Grandeurs de Radioprotection Grandeurs et unités- 3 Grandeurs de Radioprotection Now let’s discuss radiation protection quantities and their associated terminology and units. The protection quantities are the quantities that are linked to radiation exposure risk. Although they can be calculated under specified exposure conditions, they generally cannot be measured in routine field situations. Jour 3 – Leçon 1

Objectif Etudier les grandeurs de radioprotection et la terminologie associée et apprendre davantage sur la dose équivalente, les facteurs de pondération du rayonnement, la dose efficace, les facteurs de pondération des tissus, la contamination interne, la dose engagée, la dose efficace engagée, et diverses grandeurs opérationnelles We will distinguish between radiation vs tissue weighting factors, which sometimes get confused.

Contenu Dose Equivalente et débit de dose équivalente Facteurs de pondération radiologique Dose efficace Facteurs de pondération tissulaire Les rayonnements faiblement et fortement pénétrants Dose équivalente ambiante Champ de rayonnement élargit et canalisé Dose équivalente directionnelle Dose équivalente individuelle Contamination interne Dose équivalente engagée Dose efficace engagée We will distinguish between radiation vs tissue weighting factors, which sometimes get confused.

Dose Equivalente La dose équivalente dans le tissue T est donnée par l’expression: HT = ∑ WR DT,R Où DT,R est la dose absorbée moyenne dans le tissus ou organe T, due au rayonnement R. The equivalent dose (HT) is based on an average absorbed dose in a tissue or an organ and weighted by the radiation weighting factor (WR) for the radiation(s) impinging on the body or, in the case of internal emitters, the radiation emitted by the source. So the equivalent dose is the product of the average-absorbed dose ( DT,R ) in a tissue or organ (T) due to radiation (R) and a radiation weighting factor (WR) for each radiation in question. The equivalent dose is used in NCRP 116. When irradiation is from radionuclides deposited in various tissues and organs, nonuniform or partial body exposures usually occur.

Dose Equivalente En radioprotection, c’est la dose absorbée moyennée sur un tissus ou un organe Elle est pondérée par un facteur dit de pondération qui dépend du type du rayonnement : WR The equivalent dose (HT) is conceptually different from the older concept of the dose equivalent (H). The dose equivalent (H) is based on the absorbed dose at a "point" in tissue (rather than an average) which is weighted by a distribution of quality factors (Q) (now called radiation weighting factors) which are related to the LET distribution of the radiation at that point. NCRP 93 (1987).

Dose Equivalente WR dépend du type du rayonnement incident et de son énergie Cette dose pondérée , s’appelle dose équivalente, L’unité de la dose équivalente est le J/kg avec un nom spécial de Sievert (Sv) When the radiation field is composed of types and energies with different values of WR, the absorbed dose must be subdivided in blocks, each with its own value of WR and summed to give the total equivalent dose. Alternatively, it may be expressed as a continuous distribution in energy where each element of absorbed dose is multiplied by the relevant value of WR (shown in the next two slides).

Facteurs de pondération radiologique Type et gamme Energie wR Photons: toutes les énergies 1 Electrons : toutes les énergies Neutrons: energie < 10 keV 5 Neutrons: 10 keV à 100 keV 10 Neutrons: > 100 keV à 2 MeV 20 All values relate to radiation incident on the body, or for internal sources, emitted from the source. The value of 1 for electrons and muons excludes Auger electrons emitted from nuclei bound to DNA.

Facteurs de pondération Radiologique Type et Gamme d’Energie wR Neutrons: > 2 MeV to 20 MeV 10 Neutrons: > 20 MeV 5 Protons: > 2 MeV Particules Alpha, Fragments fission, Noyaux lourds 20 Note that the values of the radiation weighting factors depend on the type and energy of the radiation but are independent of the tissue or organ.

Débit de dose équivalente . Le débit de dose équivalente, HT, est le quotient de dHT par dt, où dHT est l’incrément de dose équivalente dans le temps dt: HT = L’unité est le J kg-1 s-1 avec un nom spécial (Sv s-1) . dHT dt The equivalent dose rate is simply the equivalent dose delivered per unit time period.

Dose efficace La dose efficace est la somme des doses équivalentes pondérées dans tous les tissus et organes du corps,. Elle est donnée par : E = ∑ wT HT où HT est la dose équivalente à l’organe T et wT est le facteur de pondération tissulaire T. The effective dose (E) is intended to provide a means for handling non-uniform irradiation situations, as did the earlier dose equivalent. The effective dose (E) is the sum of the weighted equivalent doses for all irradiated tissues or organs. The tissue weighting factor (wT ) takes into account the relative detriment to each organ and tissue including the different mortality and morbidity risks from cancer, the risk of severe hereditary effects for all generations, and the length of life lost due to these effects. The effective dose can also be expressed as the sum of the doubly weighted absorbed doses in all the tissues and organs of the body.

Facteurs de pondération tissulaire Tient compte de la probabilité d'effets stochastiques de l'organe ou du tissu irradié Représente la contribution relative de l'irradiation de chaque organe ou tissu au détriment total dû aux effets résultant de l'irradiation uniforme de l'ensemble du corps humain

Facteurs de pondération tissulaire Il est souhaitable qu'une dose équivalente uniforme sur tout le corps donne une dose efficace numériquement égale à la dose équivalente uniforme Réalisé en normalisant la somme des facteurs de pondération tissulaires égale à 1 Recall that the values of the radiation weighting factors depend on the type and energy of the radiation and are independent of the tissue or organ. Similarly, the values of the tissue weighting factors are chosen to be independent of the type and energy of the radiation. These simplifications may be no more than approximations to the true biological situation, but they make it possible to define a radiation field outside the body in dosimetric terms without the need to specify the target organ.

Facteurs de pondération tissulaire Tissu ou Organe WT Gonads 0.20 Moelle osseuse (rouge) 0.12 Colon Poumons Etomac Vessie 0.05 Seins Tissu ou Organe WT Foie 0.05 Oesophage Thyroide Peau 0.01 Surface de l’os Le reste There are 12 specific tissues or organs for which a tissue weighting factor has been specified. This does not include remainder organs, which are discussed later. The values have been developed from a reference population of equal numbers of both sexes and a wide range of ages. In the definition of effective dose they apply to workers, to the whole population, and to either sex.

Facteurs de pondération tissulaire Autres Organes glandes surrénales Gros intestin supérieur intestin grêle rein pancréas cerveau rate thymus utérus muscle The remainder consists of the tissues and organs shown in this slide. The list includes organs which are likely to be selectively irradiated. In those exceptional cases in which a single one of the remainder tissues or organs receives an equivalent dose in excess of the highest dose in any of the twelve organs for which a weighting factor is specified, a weighting factor of 0.025 should be applied to that tissue or organ and a weighting factor of 0.025 to the average dose in the rest of the remainder as defined in this slide.

Concept de la dose efficace La relation entre la probabilité d'effets stochastiques (principalement le cancer et les effets génétiques) et la dose équivalente dépend de l'organe ou du tissu irradié. La dose efficace combine les doses équivalentes aux divers organes et tissus du corps d'une manière bien corrélée avec la somme des effets stochastiques

Grandeurs Opérationnelles Aux fins de mesure des rayonnements, les grandeurs opérationnelles suivantes sont définies: Equivalent de dose ambiant Equivalent de dose directionnel Equivalent de dose individuel Operational quantities have been defined for practical measurements, both for area and for individual monitoring. Being based on dose equivalent at a point in a phantom or in the body, they relate to the type and energy of the radiation existing at that point and can therefore be calculated on the basis of the fluence at the point. The operational quantities are intended to serve as surrogates for the protection quantities. These operational quantities can be readily measured in the field as well as calculated and provide reasonably good estimates of the protection quantities under specified exposure conditions.

Grandeurs Opérationnelles Où les doses sont estimées à partir des résultats de surveillance de la zone, les grandeurs opérationnelles pertinentes sont l’équivalente de dose directionnelle et l’équivalent de dose ambiant Pour la surveillance individuelle, l'utilisation de l'équivalent de dose individuel est recommandée Two quantities linking the external irradiation to the effective dose and to the dose equivalent in the skin and lens of the eye are discussed in this slide. For individual monitoring, the use of the personal dose equivalent is recommended. Note that the personal dose equivalent is a surrogate for the effective dose.

Surveillance de zone Les grandeurs recommandées pour la surveillance de zone se réfèrent à un fantôme appelé la sphère ICRU. La sphère ICRU (ICRU, 1980) a un diamètre de 30 cm, équivalent au tissu avec une densité de 1 g cm-3 et une composition massique de 76,2% d'oxygène, 11,1% de carbone, 10,1% d'hydrogène et 2,6% d'azote For purposes of routine radiation protection, it is desirable to characterize the potential irradiation of individuals in terms of a single dose equivalent quantity that would exist in a phantom approximating the human body. The phantom selected is the ICRU sphere. In practical measurements, materials having elemental compositions and mass density somewhat different from those specified are often used and give adequate accuracy, although elemental equivalence may be necessary with mixed radiation fields.

Référence de la Sphère ICRU This slide shows the radiation field impinging on the ICRU reference sphere of radius 15 cm, diameter 30 cm. d is the reference depth at which the dose quantity is calculated. Champ de rayonnements

Equivalent de dose ambiant L’équivalent de dose ambiant en un point, H*(d), est l'équivalent de dose qui serait produit par le champ correspondant, dans la sphère ICRU à une profondeur (d) en millimètres sur le rayon opposé au sens du champ. Pour la mesure des rayonnements fortement pénétrants, la profondeur de référence est de 10 mm et la grandeur est notée H* (10). L’unité est J kg-1 Le nom spécial de l’équivalent de dose ambiant est le Sievert (Sv) Any statement of ambient dose equivalent should include a specification of the reference depth, d. In order to simplify notation, d should be expressed in mm. Ambient dose equivalent H*(10) measured at some place in air shall estimate Hp(10) a person would receive at that place. Unlike exposure meters, H*(10) meters need to simulate the absorbing and scattering effect of the human body through their energy response. The absorbing effect is only significant at energies lower than approx. 30 keV. The scattering effect is most important at intermediate energies and may lead to an increase of up to 50%. At higher energies, say 200 keV or more, the scattering effect is 20% or less, and can be neglected at very high energies. If you would like to measure H*(10) with an exposure meter calibrated in R, first multiply the reading by 0.01 Sv/R. Then apply a correction factor depending on the energy of the radiation. If you have no idea about the energy, use a correction factor of 1.5; this will almost certainly overestimate H*(10). For weakly penetrating radiation, a depth of 0.07 mm for the skin and 3 mm for the eye are used, with analogous notation.

Champ expansé Un champ de rayonnement expansé est défini comme un champ de rayonnement hypothétique dans lequel la fluence et ses distributions angulaires et l'énergie, ont la même valeur dans tout le volume de l'intérêt que le champ réel au point de référence For area monitoring, it is useful to stipulate certain radiation fields that are derived from the actual radiation field. The expanded field is such a field.

Equivalent de dose directionnel L'équivalent de dose directionnel, H'(d), à un moment donné, est l'équivalent de dose qui serait produit par le champ expansé correspondant dans la sphère ICRU à une profondeur (d) sur un rayon dans une direction spécifiée. L’équivalent de dose directionnel est particulièrement utile dans l'évaluation de la dose à la peau ou au cristallin L’unité est J kg-1 Le nom spécial pour l’unité de l’équivalent de dose directionnel est le Sievert (Sv) Directional dose equivalent H'(0.07, Omega) (also measured in Sv) is the dose equivalent produced in the ICRU sphere at a depth of 0.07 mm, that is skin dose. With strongly penetrating radiation, skin dose will not significantly contribute to effective dose. Therefore, directional dose equivalent is only important in case of weakly penetrating radiation, such as alphas, betas with energies lower than 2 MeV, and photons with energies lower than 15 keV Any statement of the directional dose equivalent should include a specification of the reference depth, d, and of the direction, . d should be expressed in mm. For weakly penetrating radiation, a depth of 0.07 mm for the skin and 3 mm for the eye are used. For strongly penetrating radiation, a depth of 10 mm is recommended, with analogous notation. Measurement of H‘(d,) requires that the radiation field be uniform over the dimensions of the instrument, and that the instrument have the required directional response (isodirectional response, ICRU, 1992). For weakly penetrating radiation, an instrument which determines the dose equivalent at the appropriate depth in a plane slab of tissue-equivalent material will adequately determine H‘(0.07, ) and also H‘(3, ) if the slab surface is perpendicular to .

Equivalent de dose individuel L'équivalent de dose individuel Hp (d), est l'équivalent de dose dans les tissus mous, à une profondeur appropriée (d), en dessous d'un point spécifié sur le corps, Hp (d) peut être mesuré à l'aide d'un dosimètre qui est porté à la surface du corps et recouvert d'une épaisseur appropriée d'un matériau équivalent au tissu Any statement of the personal dose equivalent should include a specification of the reference depth, d. d should be expressed in mm. For weakly penetrating radiation, a depth of 0.07 mm for the skin and 3 mm for the eye are used. For strongly penetrating radiation, a depth of 10 mm is used. The calibration of the dosimeter is generally performed under simplified conditions on an appropriate ICRU phantom. Personal dose equivalent Hp(10) (also measured in Sv) is the dose equivalent in tissue at a depth of 10 mm in the body (not in the ICRU sphere) at the location where the personal dosimeter is worn. From this definition follows that Hp(10) includes the effect of the body on the radiation field, that is absorption and scattering. Dosimeters designed for Hp(10) have to account for that absorption and scattering. Since they are worn on the body (allow some clothing in between), they have always been exposed to radiation scattered from the body, even before Hp(10) was introduced. One might suppose that personal dosimeters measured Hp(10) from the very beginning, just without knowing. In fact, this is true to some extent.

Equivalent de dose individuel L’unité est J kg-1 Le nom spécial pour l’unité de de l’équivalent de dose individuel est Sievert (Sv) Hp (10), mesurée à une profondeur de 10 mm dans les tissus mous, est opérationnelle pour la dose efficace, E Personal dose equivalent Hp(0.07) is similar to Hp(10) except that it refers to weakly penetrating radiation (skin dose). Hp(0.07) dosimeters are calibrated on rod phantoms simulating fingers, arms, or legs. Introducing personal dose equivalent Hp(10) has only little influence on practical personal dosimetry. Hp(10) explains what personal dosimetry really is rather than modifying it. Except at low energies, most personal dosimeters calibrated in R will be suited to measure Hp(10); just multiply their reading by 0.01 Sv/R. The same is true for personal dosimeters calibrated in rem as long as those rems come from photon radiation. ICRU data demonstrate that Hp(10) serves as a reasonably conservative surrogate for the effective dose under most conditions involving normally incident beams of penetrating radiation. Hp(10) is the equivalent of the concept of “deep dose equivalent” used in United States Nuclear Regulatory Commission (NRC) regulations.

Contamination interne Quand une substance radioactive se trouve à l’intérieur du corps humain par inhalation ou ingestion c’est une contamination interne La quantité de matière radioactive inhalée ou ingérée s’exprime en Bq Une contamination interne devrait être mise en contraste avec une absorption de la matière radioactive dans un organe ou un tissu spécifique Note that for the inhalation pathway, a percentage of the intake is immediately exhaled. Radioactive material entering the body by inhalation or ingestion will be eliminated from the body by both radiological decay and biological removal processes. The uptake in Bq to a specific organ or tissue is what is actually determines the dose to that organ or tissue. The uptake is determined from the intake.

Contamination interne La CIPR(60) a défini les limites de contamination interne (LAI) pour chaque radionucléide La LAI est basée sur une limite de dose efficace moyenne de 20 mSv par an The ALI for each radionuclide is obtained by dividing the annual average effective dose limit of 20 mSv by the committed effective dose (50-yr integration period) resulting from an intake of 1 Bq of that radionuclide. For purposes of dose calculation, the intake (not the uptake) is compared to the ALI. The uptake to a specific organ or tissue is accounted for in the metabolic model (and the associated assumptions) used for internal dose calculation.

Dose équivalente engagée Suite à l’admission dans le corps d'une substance radioactive, il existe une période au cours de laquelle cette substance donne lieu à des doses équivalentes aux organes ou aux tissus de l'organisme à différents débits L'intégrale dans le temps du débit d'équivalent de dose est appelée dose équivalente engagée  is usually taken to be 50 years for adults and 70 years for children. Depending on the half lives of the radionuclides, there will be effects over time from radiation doses. To account for the continuing irradiation of organs and tissues after intake, there is the committed dose concept. The committed equivalent dose, HT(), is the time integral of the equivalent dose-rate in a specific tissue (T) following intake of a radionuclide into the body. Unless specified otherwise, an integration time of 50 y after intake is recommended for the occupational dose, and 70 y for members of the public.

Dose efficace engagée La dose efficace engagée E(50) pour les travailleurs est définie comme suit: E(50) = Σ wT HT(50) où HT (50) est l’équivalent de dose engagé et wT le facteur de pondération spécifique pour les tissus ou organes The committed effective dose E(50), for each internally deposited radionuclide is calculated by summing the products of the committed equivalent doses and appropriate WT values for all tissues irradiated. For radionuclides with an effective half-life exceeding three months, the committed equivalent dose and the committed effective dose are greater than the equivalent or effective dose received in the year of intake because they reflect the dose that will be delivered in the future as well as that delivered in the year of intake. The committed equivalent dose and the committed effective dose are appropriate for all routine radioprotection purposes and should be used, for example, for assessing compliance with the annual effective dose limits and for planning and design.

Résumé Les grandeurs dosimétriques et les terminologies associées ont été discutées Les participants ont compris la dose équivalente, les facteurs de pondération du rayonnement, la dose efficace, les facteurs de pondération des tissus, la contamination interne, la dose engagée, dose efficace engagée, et diverses grandeurs opérationnelles

Où trouver plus d’information Knoll, G.T., Radiation Detection and Measurement, 3rd Edition, Wiley, New York (2000) Attix, F.H., Introduction to Radiological Physics and Radiation Dosimetry, Wiley, New York (1986) International Atomic Energy Agency, Determination of Absorbed Dose in Photon and Electron Beams, 2nd Edition, Technical Reports Series No. 277, IAEA, Vienna (1997)

Où trouver plus d’information International Commission on Radiation Units and Measurements, Quantities and Units in Radiation Protection Dosimetry, Report No. 51, ICRU, Bethesda (1993) International Commission on Radiation Units and Measurements, Fundamental Quantities and Units for Ionizing Radiation, Report No. 60, ICRU, Bethesda (1998) Hine, G. J. and Brownell, G. L., (Ed. ), Radiation Dosimetry, Academic Press (New York, 1956)

Où trouver plus d’information Bevelacqua, Joseph J., Contemporary Health Physics, John Wiley & Sons, Inc. (New York, 1995) International Commission on Radiological Protection, Data for Protection Against Ionizing Radiation from External Sources: Supplement to ICRP Publication 15. A Report of ICRP Committee 3, ICRP Publication 21, Pergamon Press (Oxford, 1973)