Dosimetric quantities

Draft document: Dosimetric quantities
Submitted by Eero Kettunen, STUK
Commenting on behalf of the organisation

STUK’S COMMENTS FOR THE ICRP DRAFT, “BASIS FOR DOSIMETRIC QUANTITIES USED IN RADIOLOGICAL PROTECTION” (21/110/05) 1. Comments on the concept of effective dose There are two novel interpretations in the draft: defining the effective dose in a specified phantom and limiting its use to prospective evaluations only. We disagree with both of these interpretations and hope that the Commission will reconsider these changes. 1.1 Defining the effective dose using the organ doses of standard phantoms instead of human beings The effective dose has until now been defined in accordance with the mean absorbed doses in various organs and tissues of the human body. It has not been defined in a specified model of a reference human being, although various models are used when the effective dose is calculated. This has been partly accepted also in the present draft (e.g. beginning of sections 5.5 and 5.8). Elsewhere in the document, however, it seems to be the intent of the Task Group to define the effective dose only with respect to a few standard computational phantoms (e.g. page 36, first paragraph and page 39, 4th paragraph). We understand that the proposition of using only standard phantoms would standardize the concept of the effective dose and reduce variability in assessing it. We consider, however, that the improved accuracy is superficial – it does not improve the accuracy of assessing the doses to actual persons or populations. Also, in principle, defining a dosimetric quantity only in computational phantoms prevents any attempts to measure the dose using real phantoms. Of course – as noted in several places in the document – these quantities cannot be measured in real persons but must be approximated by calculation. In our thought it should be acceptable to use whatever available human models to do so – different models just result in different approximations of the quantity. In particular, it is difficult to understand the reason why it would not be acceptable to use data from individual persons in assessing the effective dose (e.g. page 15, next to last paragraph and page 38, first paragraph). If such data are available and can be used practically, we think that they should be used. We recommend that the concept of the effec-tive dose (as a quantity) is left without a specification of the phantoms. The definition of the effective dose should refer only to a human body, not to its substitutes. In this respect the concept of effective dose should not try to approach the concept of the operational quantities. Instead of including the ICRP reference phantoms as a part of the definition of the effective dose, these phantoms should be introduced as a specific case of phantoms (reference conditions or reference model) that are useful for evaluating the effective dose. In this form the approach of specifying reference phantoms (ICRP refer-ence model) would properly help to standardize the calculation of effective dose. 1.2. Is it always necessary to interpret the effective dose as a population and gender average? ICRP has introduced the effective dose as a useful quantity for cases where the exposure is not uniform in the whole body. How can such exposure situations be handled in the case of external radiation using fixed male and female phantoms (of various ages) and averaging over the phantoms (Eqs. 3.6 and 5.9)? There is no way of unambiguously locating the phantoms of different size in the radiation field. This problem is easily solved, in principle, if it is allowed to estimate the effective dose in a single person, by taking into account the known exposure conditions of the person in question. We also point out that effective dose has been used for many purposes that may be out-side the original intent of the quantity. One such application is the exposure of patients to radiation in diagnostic x-ray examinations; there are hundreds of papers (including UNSCEAR reports) using effective dose to express the exposures of the patients. This indicates clearly that effective dose – or a related (presently non-existent) quantity – is needed also for this purpose. The effective dose is not directly related to the risk of the patient, but in lack of age- and gender-specific risk-related tissue weighting factors it is still the best available single dose quantity for this purpose. Apart from the note that it does not directly express the risk to the patient, we cannot see why the quantity couldn’t be used for the patients, too. It is reasonable that the tissue weighting factors have been set by considerations of the radiation detriment on the population level. This should not be taken to imply that the effective dose of individuals should also express the risk in the individuals. However, the Task Group seems to use this argument against using the effective dose to actual persons. We see this matter differently: to apply to individuals the effective dose needs not be an accurate descriptor of risk (taking into account the gender- and age-dependence of the risk) but a reasonably defined equivalent of whole-body dose. The set of tissue weighting factors can be considered to broadly describe the significance of the doses in the various organs. We suggest that the definition of effective dose would be modified so that it could also be calculated for individual persons or for male or female population groups. This would only require guidance on how the doses in gender-specific organs should be treated. One suggestion is that the tissue weighting factors are kept as proposed, but the doses to those gender-specific organs (breasts, ovaries, testes, uterus, prostate) that are not present or not in risk in the specific person or group, are treated as being zero. It would be illogical to include doses in organs that the person or group does not have. 1.3. Restricting the use of effective dose to prospective dose evaluation The draft states that effective dose is intended to be used only in prospective dose assessments (the last paragraph on page 38, the first paragraph of section 5.5 and the last paragraph on page 46). We note, however, that effective dose is extensively used also for regulatory purposes to set dose limits (e.g. section 5 on page 33 and 4th paragraph on page 38). Thus the effective dose of a specified person needs sometimes be evaluated retrospectively to see whether the dose limit has been exceeded. Of course, if the intent in the retrospective dose assessment is to end up with an actual risk assessment, as discussed in the text, it is better to make the assessment based on the organ doses. The same applies, if the dose in any organ may be high enough to cause deterministic injuries. Such detailed information of the low risk from low doses is not usually required, however. It is often sufficient – and more simple – that the person’s exposure is described with a single number, even if it is not an accurate descriptor of the individual radiation detriment. Effective dose, calculated for the person(s) in question (as accurately as is practicable) or for a standard phantom (if specific data on the person is not available or cannot or need not be used), will then accomplish the task nicely – as it has done in the past. If it is explicitly stated that this is not proper, what quantity should be used instead, especially in the case that regulations specify dose limits in terms of effective dose? 1.4. Summary of our comments on effective dose In summary, we hope that ICRP reconsiders its position in changing the definition of effective dose in the way inferred from the draft. The concept of effective dose has proved to be useful and should not be modified in the manner implied. The concept could be developed by defining it so that gender-specific phantoms could be used. It is not always necessary to take the average over male and female data: depending on the case, effective dose could sometimes be calculated for male or female persons as well. The minimum needed for this would be just deciding how to deal with the gender-specific organs (e.g. by assigning the non-existent organs a dose equal to zero). We also suggest that the earlier interpretation is kept: organ doses in humans are meant, in principle. It should be acceptable to calculate effective dose - to various degrees of approximation - by using different kinds of phantoms or models of the human body. We also hope that ICRP will reconsider the unnecessary limitation of the concept to prospective dose assessments only. 2. Other comments on substance matters (Page 4 and later): The term “tissue reactions” is not an optimal choice, we think. The implications of the term are more general than the meaning specified in the draft. There are reactions of some kind also at dose levels much lower than intended in the definition of the term. For example, on page 10, 3rd paragraph, “biological reactions” are mentioned when discussing stochastic health effects. It is likely that the adoption of the new term would cause more misunderstandings than the presently used term “deterministic effects”. (Page 8, next to last paragraph): We do not favour the proposition of defining the quantity “RBE-weighted dose” and stating the RBE value used along with the value of the quantity. It would be better to separately state the absorbed dose and the RBE value that can be used with it. (Page 18, 2nd paragraph, Sections 3.4.4 and 5.2.1): The use of lead aprons in medical radiology is a practical rule. It would be useful to mention the implications of this on the relationship of the monitoring quantity and the effective dose. Neither does section 5.6 discuss external exposure from non-homogeneous radiation fields and partial body exposures at all. (Page 20, Section 3.4.4): The definition of the personal dose equivalent differs from the definition in ICRU 51. (Page 37, eq. 5.8): The concept and use of the effective dose to reference persons of different ages are not explained in detail. Elsewhere in the document one gets the impression that such age-dependent effective doses are not tolerated (see, e.g. page 4, 4th paragraph). (Page 40, last paragraph): The “gender- and age-averaged effective dose” is discussed without specifying it sufficiently. Equation 5.9 considers only the gender-averaging part and is incomplete: there are also other gender-specific organs in the tissue weighting factor list than just the breast (ovaries, testes, uterus and prostate). The treatment of these organs should be clarified: gonads mean the testes in males and ovaries in the females, the remainder includes the prostate only in males and the uterus only in females. (This seems reasonable, although p. 33 (3rd paragraph) says that the remainder weight is divided equally between the 15 specified tissues. However, only 14 of them are present in a phantom of a given gender. The weight of each should then be approximately 0,00857 instead of the value 0,008 given in the text.) We also note that the weighting of the testes and ovaries seems to be different from ICRP 2005 draft recommendations (where the mass-weighted average is used – for reasons unknown to us). The discussion below eq. 5.9 seems to be non-symmetric because only the ovaries are discussed. (Section 5.8): We disagree with the idea of neglecting small doses in the calculation of the collective dose. The need for their inclusion is a direct consequence of the LNT hypothesis and necessary for controlling small exposures of a large number of people. It is necessary for both justification and optimisation purposes when considering practices that will expose the population widely. How can such practices be assessed if these small doses (< 10 µSv) are taken as zero? It can also be noted that the dose limit for the public might be exceeded if there were 100 such wide-exposure sources, each with a collective dose of zero. This idea of neglecting low doses is in conflict with the discussion elsewhere, where it is stated that the whole system is based on the LNT assumption (which is seen well indicated although scientifically unproven, e.g. on page 44). (Page 56, Table 2): There are several tissue weighting factors different from ICRP 2005 draft recommendations. All the values and foundations should be checked carefully. 3. Comments on the glossary The glossary should be checked thoroughly. The terms should be written in lower-case letters (as they generally are in terminological glossaries). Especially, the names of units (becquerel Bq, gray Gy, sievert Sv) should be lower case to show the correct way of writing according to the SI system. - Activity, A: the expected number of spontaneous nuclear… - Annual limit on intake: check the sentence. - Collective effective dose: the use of upper and lower limits for the integration is unacceptable. - Intake: This word is used in two different meanings – as a general concept and as a quantity. The explanation in the glossary represents more the general concept, or the definition of uptake (enters the body…through the gastrointestinal tract). For definitions of intake and uptake, see ICRP publication 71. - Kerma, K: …the sum of the initial kinetic energies… - Linear energy transfer (LET): The definition here is not correct. See ICRU reports. - Particle fluence: …is the expected number of… - Protection quantities: change “values” to “quantities”. - Radiation quality: the common use of the term does not refer to the biological effec-tiveness, but to the type and energy of radiation. Therefore, change “radiation quality” to “quality factor”. - Reference person: The reference should be ICRP 2002. The reference does not define the reference person completely, however. - Reference value: the term is used (also by the ICRP) for many other values in addition to the specified use here. - sievert (Sv): Add the quantity “dose equivalent” in the list. We suggest that the names of units would be written with lower-case letters also in the glossary. -Specific absorbed fraction: Replace “1 kg” by “unit mass”. -Tissue weighting factor: …to the total stochastic detriment…. Also, it might be con-sidered if the word “represent” should be replaced by “approximate”. 4. Editorial comments The use of capitals in section titles should be uniform throughout the paper. Extra capitals should not be used without good reasons. (Page 2): Remove extra capitalization from “Stochastic effects” and “Tissue reactions”. (Page 6, 2nd paragraph): Delete the explanation of LET in parentheses. It was explained in the previous paragraph already. (Page 6, 3rd paragraph): There is no ICRP 2005 in the reference list. (Page 7, 2nd paragraph): Move the sentence “No values different from 1…” to the previous paragraph, after “…than low-LET radiations.” (because N is not applicable to equivalent dose but to dose equivalent). (Page 7, last paragraph): Replace “…protection system and in accident situations radiation…” by “…protection system (in accident situations) radiation…” (Page 8, first line): We propose different punctuation for clarity: “These result from the impairment of the integrity and function of organs and tissues; clinically observable damage occurs above a threshold dose, and the extent of any damage depends upon the absorbed dose and dose rate, as well as on radiation quality.” (Page 9, 5th paragraph): …dN is the expected number of…, …fluence is sometimes alternatively… (Page 10, three first paragraphs): Unnecessary repeating of previous text. (Page 10, 4th paragraph): … below (see Section 5.4). (Page 10, last paragraph): The absorbed dose cannot be determined from a primary standard just by computation, without a measurement. The sentence should be clarified. (Page 12, 3rd paragraph): This approach implies that at low doses, … , and hence the additivity of doses, are being assumed for radiological protection applications. (Or, alternatively: This approach assumes that at low doses…) (Page 12, 4th paragraph): Spelling error in …connective tissue… (tissure). (End of page 12): Remove hyphenation from “alpha particles” and “beta particles”. (End of page 14): … the radiation-weighted dose … (see also Section 5.4). (Text on page 14 and later): There is variability in using the hyphen: “radiation-weighted dose” or “radiation weighted dose” . Both are used in the text. (Page 15, 3rd paragraph): The reference should be specified as ICRP 1996b. (Page 17, 3rd paragraph): “…controlled or restricted areas…” should be “…controlled or supervised areas…” (Page 17, 5th paragraph): The reference should be specified as ICRU 1993b. (Page 18, 3rd paragraph): The expression “…charged particle build-up is only performed in…” is unclear. Spelling error in …build-up… (built-up). (Page 21, 3rd paragraph): The first sentence is difficult to understand. The correlations of the words “their” are not clear. (Page 23, 1st paragraph, page 25, 3rd paragraph, and page 29, 3rd to last paragraph): Change “radiation quality” to “quality factor” or “RBE”. Reason: radiation quality of a specified radiation type refers to the energy of the radiation, not biological effectiveness. (Page 24, next to last paragraph): Spelling error in …value… (vaue). (Page 25, 3rd paragraph): Spelling error in …following… (folloewing). (Page 27, 2nd paragraph): The references should be SSK 2005, Dietze and Alberts 2004. (Pages 27 and 28, eqs. 4.6 and 4.7): In principle, logarithms can be calculated only from dimensionless numbers, e.g. from En/MeV. If En means just the numerical value (without unit), the relevant unit (MeV) must be specified. (Page 28, 1st paragraph): Spelling error in …is… (is0). (Page 28, 3rd paragraph): Spelling error in ….for very… (forvery). (Pages 30 - 31): Consider wordings “alpha particles” and “alpha emitters”. In the quotation from ICRP 92, check the hyphenations of “á-rays”. (Page 33, 4th paragraph): …radiation-weighted dose or effective dose. Neither quantity can be… (Page 34, 3 - 5th paragraph): In IEV 393 the names of these quantities are “specific activity”, “volume activity” and “surface activity”. See also ICRU 65 for “activity density” and “specific activity” (with another meaning and definition). (Page 35, next to last paragraph, end of page 37): The abbreviation AMAD has not been explained. (Page 36, 1st paragraph): Such phantoms are usually called voxel phantoms. There are also other types of computational phantoms. (Page 36, eq. 5.6): In order to have the dimensions of the equation correctly the value 2400 should include also its unit, i.e. 2400 m3. (Page 37, eq. 5.7): In order to have the dimensions of the equation correctly the value 2000 should include also its unit, i.e. 2000 h. (Page 37, Section 5.3): “operational exposure” should be “occupational exposure”. (Page 37, eq. 5.8): The equation relates only to the case of radioactive materials. Why is other external radiation omitted? (Page 39): ICRP 1996 should be ICRP 1996b, and ICRP 2003 should be ICRP 2002.