Operational Quantities for External Radiation Exposure

Draft document: Operational Quantities for External Radiation Exposure
Submitted by Dr. M.S.Kulkarni & Mr. R.B.Rakesh, Bhabha Atomic Research Centre
Commenting on behalf of the organisation

The protection quantities have been revised, owing partly to the changes in application and partly to the changes in the particle types and range of energies involved. The operational quantities require revision for these reasons, but also for the limitations of the previous quantities. As per the draft report, H*(d) and Hp(d) are no more valid quantities and personnel and ambient dose (effective dose-maximum) are the revised relevant quantities.
There are certain points in the draft report which need to be clarified or addressed in the revised version.  Suggestions/comments on the report are given below:
General comments:
1. As the protection quantities are not measurable, it is advisable to define a quantity which is both measurable and serves as protection quantity. Alternatively, protection itself may be redefined in terms of operational quantity itself. Quantity which cannot be measured has little relevance in defining parameters and lacks reliability. Therefore, the quintities are required to be frequently redefined and modified. It leads to inconsistency in these quantities and issues related to addition of previous measurements performed using older quantities.  Simulations must be validated to the extent possible. It is to be noted that effective dose as a single risk related quantity for stochastic effect, is full of uncertainties related to sex, age, life style, race, geography etc.
2. The report does not talk about the difference between weakly and strongly penetrating radiations and their contribution to the dose.
3. The report is also silent on the contribution of high energy electrons to effective dose, as it is no more a weakly penetrating radiation.
4. Although the use of Gy instead of Sv for tissue reactions is laudable, it would be difficult to use Gy for tissue reactions involving high LET irradiations. It appears that the definition of absorbed dose defined for skin and eye lens holds only for photons and electrons and not for other radiations. For high LET radiations, Sv/Gy need not be 1.
5. The use of personnel dose and approximation of E is flexible, but in practical conditions, to find the point of maximum effective dose particularly with different field geometries, energy, radiation types etc is difficult. In general, the basis of operational quantities was based on the concept that Hp(0.07) is equal to or greater than Hp(10) for photons. This indicated that 0.07 mm was the point of dose maximum. By considering this, it can be concluded that 0.07 mm depth (for energies considered previously) will be the point of maximum effective dose.
6.  Conversion coefficients and the associated quantity should have clear distinction with different symbol rather than just capital and small letters.
7.  For monitoring purposes especially for photons, although fluence is a fundamental quantity, particle fluence to the protection quantities conversion coefficients are hard to use.  Further discussion on this would be useful.
8.  Sections 5 &7 may be part of appendix.

Specific Comments:
Line no: 239-240 sentence may be checked.
Line Nos.303-305: the statement is consistent with the ICRP reports but needs to be relooked. It does not address the point that how the stochastic effect is accounted if there is no deterministic effect (threshold > 100 mSv except for CA) or deterministic effect has been fully managed by medical intervention. Does it mean if deterministic effect is managed or there is no deterministic effect (>100 mSv, less than threshold dose), there will be no stochastic effect? Whether doses above 100 mSv will not form part of life time dose records? It is also important to note that numerical value of dose (> 100 mSv or < 100 mSv) is known only after evaluation based on calibration for respective operational quantity. It would be difficult to report the dose > 100 msv as absorbed dose based on calibration for operational quantity.
Line Nos. 307-308 “The RBE values will depend on the radiation type and energy, and can differ for different biological endpoints and different organs or tissues (ICRP, 2007)”.
What should be RBE value in such situation? How to account for calibration? Calibration in terms of operational quantity will not give correct absorbed dose.
Line No 313-316: In field, it may not be feasible to get information all the time, particularly in complex radiation environment such as reactor where the radiation type & energy has significant temporal as well as spatial variation in many locations. Information, wherever is possible can be obtained after detailed investigation. Till the time, a dose needs to be assigned which would form the basis for further investigation.
Here also issue related to calibration needs to be addressed.
Line No 440-457: Conversion coefficient (Line NO 440-457) are only upto 50 MeV, they need to be calculated for higher energies as well.
Line Nos.473: no section like 3.3.3 exists in the report. It should be 3.3.2.
Line Nos. 477: “limits recommended for the skin apply to the absorbed dose to ------“. Does it mean neutron/high LET radiation has same effectiveness for skin/eye lens reactions? E.g.  10 mGy neutron dose to skin will have similar reactions to 10 mGy of X ray dose to it? The concept needs to be reviewed.
Line Nos.486-487: “The sum is performed over organs and tissues considered to be sensitive to the induction of stochastic effects “few issues with this definitions are,
1. As operational /protection quantities has been defined for low doses (< 100 mSv). At higher doses, tissue reaction has been considered and absorbed dose is measured. Since the limit for skin dose > 100 mSv, it should also be given in terms of absorbed dose.
2. skin/ eye lens are considered to have small or no contribution ( line 471). Therefore dose to such organs would not be part of effective dose. Instead, all tissues should be considered with appropriate WT for such organs having small/negligible contribution.
Line Nos. 491-493: issue related to definition of ambient dose are
1. Maximum Effective dose to what? Phantom/body and reference point for it. Unless Emax is defined h*Emax may not establish relationship with fluence.
As indicated in next para, the adult reference phantom should be included in the definition itself
2. It is not clear that how the maximum value of effective dose Emax is determined. Geometry / conditions for Emax needs to be elaborated.
Line Nos. 513: absorbed dose unit would be in terms of mGy, and that of protection quantity and dose limits are in terms of mSv. How the mGy and mSv will conform is a basic question and needs to be clarified.
Medium of radiation field needs to be mentioned.
Line Nos. 518: for directional absorbed dose to the lens of eye, phantom has not been defined or mentioned. However the conversion coefficient is calculated for a specific phantom. Therefore for directional absorbed dose to the eye lens should also be defined for same phantom.
Line Nos. 549-50: for effective dose, adult reference phantom has been used. It would be ideal if the phantom has 1:1 correspondence with the human body, as the whole-body dose and skin dose is estimated for the same individual and there is always a skin dose component in the whole body. Defining two different phantoms will make the skin dose component in WBD different from that measured using dedicated phantom / detector for the same radiation and field.
It has been noted in the context of operational quantities that definition based on ICRU 4 element tissue phantom is hard to obtain. The problem associated with 4 element ICRU tissue phantom would continue to remain and will not be solved as outlined.
Personal dose:  has been defined for parallel broad beam in adult reference phantom for radiation incident from left to right as maximum value of effective dose (ie at dmax) using particle fluence.
Although this definition seems to be ok, it would be rather difficult for designing universal dosemeter at least for the range of Ep, where single dosemeter is used. In practical field, the estimation will not be correct as the maximum effective dose will change with effective EP
Line Nos. 637-638
reference for solid angle, for both direction absorbed dose and personal doses needs more clarification. It may be useful to demonstrate through diagram.
Line Nos.642
reference direction may be the direction of maximum fluence on the surface

Line Nos.644
with respect to body plane or perpendicular to the body, is not clear.

Appendix A

For practical application, it is necessary to provide more information on
1)    phantom used
2)    type of beam (narrow/wide/collimation) for proper use of conversion coefficients
3)    measurement point on phantom i.e detector location
4)    distance between source and phantom/detector