total Average annual effective dose commitment/nSv... 

---