1 Introduction

Committee 2 of the International Commissio n on Radiological Protection(ICRP) has responsibility for secondary standards for both ex ternal radiation and inco rporated radionuclides. In 1987, ICRP set up a Task Group of Committee 2 on Age-dependent Dosimetry (AGDOS) which was charged with the responsibility of working with the Task Group on Dose Calculations(DOCA L)to calculate dose coefficients(doses per unit intake) for members of the public. This task covered the development of age-dependent physical and biokinetic models and the selection of appropriate biokinetic parameters. The terms of reference of AGDOS were ex tended by the Commission in September 1993 to include a revisio n of dose coefficients for the working population. It became the Task Group on Internal Dosimetry (INDOS). This paper summarises the prog ramme of work of Committee 2 on internal dosimetry in the context of both public and worker exposure.

2 Members of the Public

A series of reports have been published on age-dependent doses to members of the public from intakes of radionuclides. These aimed to g ive dose coefficients for selected radionuclides based on the following objectives :

· to select those elements for which age-dependent biokinetic data are needed;

· to identify the most radiologically significant radioisotopes of the above elements that mig ht be released to the environment due to various hum an activities;

· to develop age-dependent models and parameters for describing the biokinetics of these elements;

· to assess dose coefficients, ie, doses per unit intake, for intakes by ingestion and inhalation of the radioisotopes considered, for members of the public of various ages, including the embryo and fetus;

ICRP Publications 56, 67, 69 and 71^[1~4] have given age-dependent ingestion and inhalation dose coefficients for selected radioisotopes of 31 elements. The contents of these repo rts are sum marised in Table 1. For the calculation of the inhalation dose coefficients given in Publication 71 the human respiratory tract model was used for an aerosol with an activity median aerodynamic diameter(AM AD)of 1 μm^[5]. A compilation of these ingestion and inhalationdose coefficients was subsequently issued as Publication 72^[6].Additionally, this report g ives dose coefficients for radionuclides of a further 60 elements for which age-dependent models have no t been developed. The biokinetic models used for these additional elements are those g iven for adults in Publication 30^[7~9] and allow ance has been made only for changes in body mass and for doses from material in the excretion pathways. The dose coefficients must therefore be used with care for infants and children. These dose coefficients are also given in the International Basic Safety Standards^[10] and in the new EURATOM Directive^[11].

The dose coefficients given in the reports are committed doses calculated to age 70 years and are based on a unit intake of the radionuclide at six different ages; infants of 3 months, children of₁ y ear, 5 years, 10 years, 15 years and adults. These age g roups are assumed to be representative for assessing doses from intakes of radionuclides by the general population. It is considered that these age-specific dose coefficients may be reasonably applied to the age ranges of 0 -12 months, 1 -2 years, 2 -7 years, 7 -12 years, 12 -17 y ears and for the adult of mo re than 17 years of age.

Table 2 gives examples of ingestion dose coefficients for intakes of tritiated water, ¹⁴C, ⁹⁰Sr, ¹³¹I, ¹³⁷Cs and ²³⁹Pu for three month infants, 1 and 10 year-old children and adults. Inhalation dose coefficients are given in Table 3. In general, dose coefficients are higher in the younger age g roups than in adults because of the smaller body m ass, although the difference betw een the various ages is very variable. Thus, for ingestion of ²³⁹Pu there is a 17 -fold difference betw een the three month-old infant and the adult, which largely reflects the differences in f₁ values for the two ages (f₁ =10^-2 and 10^-3respectively). For ¹³⁷Cs(f₁ =1 for all ages) the rate of loss of caesium from the body is faster in children than in adults and as a consequence there is little overall difference in the dose coefficients for the different ages. For ¹³¹I (f₁ =1) the half-time of retention in children is less than in adults but the fractional uptake by the thy roid gland (0. 3)appears to be independent of age. The effective half-time in the gland is, how ever, largely determined by the phy sical half-life of the isotope(8 days) and as a consequence the 6 -8 -fold difference in the dose coefficients for the different ages mainly reflects the differences in thy roid mass.

A further document is being developed on the reliability of dose coefficients for the public. The aim is to review parameter values used in the biokinetic and dosimetric models for a range of elements and to examine how uncertainties in the models influence the reliability of the point estimates of committed effective dose, and hence of radiation detriment. The repo rt will examine inter alia variability in human studies, the difficulties in extrapolating the results of anim al experiments to man and the relevance of in vitro solubility studies. The focus of the report will be to examine how uncertainties in the biokinetic and dosimetric models for ingestion, inhalation and systemic activity influence effective dose. It will also identify areas for future work.

3 Occupationally Exposed Workers

Adoption of the 1990 recom mendations of IC RP in Publication 60^[12] necessitated a revision of the Commission' s secondary limits. In o rder to permit immediate application of these new recommendations, revised values of the Annual Limits on Intake(ALIS) based on the methodology and biokinetic information from Publication 30^[7~9], but which incorporated the new dose limits and tissue weighting factors, W_T, were issued as Publication 61^[13].

A replacement for Publication 61 was subsequently issued as Publication 68^[14] which takes into account the new model for the human respiratory tract^[5] as well as revised biokinetic models for sy stemic activity (Table 1). Some modifications to f₁ values have also been made. Previous calculations of inhalation dose coefficients for wo rkers have been based on an AMAD of₁μm^[7]. An AMAD of 5 μm is now adopted as a more appropriate default particle size for the workplace(Dorrian and Bailey^[15]), althoug h dose coefficients for an AMAD of 1 μm are also given. Fo r the purposes of the report, compounds with lung clearance given as Classes D, W or Y in Publication 30 are assig ned to the default lung absorption Types F, M and S respectively as specified in Publication 66.

For radionuclides inhaled in particulate form it is assumed, as is the Publication 30 lung model, that entry and deposition in the respirato ry tract are governed by the size distribution of the aerosol particles. The situation is different for gases and vapours, for which the radionuclide has a specific behaviour at its site of entry to the respirato ry tract, depending on the chemistry of the compound. For Publication 68, the behaviour of gases and vapours assumed in Publication 30 has been retained, but is represented using the formalism of Publication 66. The new model assig ns gases and vapours to three classes :

· Class SR -1(soluble or reactive : deposition in ex tratho racic region, subsequent retention in the respiratory tract and absorption to blood determined by specific chemical properties of g as or vapour). Retention in respirato ry tract tissues, and uptake to the sy stemic circulation may be less than 100 % of the inhaled activity(eg, Co).

· Class S R -2(hig hly soluble or reactive: deposition thorought the respiratory tract. Generally com plete and instentaneous systemic uptake of the inhaled activity (eg, ³H₂O, iodine vapour).

· Class SR -0(insoluble and non-reactive: negligible deposition). Here there is external irradiation from submersion in the cloud of gas, and internal irradiation from gas within the respiratory tract(eg, ³⁷Ar, ⁸⁵Kr).

For occupational exposures, the 1990 recommendations of ICRP limit the effective dose to 100 mSv in a five year period (giving an average annual value of 20 mSv) with a limit of 50 mSv in any single year. In the caes of internal exposure, the recom mendations indicate that the Annual Limit on Intake (ALI)should be based on a committed effective dose of 20 mSv. Publication 68 does not, however, give ALIs as these are now seen has being industry or even building specific and managements need to establish appropriate ALIs, taking account of all the characteristics of materials in their workplaces.The new respiratory tract model is designed to facilitate application of material specific parameter values for inhaled radionuclides.

To supplement Publication 68, a Technical Report is being prepared by INDOS giving guidance on the practical application of the Human Respiratory Tract M odel to situations in which information is available which enables more accurate dose assessments to be made than would be the case with reference parameter values.

A revision of Publication 54 on Individual M onitoring for Intakes of Radionuclides by Workers^[16] has been prepared by a Working Party of Com mittees 2 and 4 to provide information for interpreting monitoring data consistent with these models. The report will give inform ation for selected radionuclides of a limited range of elements(H, Fe, Co, Sr, Ru, I, Cs, Ra, U, Np, Pu, Am, Cm and Cf) that are important for protection purposes. The models used for the calculations will be those adopted for Publication 68. The Working Party is also developing a CD-ROM to facilitate the interpretation of bioassay data. The report has been approved for publication by the Commission and should be issued late in 1997.

A long-term task of INDOS and DOCAL is a full revision of Publications 30 and 54. With the move to physiologically-based biokinetic models, the aim now is to develop models that are appropriate for both bioassay interpretation and for dosimetry calculation. Thus the full revision of Publication 30 will take into account the work done for the revision of Publication 54. As it will be necessary to review the biokinetic models for all the elements that have not been review ed in Publications 56, 67, 69 and 71, it is considered unlikely that the work could be completed before the year 2000. I t is intended that in the revision of Publication 30, dose coefficients will be given for specific chemical forms of various radionuclides, in addition to those based on the default absorption Types. The Technical Report on the application of the Human Respiratory Tract Model will provide information on the likely approach to be adopted in applying material-specific data in the calculation of inhalation dose coefficients.

4 Compilation of Dose Coefficients on CDROM

Publications 72 and 68 give only committed effective and in the case of inhalation covers only inhalation of aerosols of 1 μm and 5 μm Activity Median Aerodynamic Diameter (AMAD). A CD-ROM is being developed by ICRP that will give inhalation dose coefficients for a range of particle sizes (0. 001, 0. 003, 0. 01, 0. 03, 0. 1, 0. 3, 1, 3, 5and 10 μm AMAD) as well as ingestion coefficients. It will give equivalent doses to all tissues with specific tissue weighting factors, W T and effective doses for a range of integration times (1, 7, 30days, 1, 5, 10, 20, 30and 45years)together with committed equivalent doses and committed effective doses. It will also include similar data for the radionuclides for which only committed effective doses are given for workers in Publication 68. The CD-ROM will give the biokinetic models for the elements, as well as the text from Publications 68 and 72. It is expected that the CD-ROM will be issued by the Com mission early in 1998.

5 The Embryo and Fetus

Whilst dosimetric models for infants and children can generally be based on those developed for adults, with appropriate modifications to biokinetic parameters, in the case of the embryo and fetus fundamentally new models need to be developed. Such models are important for assessing doses to the fetus for women who may be either occupationally ex posed o r given radiolabelled drug s for clinical reasons. In the 1990 Recommendations of ICRP, particular emphasis is placed on measures to control exposures in utero for women who are occupationally exposed. This arises because of the recognition of the g reater sensitivity of developing tissues to ionising radiation and the need to treat the fetus broadly as a member of the public.

A further report is, therefore, in preparation that will give dose coefficients for the embryo and fetus from intakes of radionuclides by the mother. It will cover radionuclides of the 31 elements covered in Publications 56, 67, 69 and 71 and will apply to bo th members of the general public and to workers. In the development of biokinetic models, tw o approaches are being used. Where sufficient information is available, element-specific models are being developed. This applies, for example, to tritiated water, caesium, iodine and the alkaline earths. Where only limited data are available, a generic modelling approach will be used based o n information on relative concentrations of radionuclides in fetal and maternal tissues obtained principally from anim al studies. As part of the development work needed for this report DOCAL is developing dosimetric models that will allow the calculation of doses to the embryo and fetal tissues from radionuclides deposited either in the tissues of the embryo/fetus, in the placenta o r in the mother. To provide data that can be used for assessing a range of possible intake scenarios dose coefficients will be g iven for acute and chronic intakes by the mo ther at various times bo th before conception o r during pregnancy. Ingestion dose coefficients will be given for a range of f₁ values while inhalation dose coefficients will be given for both 1 and 5 μm AMAD aerosols; the default values for members of the public and workers respectively.

For acute exposures, intakes of radionuclides will be taken to occur at the start of weeks 1, 5, 10, 15, 25 and 35 of the pregnancy and at 6 months and 2 1/2 years before conception. For chronic exposures, intakes are taken to occur during the year of preg nancy, starting from conception, for 1 year before conception or for 5 years before conception. This range of intake scenarios should allow doses to the offspring to be calculated for any pattern of intake by the mo ther. It is proposed that in the final repo rt equiv alent doses to the date of birth will be given for the brain from 8-15 weeks of gestation and for the tissue receiving the highest dose. The effective dose to birth will also be given using the W_TS recommended by ICRP in Publication 60. Whilst these values are not strictly appropriate for exposures in utero, they are to be used as no alternative weighting factors are available and the calculation of effective dose provides a useful quantity for comparison with doses to the adult. Effective doses (to age 70 years) received after birth will also be given, together with the to tal effective dose (before and after birth)received by the offspring. It is ex pected that the repo rt will be ready for publication late in 1998.

6 Dosimetric Model for the Human Alimentary Tract

The present model of the gastrointestinal tract applied by ICRP in the preparation of dose coefficients was first published in 1966^[17]. Whilst it has provided an essential basis for dose calculatio ns for mo re than 30 years, there is now a need to develop a new dosimetric model for the human alimentary tract which takes account of more recently published information. A Task Group has been established to develop a new model for the human alimentary tract. The work will cover:

· definition of the anatomical regio ns needed for dosimetry;

· review and evaluation of information on the movement of materials throug h the whole of the alimentary tract, including the mouth and oesophagus;

· the possible retention of radionuclides in the gut wall and absorption from different regions;

· review of inform ation on the location of cells at risk and methods for estimating radiation doses;

· provision of reference parameters for the relevant biokinetic and anatomical parameters for v arious age g roups; · consideration of uncertainties in dose calculations.

It is expected that the Group will complete its report in about four years time.

7 Other Issues Under Review

Dosimetry of Auger Emitters

In calculating doses for Auger electron emitters, the assumption presently made is that energy is deposited uniform ly throughout cells and tissues. In some circum stances, however, there may be concentration of Auger emitters in particular parts of the cell and therefore inhomogeneity of energy deposition. In particular, where radionuclides become localised within the nucleus, it is possible that higher local doses to sensitive sites would be underestimated on the basis of calculating average tissue dose. The Committee is keeping a watching brief o n this problem and intends to stimulate work in both Committees 1 and 2, with the aim of developing mo re work in this area.

Retrospective Dosimetry

The Committee is to keep under review information available for retrospective dosimetry that will permit the assessment of doses resulting from the release of radionuclides into the environment, either in routine situations o r after accidents such as that at Chernobyl.

Phantom Development

The present phantom used for dose calculations is the M IRD phantom which represents the body as a set of geometrical shapes. DOCAL has considered the question of developing phantoms for dose calculations based on medical im aging data. These would have the advantage that they could cover arange of ages, they could be used both for ex ternal radiatio n and internally incorporated radionuclides, they would be more flex ible in use and more realistic. In particular, if further org ans are identified for which dose calculations are needed they could be readily specified in the model. DOCA L will wo rk tow ards developing new phantoms but it is expected to take some years before a family of reference phantoms will be available.

Transfer of Radionuclides in Milk INDOS is to develop parameters for the transfer of radionuclides to mo ther' s milk and dose coefficients for intakes by the offspring.

The issue of dose coefficients for the offspring can have a potential im pact on advice for maximum levels of radionuclides in foodstuffs. The task is to cover:

· assessing the transfer of radionuclides to breast milk follow ing intake by the mother;

· determining f₁ values for intakes of radionuclides in milk by the newbo rn child (appropriate f₁ values may well be higher than those adopted for the 3 month infant as it is know n that gut absorption falls rapidly in the first few weeks and months of life); and

· calculation of dose coefficients for the transfer from food to mother' s milk to the new bo rn.

Intakes of Radionuclides Through Wounds

To date, ICRP has no t given advice on the interpretation of wound monito ring data follo wing accidents involving radionuclides. The biokinetic models that have been developed for various radionuclides are, how ever, applicable to the soluble component of any deposit in wounds that enter the blood circulation. Committee 2 has considered the need to give advice on doses from material deposited in wounds. The United States National Committee on Ra-diological Protection and Measurements (NCRP)has recently set up a committee to review the problem of wound contamination. ICRP will, for the present, follow the work of this committee.

Radon

The human respiratory tract model is not at present readily applied to the dosimetry and risk assessment for exposures to radon and its decay products. The issues are complex and are influenced by the reference values adopted for deposition and clearance in the respirato ry system as well as the radiation weighting factor, W_R for α particle irradiation and the dose and dose rate effectiveness factor (DDREF). At present, exposure limits for radon recommended by ICRP^[18] are based on dose-response relationships from epidemiological studies. Committees 1 and 2 are to review the epidemiological and dosimetric approaches to assessing the consequences of exposure to radon with the aim of harmonising the two approaches.

Sensitive Cell in the Skeleton

A joint Working Party of Committees 1 and 2 is review ing information on the calculation of doses for bone seeking radionuclides.For the skeleton, doses are calculated to cells ly ing within 10 μm of the endosteal bone surfaces. This followed a recommendation in ICRP Publication 11^[19] which reviewed the radiosensitivity of tissues in bo ne. Since Publication 11 was issued further information has become available which indicates that the sensitive cells for bone tumour induction are present no t only near endosteal surfaces but also throughout the bone m arrow and bone matrix. It is proposed to review these data in the contex t of epidemiological and experimental data on bone tumour induction. The aim is to give advice on the location of sensitive cells in the skeleton for dose calculation and risk assessment.