Operational Quantities for External Radiation Exposure

Draft document: Operational Quantities for External Radiation Exposure
Submitted by Takatoshi Hattori (Chair of International Affairs Committee), Japan Health Physics Society
Commenting on behalf of the organisation

General comments

We recognize that ICRP and ICRU will continue to use measurable operational quantities for not underestimating protection quantities even in higher energies and also will provide the theoretically reasonable estimates of the protection quantities.
The JHPS Expert Group created in 2007 on the dose concept for radiological protection suggested that the ICRU phantoms defining operational quantities should be unified and simplified, and also the voxelized anatomical phantoms in ICRP publication 110 should be used for radiological protection of high energy radiation. JHPS accept that the approach for operational quantities presented in this report agree with the JHPS Expert Committee’s suggestion.
However, the following questions will be emerged from the practical points of views if more clear explanations would not be provided on limitations of the use of new operational quantities based on protection quantities.

 -  It is questionable to promote the use of new coefficients converting to effective doses 
   for radiological protection practice since the definition of the effective dose has 
   changed during the last 40 years. The values of tissue weighting factors based on 
   detriment would not be stable since recent scientific data will change the detriment. 
   The limitation of use should be carefully noted by emphasizing the underlying 
   uncertainty of the protection quantities to avoid the misuse for risk calculation.

 - Precise conversion coefficient values with three effective digits are unnecessary for 
   control of effective doses. Considering the intrinsic uncertainty of the effective dose 
   for controlling stochastic effects, the significant figure of the conversion coefficients 
   would be 1 or 2 as indicated in the rounded numbers of radiation/tissue weighting factors.

-  More deep and careful investigations following the discussions in past ICRP reports
   should be made before the new operational quantities are published. The currently
   used conversion coefficients and the new conversion coefficients h*Emac were derived
   from maximum values in the range of relevant physical quantities.
   Both have no substantial difference. It should be recognized that change of the
   determination method with little change of the practical consequences would be
   confusing and frustrating for many users. 

-  The practical benefit of using the new operational quantities is unclear while it will bring
   some confusions that could not be justified in balance of the advantage. The 
   quantities still have large uncertainties. The benefits and limitations 
should be clarified,
   although the current operational quantities would be enough in 
a practical sense.

-  Regarding the uncertainty on evaluation of the eye lens doses, it should be noted and 
   investigated that many people put glasses or contact lenses when working. The average
   effects of those artificial materials should be incorporated in to the dose calculation as well as
   precise geometries. Otherwise, the presented coefficients would not reflect the real,
   representative situations of eye lens exposure.

 -  The same concern for the eye lens is shown on the effects of clothes in calculation of skin
    doses. It is requested to consider the shape and thickness of representative working clothes
    (so-called “uniform”), so that the calculation conditions would represent well the actual
    exposure situations.


-  The dose limits for the lens of the eyes and skin are still expressed with “Sv”. It is
   hoped that the new dose limits using different units indicating RBE-weighted values
   (e.g. Gy-Eq) would be presented in this report.

-  There is no description about areal and personal monitoring instruments.
   A new section on characterization of the monitoring instruments is desirable.


Specific comments


For section 3
- The relation of the definition, unit, and value between the current operational quantities and
  new ones should be mentioned.


For section 4
-  Some conversion coefficients are unrealistic for operational quantities
   a) Not need the conversion coefficients for over 90 degree.
   b) Not need other angle values except 0 degree for the extremity and finger
   c) For the neutron, conversion coefficients should be given for only ambient dose and
       personal dose, not for extremity and finger.


-  The conversion coefficients of d ’lens should be calculated not only for right and left angles
   but for upper and lower angles because the incident angle Ω is defined in terms of the
   components of the two kinds of angles as mentioned in section 3.4.


-   A relationship between conversion coefficients to personal dose and to personal absorbed
    dose in the lens of the eye should be mentioned clearly like relationships between H*(10)
    and h*, or between hp(3) and dplens. in Figure 4.1 to 4.6.


-  For the photon energy rage of common radionuclides ( 100 keV to 1.5 MeV), the numerical
   values of conversion coefficient from air kerma to personal dose are 7 to 16 % lower than the
   values of conversion coefficient from air kerma to personal absorbed dose in the lens of the eye.
   This means that workers are restricted by the equivalent dose limit to the lens of the eye,
   even if they are exposed to photons in a homogeneous field (if the current dose limit to the lens
   of the eye expressed in ‘mSv’ is used as absorbed dose limit expressed in ‘mGy’).
   This is unsuitable for the current radiation regulations, radiation management method and
   radiation protection system. Thus it should be promoted that it is necessary to pay attention
   to a usage of the conversion coefficients to radiation protection.


For section 5
-  The impact of new operational quantities may be large for medical fields, so should be mentioned.


-  The draft explains “measureable” in Chapter 5 (Line 176-1186).
    We are sure that the most important issue is to change determination of operational quantity,
    from “dose at a point in the ICRU sphere or human tissue” to “dose based upon protection
    Any measurements can actually give radiation dose over a limited volume. Thus, it can be
    controversial whether the suggested operational quantities are measureable or not by taking
    account for actual measurements.
    ICRU should explain the reason why suggested operational quantities are “measureable” in
    Chapter 2 before definitions of the new operational quantities.


For section 6
-  ICRU suggests a “PMMA phantom with the size of 30cm×30cm×15cm” for calibration of
   dosimeter that measures “personal dose”. (Line 1242-1243) ICRU Report 57 (ICRP Publication
    74) states that the phantom with this size substitutes a “human torso”. The current personal
   dose equivalent, Hp (d), is defined as “a dose at the point of depth (d) in human body tissue”.
    As the dosimeter might be set on the body surface of a torso, the use of this phantom is
   reasonable for the current quantity, Hp (d). On the other hand, the newly suggested personal
   dose is defined on the basis of the radiation dose over a whole body. Thus, ICRU should explain
   the “validity” for the use of PMMA phantom with the size of 30cm×30cm×15cm.


-  Personal absorbed dose in the lens of the eye is defined at a point on the head or body (Line
   592).However, if the quantity is defined for personal monitoring, the personal absorbed dose
   in the lens of the eye should be defined at a point on the head, not on the body. The phantom
   for dosimeter calibration also might represent the head size in Chapter 6 (Line 1244 -1246).