ICRP has brought out draft on very important topic use of effective dose as radiological protection quantity. This was required since long to clear misunderstanding or improper interpretation and use. It is also important to give sufficient time to the stake holders to study and comment on the draft. It is expected that ICRP will go through the comments and ask clarification if required or incorporate the comments or modify the draft accordingly.
We have gone through the report and have some suggestions as given below
Abstract Line 96-101 and section 5: Derivation of effective dose for comparison of non -whole body (partial) dose, which is the case in most of the medical examination, is not the correct practice and should not be encouraged. Since estimation of effective dose E, would be based on tissue weighting factor which itself is inherently based on uncertainties such as sex, age, ethnicity etc and exposed organ dose. Measurement of organ dose will be another issue as volume exposed, non-uniformity in field over the exposed volume etc needs to be accounted. Therefore, such intercompares based on E should best be avoided and more reasonable quantity as discussed in para 96 and such other quantities depending upon examination may serve the purpose in better way.
Comparison of alternative techniques may better be dealt through justification of practice and optimisation, as attributable risk at the level of diagnostic examination doses, LNT, DDREF, estimation of organ dose, age, sex, ethnicity, life style and many other confounding factors would make it meaningless. Brenner has brought out some solution in this direction.
ICRP may issue separate guideline on this.In any case, if ICRP intends to use E for medical examination, it should include the guideline for its use in section 5.
Line 113- 115
Although limit on equivalent dose is for tissue reaction, it needs to be further considered that, due to uncertainties associated with tissue weighting factor, E will have more uncertainties. ICRP has correctly stated that dose limits or risk coefficients should not be used for calculating or predicting risk; organ dose/equivalent dose would result into smaller value of E, making comparison less realistic. It is also well accepted that the estimation of risk coefficient has in built uncertainty of factor of 2. Such concept needs further review and use of E for even approximate risk estimation should be discouraged, more so for organ/equivalent dose.
Absorbed dose in place of equivalent dose
Use of absorbed dose instead of equivalent dose for limits on tissue reaction is important issue. The main reason is said to be unit which is same for effective dose and equivalent dose and further realisation that WR is overestimated for tissue reaction and equivalent dose is highly conservative.
The proposal seems to be scientifically more complex and may lead to interpretation in different ways in different places. For example, ICRP says that if equivalent dose for specific organ, say skin dose, is significantly high, appropriate weightage is to be given for such organ specific dose to effective dose. In case of skin dose, it says the weighted skin dose (over the whole skin) may be multiplied with WT to be account in E. As per new interpretation such concept may create more confusion.
It is also important to convince the stake holder that tissue reaction will be nearly same for different radiation, for example, 100mGy of gamma of suppose 1MeV will have same tissue reaction as 100 mGy of Neutron (intermediate or high energy) for a given tissue.
Although the ambiguity between equivalent and effective dose will be solved but this is not going to solve the fundamental problem and may not be conceptually correct. as noted, the absorbed dose does not take into account the effectiveness of radiation in causing the damage. it has also been noted even for tissue reaction, type of radiation matters albeit to a lower extant compared to the stochastic effect. it does not say there is no difference or the WR for neutrons or alpha or heavy charged particle is closer to unity. if the WR for N, alpha, fission fragments etc for tissue reaction is not close to photon, it will be wise to give separate name for the unit of equivalent dose other than Sv,or Gy which essentially will differentiate all the three even though the basic unit remains J/kg.
We feel, to overcome the difficulty, a separate symbol/unit such as Heq or Deq etc may be defined to take into account the radiation effectiveness (which may be different from the one recommended for stochastic effect). In any case the uncertainty needs to be taken into account and as practiced, conservative estimation would be used.
Use of E above 100mSv
It is correctly pointed out in Parra, 46 that there is no reason why E should not be used above 100mSv. It is a welcome argument. In fact the whole dose range for which stochastic effect is considered, E should be allowed to be used. as it has been defined solely for stochastic effect. This is also important to note that at higher dose both tissue reaction and stochastic effect would be expected. Tissue reaction, being the immediate concerned, is taken into account and medical intervention is applied to control the reaction. But the medical intervention would not reduce the probability of stochastic effect. It is also important to note that the dose in A-bomb survivor was >100mSv which forms the basis for the nominal risk coefficient and stochastic effect.
Therefore, use of E at higher dose should be emphasised.
Use of E in partial exposure
Parra 46, point 1, 973-977
Use of effective dose, in cases where dose to some organ (eg-thyroid) is much higher, must be limited and there must be clear guideline. concept of effective dose, for radiation protection purpose should be used when significant (may be more than half) portion of the body is exposed. small exposed mass/organ or organs with low value of WT will lead to small value of Effective dose but may cause severe tissue reaction in the exposed tissue. therefore, guideline should be established on when to use effective dose in case of partial exposure.
Use of DDREF
As discussed in Parra 46, point 2, the dose rate at which DDREF should not be applied needs to be specified in clear terms. It should discuss DDREF as a function of dose rate.
Skin Dose
In the context of variation in skin thickness with age and also with reference to task group (1991a); basis for considering 70µm skin thickness as most appropriate. it may not be that conservative at least for lower age group,for whom, the skin damage will matter most.
Partial skin exposure: Conceptually, the approach described in para 56 is correct. But from practical point of view, it is hard to practice, particularly for external source (other than surface contamination). therefore, it would be easier to give guidelines on application of averaging.
Operational quantity: this section may be worked out after taking into consideration the final report of the ICRP on the operational quantity.
Collective dose
Against the stipulation mentioned in para 66 and other ICRP reports, most of the epidemiological studies particularly related to radiation worker, use the collective dose or its distribution to estimate the risk and study the LNT. ICRP accepts finding of such studies and uses for its recommendations. This contradiction should be cleared. If collective dose is not be used, then appropriate recommendation on epidemiology should be included.
Use of collective and individual dose estimate to optimise the protection as stipulated in parra 69 is highly impractical as increasing the number of worker, the individual dose could be reduced keeping the collective dose same. There, it is also important to optimise the number of workers involved. Ultimately a very high increase in number of worker may reduce the individual dose to trivial level. Risk in such cases optimisation/risk based on collective dose would not represent the correct risk.
Further as stipulated in parra 92 line 1551-1555 ‘there can be situations where the estimation of health effects from collective effective doses can be useful for planning of radiation protection actions’ needs to be clarified as, collective dose in itself may be misleading as it does not give information about number of worker, dose to individual and its distribution. In situation of nuclear accidents or its advance planning, the collective dose alone would not be useful as stated. in such situations, knowledge of the number of person is more important as the dose will be nearly uniform in clusters when direction and distance are taken into account. Therefore, groups of persons may have nearly uniform dose but the knowledge of number in the population group would be required.
Further as stipulated in parra 92 , 1558-1564, whether epidemiological studies may provide statistical significant results is also against its own decision on estimation of risk using collective dose. ICRP should clarify whether at low doses epidemiology should be considered at all keeping into consideration the uncertainty associated with the system.
As stipulated in para 116, that the age specific risk variation is between 4-9 times for ages 0-9 years and 60-69 years with mean around 30-39 years. Is single risk coefficient and dose based epidemiology will serve any purpose in this high variation.
Specific comments
309-312
Calculation of E is basically a theoretical/conceptual consideration. In practical, it is measurement for uniform exposure to whole body, or partial or organ/tissue specific exposure. The measurements, using proper calibration is said to be the estimate of E for a reference person which does not exist and no risk data, whether LSS study or any epidemiology, exist for reference person.
It is also important to consider that using the risk coefficient and dose to an individual, risk of reference person is estimated not to the individual exposed who might have quite different risk keeping in view the large number of compounding factors. One can rightly question the relevance of such concept for himself.
326-330
''at low doses and dose rates, simplified risk adjustments do not fully reflect our scientific understanding of radiation risk.'' but the validity of E , in general is restricted to this range itself. Therefor E seems to be dosimeteric quantity in uncertain area lacking scientific understanding? better approach with some understanding should be adopted.
370-371
Whether the smaller difference between radiation type is negligible and whether the radiation weighting factor can be derived as a function of dose and dose rate rather than type of radiation?These issues should be looked into for better understanding.
Fig. 3.2
WT should be applied before averaging equivalent dose, as both male and female has difference in organ and their significance to quality of life. In fact, the effective dose to male and female should be averaged to get the dose to reference person.