Booz, J.

Kellerer, Albrecht M.

English

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Radiation 'P'rotec~ion Dosimetry Vol. 12 No.2 p. 79-$1 ~1985' Nudear Technc.logy Publishin.g THE NEW OPERATIONAL QUANTITIES FOR RADIATION PROTECTION A. M. Kellerer Institut für Medizinische Strahlenkunde der Universität Würzburg Wiirzburg, Federal Republic of Gennany /NV/TED PAPER Abstract - Philosophies and quantities for radiation protectioI1 have often been subjected to changcs., and same of the developments are traced which ultimately led 10 recenl proposals by ICRU. Development in Ihe pasl ha •• argely been towardsclarification anrl genenl\isation of definitions. The presentchanges, however, reflect a more fundamental issue, the transition from tbe 1imitation system to the ass.essment system in radiation proteetion. The index quantities were suitable tool5 to ascertain compliance with the limitatioll system of radiation protection. The I1ew quantities proposed by ICRU are suitable estimators for effective dose equivaJent. which is an essential quatltity in the assessment system of radiation protection. A synopsis of the definitions is given. INTRODUCTION The history of radiation proteetion is also (he history of a succession of different quantities - or units, as they used to be called before the IeRU Came into existence. In the first decade of the century radiation dose was specified in terms of certain pills of yellow.green changeable colour, who.e ehemical composition was carefully concealed. Later, the 'pastille uni!' of liolzknecht and the subsequent one of Sabouraud were succeeded by less picturesque but not always less enigmatic quantities. As a rule, the eoncepts of radiation proteetion tended, and still tend, 10 retain elements of vagueness on different levels, Quantities can suffer from confusion, caused by technical deficiencies in definitions and by the degradation of definitions when they are put to practical and impraclical uses. More important are problems that arise from the very concepts and philosophies of a branch of scienee. It may be appropriate, beiore presenting a new set of quantities, to indicate briefly some of the ambivalences of the latter type in radiation prot~tion, and to relate theru to the definitions now chosen. TRANSITION FROM THE LIMITATION SYSTEM TO THE ASSESSMENT SYSTEM When I CRP classified the effects oi ionising radi at ions into two typest I), it took aecount of areal difference witb important im plications for the practice of radiation proteetion. Tbe choice of the desigoations "stochastic" and "non-stochastic" may 79 not have been entirely fortunate, and there ruay also be inslances- e .g. eertain prenatal effeets - where the classification remains doubtful. Nevertheless, the dislinetion can help darify the aims of radiation proteetion. Initially, radiation proteetion was merely concerned with the non-stochastic effects of high doses of ionising radiations, lt was, lherefore, natural to introdu~e a system of dose limitations with the intention of excluding any harmful effects of ionising radiations. Later,·hereditary damage and radiation cancerogenesis were recognised as risks of ionising radiations that could neyer be excluded with absolute certainty. Therefore, a different philosophy was conceived, which does not aim to eliminate risks, but 10 reduce them 10 levels that are deemed acceptable. Any optimisation must, of course, have the twofold aim of keeping the prob ability of harmful effects in an individual small, and of minimising - in a practicable manner - the total risk in an e"posed cOllective of person,. Therefore, in such a system, one aims not only at. the limitation of the exposure of individuals, but also a1 an assessment of overall exposure. Rossi has reeently given an account of the two principal systems in radiation protection, which he refers to as the limi talion and lhc assessment systems(2). Dose equivalent was introduced as a quantity for the limitation ,yslem. It was defined to apply to any specified point in an exposed body, and no delailed advice was given, where the relevant points of measurement in the body would bave to be chosen, if compliance with regulations had to be ascertained, 11Ie most conservativc and the common in terpretation was, that the dose eq uivalent had to be A.M.KELLERER kept - with some reasonahle averaging over small volumes - below specilied limits at any point in an exposed person. In practice this led 10 difficulties, at least under certain circumstances. If a small detector measured absorbed dose or dose equivalent in an unknown radiation lield, the result of the measurement would apply to the detector Of to struetures of comparable geometry, but it could not necessarily be related to values of dose that would be produced at various points in a human hody positioned at the loeation ofthe measurement. ICRU then introduced the index quantities(3) which are closely related to the maximum dose equivalent in a human body. The dose equivalent index is the maximum dose equivalent occurring in a spheneal phantom that is cenlred al the point of interest. Artificial as this coneep! may have appeared at the time, it was in fact merely a natural extension and an explicit statement of common pragmatic procedures; instruments had long been designed with the intention of determining a dose which included buildup and backscatter due to the body*. In common cases an actual exploration of the ICRU sphere was, of course, neither recommended nor required; the index quantities merely served to define formally the quantity that was previously used or aimed at intuitively. A few years ago, when the assessment system superseded the limitation system, inadequacies of the prevalent praetice began to be feit. Ostensibly, the criticism was directed al the index quantities, but inherently - and apart from a general desire for simplicity in measurement - it was mOlivated by a trend IOwards operalional quantities that were less eonservative and more c10sely linked to the a.sessmen! system of radiation proteetion, which requires realistic estimates rather than mere upper bounds of factual values. Tbe effeetive dose, i.e., the effective dose equivalent, H" introduced as a weighted average of the dose equivalents to the organs of a person(1), is-except for minor lechnieal deficiencies of the definition - a suitable. weU-defined and by now widely aecepted quantity. It is sometimes asserted that H. is not directly measurable. However, tbis qualifieation is sufficiently loose to apply, depending on the eonception of a "direct measurement", to any quantity. In fact, H, is difticutt to determine with high aceuracy, but it rarely requires such adetermination. In praetice, erude estimates of the effeetive dose are u,uaUy sufficient. Various instruments or "Even with the early dosimetrie methods. which hardly met pre~n\ metrologlcal standards, tbe presence of the bDdy had 10 be erudely simulated .• llhough Ihis proeedure was still purely empirie.l: "When in the holder. the pastille .. . should have a piece of metal as a backing. if its indications are to be accurate"(4-}. computations can provide such estimates. However. the need for simplifications and approximations can not obviate the need to base measurements and design criteria for instruments On rigorous definitions and clearly stated conventioos. lt was, therefore, feit desirable to develop certain concepts and quantities that could be u,ed in radiation protection practiee and that could serve as estimators of the effective dose., or of potential effeetive doses, in persons. An ICRU Report Comrnittee, chaired by T. E. Burlin and sponsored by G. Cowper and D. Harder, has covered the groundwork that led to the present reeommendations. From this extensive study two further comrnittees of the ICRU and the Main Commission have then distilled definitions which were intended to be sufficiently simple and clear to speak for tbemselvcs(51. Under the assumption that this aim has been reaehed, they are here eited with a few comments in abbreviated form. DEFINITIONS OF THE NEW QUANTITIES The new quantities for environmental (area) monitoring are compromises between authenticity and abstraction. For conceptional simplicity and for praeticability of measurement they are defined as point functions, i.e., their values al a specified point depend only on the radiation field at this point. Nevertheless, they are related 10 an extended, remolely anthropomorphic phanlom - the ICRU sphere. To resolve this apparent contradiction, the somewhat artificial concept of an expartded field is required; it is the uniform radiation lield that agrees with the actual field at the specified point. Tbe principal quantity for area monitoring is, moreover, designed to be independent of the angular distribution of the radiation field, which requires the further abstraction of an aligned, expandedjield. This is the uniform, unidirectional field that has the same fluence distribution as the expanded field. Using these two auxiliary coneepts, one can define a quantity for the environmental monitoring of penetrating radiation: Tbe ambie", dose equivalent, H*, at a point in a radiation field. is the dose equivalent produced by the aligned, expanded field in the ICRU sphere 10 mrn below the point of normal incidenee. For individual monitoring of penetrating radiations one can use a partly amdogous quantity; Tbe individual dose equivalent, penelrating, Hp, is the dose equivalent in soft tissue 10 mm below a specified point on the body. 80 In certain circumstances, a depth different from the reeommended vatue of 10 mm may be chosen for H· or Hp: it must then be indicated. H* and Hp can serve as estimates, usually conservative, of the effective dose if properly OPERA TIONAL QUANTIT1ES FOR RA DIA T/ON PROTECTION applied. For most penetrating radiations they can also serve to ascertain eompliance with the limits for organ doses. For weakly penetrating radiatioI;ls, where the skin (or the lens of the eye) may be the Iimiting organ, one requires additional quantities: The directional dose equivalen/~ Hf. at a point in a radiation field, is the dose equivalent produeed by the expanded field in the [eRU sphere at a depth of 70 I-'m on the radius in a specified direction. Tbis quantity can be used for environmental monitoring, and the corresponding quantity for personal monitoring of weakly penetrating radiation is entirely analogous: Tbe individual dose equivalent, superficial, H" is the dose equivalen! in soft tissue 70 J.'m below a specified point on the body. With the quantilies H' and H" there too may be ciTeumstances. where a depth different from the reeommended value is chosen; it must then be indicated. CONCLUSION H* and Hp serve as estimators of the effeetive dose that either would be produced in a person al the monitored loeation or was produeed in the person monitored with the individual dose meIer. H· is independent of the direelional distribution of the radiation fieId. To aecount far the warst case, usually frontal irradiation, the definition must, therefore, be inherently conservative. Large values of H '/H, oeeur. particularly, for neutrons between 100 keV and 1 MeV. Tbis could be a reason to reeonsider tbe reeommended depth for H'. Nevertheless, the degree of conservatism is for H'" I at REFERENCES least for unidirectional fields, less than that of the index quantities. In eertain instanees H' could even slightly underestimate the effective dose. The relation between Hp and the effective dose depends, of course, on the proper placing of the personal dosemeIer. By only using a single dosemeter one monjtors. in effect, merely the frontal half-space of the person. Additional precautions may, therefore. be required to ensure that no irradiations remain undetected_ From this point of view, it is somewhat undesirable that the definition of Hp implies reduced response to lateral incidence for certain radiations. For this reason a modified definition was considered which eould lead to a more nearly isotropie response Over tbe frontal half-space. However, it was then feit that commonsense ftexibility in the implementation ofthe definition will suppress undesirable. formalistic. extra steps that might be taken to reduce the effective acceptance angle oi a personal dosemeter. If a doserneter res ponds properly to frontal irradiation, its suitability will not be reduced if its lateral response happens to be somewhat larger than isrequired by the definition. Little needs to be said about the quantities H' and H,. Tbeir definitions are effeetively equivalent and they require identical calibration procedures in the case of weakly penetrating radiations. H' mayaiso be of interest for larger values of d and would. in this case, be largely equivalent to Hp_ However, an instrument used to measure H' would then - in the same way as an instrument for H· - demand built-in characteristics that represen! the spherieal phantom stipulated in the definition. Personal dosemeters require only part of such characteristics, sinee they are worn on the body and calibrated on its surrogate, the ICRU sphere. 1. ICRP. Recommendations of the International Commission on Radiological Protection. Publication 26 (Oxford: Pergamon) Ann. ICRP 1 (1977). 2. Ross!. H. H. Limitation and Asses.sment in Radiation Protection, and Measurements. Lecture No . .8 (Bethesda: NCRP Publications) (1984). 3. ICRU. Radiation Quantitj~s and Units. Report33 (Washington, De: International Commissionon Radiation Units and Me,surement.) (1980). 4. Jones, H. L. X-Ray Treatment. Encyclopaedia Britannica 11th edn. Vol.:I8, 887-889 (1910). 5. ICRU. Determination 01 Dose Equi'lalenrs Resultingfrom External Radiation Sourees. Report 39. (Washington, De: International Commission on Radiation Units and Measurements) (1985). 81 

The new operational quantities for radiation protection

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