Dosimetric quantities

Draft document: Dosimetric quantities
Submitted by Prof Dr W-U Müller, Dr J Böhm, SSK (German Comm. on Radiol. Prot.)
Commenting on behalf of the organisation

[THE FOLLOWING COMMENTS CONTAIN SOME SYMBOLS, FIGURES, AND EQUATIONS THAT CANNOT BE REPRESENTED IN THIS VIEWING BOX FORMAT. THEY ARE AVAILABLE ON REQUEST FROM THE SCIENTIFIC SECRETARIAT] Introduction The comments focus at the consistency of the proposed new concept of dosimetric radiation protection quantities with particular attention to external radiation. The concept described by the task group of ICRP slightly differs from that published earlier(1). The earlier version was recently reviewed(2). Some proposals for changes are in accordance with this review. Operational quantities representing the protection quantities At present, dose limits are recommended for external exposure for the quantities effective dose, equivalent dose in the lens of the eye, the skin, and the hands and feet. The control of the dose in the lens of the eye and the dose to the hands and feet should be added to the table given on page 17 of the draft. One should carefully examine whether H'(0.07, Ω) and Hp(0,07) would be the appropriate operational quantities for the control of all kinds of radiation (including photon radiation). Low energy photons are too much attenuated by 10 mm tissue and therefore Hp(10) is not a conservative estimate for measurements at 3 mm depth. It is welcome that the confusingly large variety of operational quantities H*(d), H'(d, Ω) and Hp(d) with potentially different d values is now reduced to the few essential ones. The smaller the number of operational quantities to represent the protection quantities, the lower are the expenses for routine monitoring as less types of monitoring instruments are required. But the operational quantities should not only be denominated by physical symbols but also by simple names (compare also (5)). It is proposed to name • H*(10) as ambient dose equivalent, • H'(0.07, Ω) as directional dose equivalent, • Hp(10) as personal deep dose equivalent; deep is preferred to penetrating because deep characterises a “deep” depth of 10 mm below a specified point on the body and not the penetrability of the radiation. • Hp(0.07) as personal surface dose equivalent; surface is preferred to superficial because (similar as above) surface characterises a depth of 0.07 mm (which is close to the surface) below a specified point on the body and not the penetrability of the radiation. The point quantity Hp(0.07) can be represented by its average quantity (averaged over 1 cm²) as ICRP recommends a skin dose average over any 1 cm², regardless of the area exposed, whereby the nominal depth in skin is 0.07 mm. • H'(0.07) as maximum directional dose equivalent. As stated on page 20 of the draft,  in the bracket of the quantity H'(0.07, ) is often not specified. Consequently, it should therefore be omitted. The angular dependent quantity H'(0.07, ), in general measured with portable monitors, can be represented in radiation protection practice most frequently by its maximum value at the point of interest as a health physicist will turn around his monitor in the radiation field looking for safe data for the optimisation of preventive measures. Usually only this maximum value is recorded. The concept of the dose quantities with the names proposed above is shown in the figure for external exposure. On the left side one can see the operational quantities which are all dose equivalent quantities. On the right side one has the limiting quantities which are all protection quantities for which dose limits are recommended. In Figure 1 of the ICRP draft the limiting quantities equivalent dose in the lens of the eye, the skin, and the hands and feet are missing and should be supplemented. Figure [NOT VISIBLE HERE]: System of quantities for use in radiological protection for external exposure. The nomenclature of the protection quantities is according to draft, the nomenclature of the operational quantities according to the proposals in the beginning of this chapter. In the case of internal exposure the operational quantities are supplemented by activity quantities in combination with models and computations; the protection quantities are supplemented by committed doses HT() and E(), and by the collective effective dose, S. Measuring the unmeasurable The effective dose is principally not measurable, but may be calculated for external exposure from spectral fluences of the particles of the radiation field incident on the human body. The measurement of the spectral fluences with successive calculation of the effective dose is much too difficult for routine radiation protection. As a way out, operational quantities for dosimetric measurements may be used which in most cases enable a sufficiently accurate dose determination with only fair knowledge of the radiation fields. The general conclusions of the Joint Task Group was (3,4) that the operational quantities provide a satisfactory basis for most measurements for radiological protection against external radiations. In routine personal monitoring the following simple method is commonly applied. If the measured personal dose is obtained by a properly functioning personal dosemeter worn at the representative place of the body and the dose value is far below the dose limit, the measured Hp(10) value is regarded as the value for effective dose E. For high personal doses (i.e. if the values approach or exceed the annual dose limit) or in strongly inhomogeneous fields, the application of dose modifying factors for the conversion of the measured personal doses into E according to the irradiation conditions at the work place is recommended (see(6)). If this procedure results in an effective dose above the limit, more specific circumstances of the exposure and information on the individual are needed to calculate organ and tissue doses. The statement of eq. (5.4) [NOT VISIBLE HERE] in the draft without giving the background information contained in the preceding paragraph and without explicit warning is dangerous: eq. (5.4) is an approximation valid only below dose limits. It is recommended both to include this warning in the written text and to replace the equality sign by a respective sign indicating an estimator or approximate equality. Otherwise eq. (5.4) might be applied without any restrictions with respect to the value of the measured personal dose. Such use of eq. (5.4) would, however, be consistent with relation to other everyday hazards whose limits are specified directly in measurable quantities and not in terms of the biological effect or risk to an individual. It is remarkable that the use of Hp(10) and H*(10) being a very conservative estimate of E for photons with energies between 10 keV and 40 keV is regarded in the draft to be very welcome to compensate the value of wT = 1 for the RBE much higher than 1 in this energy range. Attention has to be paid for photon radiation with very low energies (energies below about 15 keV). Even if eq. (5.4) would yield very low E values, e.g. the dose limit of the skin dose might be exceeded. It should be mentioned that for the routine monitoring of air crews Hp(10) in eq. (5.4) can be replaced by H*(10) which can be calculated in good approximation from the relatively well known distribution around the earth only slowly varying with the solar cycle. Tissue reactions The new term "tissue reactions" does not provide an obvious improvement compared to the term "deterministic effects" that has been used for decades and is well established in the scientific community. If there is no clear advantage or necessity, changes in terminology should be avoided. Use of the quantity N in different context Various quantities N are used in various contexts throughout the document: On page 7, equation (2.1), N is "the product of all other modifying factors". On page 9, equation (3.1), dN is "the number of particles incident upon a small sphere …" On pages 33-34, equation (5.1), dN is "the expectation value of the number of spontaneous nuclear transitions …", and On page 43, equations (5.11) ff, dN is "the number of individuals who experience an effective dose …" This should possibly be avoided. For "the product of all other modifying factors" it is proposed to chose a completely different name, whereas denominating "numbers of" something by the letter N seems quite logic. Here a distinction could be achieved by applying subscripts, e.g. Np, Nt, and Ni for the number of particles, nuclear transitions, and individuals, respectively. Organs contributing to effective dose On page 32 (line 12), kidneys are mentioned as organs for which specific wT values are now given in addition to those from ICRP Publication 60; on page 56, however, kidneys are listed among the Remainder Tissues. References 1. International Commission on Radiological Protection. 1990 Recommendations of the International Commission on Radiological Protection. ICRP Publications 60, Annals of ICRP 21 (1-3) (Oxford: Pergamon Press) (1991). 2. Böhm, J. and Thompson, I. M. G. Adaption of the Present Concept of Dosimetric Radiation Protection Quantities for External Radiation to Radiation Practice. Radiat. Prot. Dosim. 109(4), 311 (2004). 3. International Commission on Radiological Protection. Conversion Coefficients for Use in Radiological Protection Against External Radiation. ICRP Publications 74, Annals of ICRP 26(3-4) (Oxford: Pergamon Press) (1997). 4. International Commission on Radiation Units and Measurements. Conversion Coefficients for Use in Radiological Protection Against External Radiation. ICRU Report 57 (Bethesda, MD: ICRU Publications) 1998. 5. Goldfinch, E. P. Measurement ‘Quantity’ for Extremity Doses – What’s in a Name? Radiat. Prot. Dosim. 69(30), 163-164 (1997). 6. Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit. Berechnungsgrundlage für die Ermittlung von Körperdosen bei äußerer Strahlenexposition. Veröffentlichungen der Strahlenschutzkommission Band 40 (Urban & Fischer München, Jena) (2000).