|NEWS AND INFORMATION
|Year : 2021 | Volume
| Issue : 2 | Page : 116-119
ICRU report 95 – Operational quantities for external radiation exposure
Radiation safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 500085, India
|Date of Submission||29-Sep-2021|
|Date of Acceptance||29-Sep-2021|
|Date of Web Publication||23-Oct-2021|
Radiation safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 500085
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Sneha C. ICRU report 95 – Operational quantities for external radiation exposure. Radiat Prot Environ 2021;44:116-9
ICRP/ICRU have released a joint report that changes the concept of the operational quantities, for area and individual monitoring in a major way. The reasons for the changes and the proposed new quantities are explained in this joint report.
ICRP has defined protection quantities equivalent and effective dose for implementation of dose limits. Dose limits for eye lens and skin are defined for the mean absorbed dose weighted by the radiation weighting factor in terms of Sv. This is being changed by ICRP in the near future. A glimpse of future ICRP view is given in this report where the dose limits for skin and eye lens are defined in terms of absorbed dose with units of Gy. The concept of effective dose remains unchanged. The rationale behind this is that the radiation weighting factors are derived for stochastic effects rather than tissue reactions whereas the dose limits for skin and eye lens are based on tissue reactions.
In view of the fact that protection quantities were not measurable in the field, it was necessary to define operational quantities which could be related by measurement to the radiation field. Operational quantities were therefore meant as a bridge between the protection quantities and the radiation field.
However, for some energies of photons and neutrons, the operational quantities were over-estimates of the effective dose. For higher energies which were not very prevalent previously, the operational quantities were underestimates. After the publication of the voxel phantom in ICRP 116, and the wide availability of Monte Carlo computational techniques, the use of the operational quantities as currently defined became questionable.
Chapter 1 is introductory in nature and gives the outline of the report. Chapter 2 describes the deficiencies of the existing system of operational quantities. The phantoms used for estimating operational quantities were different from the phantoms used for estimating protection quantities. So essentially, it was necessary to define three phantoms, (i) the anthropomorphic phantom (later followed by the voxel phantom in ICRP 116) for the computation of effective dose, (ii) the physically unrealizable ICRU sphere for defining the area monitoring quantities and tissue for defining individual monitoring quantities, and (iii) the slab, cylinder, or rod phantoms for measurement of the personal monitoring quantities.
Conversion coefficients were not available for higher energies of photons and neutrons. The method of calculation of conversion coefficient, which was using kerma approximation, also was felt to be inadequate for high energies since there were large differences between the calculated values both with and without kerma approximation. Values calculated with kerma approximation implied overestimation of effective dose and those calculated without kerma approximation clearly indicated underestimation of effective dose.
Chapter 3 gives the details of the current recommendation. There has been a “change of paradigm.” Conversion coefficients are now directly related to the values of the protection quantities. The operational quantities are redefined as the products of radiometric (or dosimetric) quantities and conversion coefficients. Now, the phantom for calculation of protection quantities and operational quantities (effective dose and dose to eye lens) are identical. Therefore, the numerical values of the protection and the operational quantities are also identical for several energies and angles.
The ambient dose H* is now defined as the product of the particle fluence at a point, Φ, and the conversion coefficient, H*, relating particle fluence to the maximum value of effective dose, Emax, for various irradiation conditions. The unit remains Sv. The directional absorbed dose in the lens of the eye, D'lens(Ω), at a point in a radiation field with a specified direction of incidence, Ω, is the product of the particle fluence at that point, Φ(Ω), and the conversion coefficient, d'lens(Ω), relating particle fluence to the value of absorbed dose in the lens of the eye. Since it relates now to the absorbed dose, the unit is Gy. Similarly, the directional absorbed dose in the local skin is now defined in terms of Gy as product of the particle fluence at that point, Φ(Ω), and the conversion coefficient, d'localskin(Ω), relating particle fluence to the value of absorbed dose in the local skin.
The conversion coefficient for ambient dose H* is calculated for exposure of the whole-body ICRP/ICRU adult reference (voxel) phantoms to broad uniform parallel beams. The conversion coefficient for directional dose to eye lens D'lens is calculated for exposure of the stylized eye model and the whole body for broad uniform parallel beams of the radiation field. The conversion coefficient for directional dose to local skin is calculated for exposure of a specified phantom.
The personal dose, Hp, (in Sv) at a point on the body is the product of the particle fluence incident at that point, Φ, and the conversion coefficient, hp, relating particle fluence to the value of effective dose, E. The personal absorbed dose (in Gy) in the lens of the eye, Dp lens, at a point on the head or body is the product of the particle fluence incident at that point, Φ, and the conversion coefficient, Dp lens, relating particle fluence to the value of absorbed dose in the lens of the eye. The personal absorbed dose in local skin, Dp local skin, (in Gy) is the product of the particle fluence incident on the body or extremities, Φ, and the conversion coefficient, Dp local skin, relating particle fluence to the value of absorbed dose in local skin.
The phantoms for personal dose are identical to the phantoms for ambient and directional dose. In addition, cylindrical phantom for extremities and rod phantom for finger are also defined for personal absorbed dose to skin at extremities.
Chapter 4 gives details of the calculations of conversion coefficients. The details of the Monte Carlo radiation transport codes used for the calculations are given in the Appendix. In general, the base of all calculations is fluence, but conversion coefficients from air kerma are also given for photons of energy up to 50 MeV. The phantoms are irradiated in vacuo with broad uniform parallel beams, and in some instances, to rotational and isotropic fields. All calculations are performed with full transport of generated particles. Personal dosimeters and area monitoring instruments for photons are customarily calibrated in a setup where charged-particle equilibrium is established. For this purpose, additional conversion coefficients for photons with kerma approximation are also given.
Previously, conversion coefficients were available only for photon, neutron, and electron beams. Now, in addition to these, conversion coefficients are also given for positrons, protons, muons, pions, and alpha particles. Previous values of conversion coefficients are compared to the currently recommended values. Since the current values relate directly to effective dose, the comparison is essentially of the previous values, Hp(10) and H*(10) to effective dose. At low energies of photons, less than 70 keV, as was known previously, there is a significant overestimation of effective dose. Between 70 keV and 3 MeV, the effective dose is overestimated since the 10-mm depth chosen for H*(10) and Hp(10) was meant to reflect the maximum dose equivalent. However, at higher photon energies, the dose at 10 mm underestimates effective dose. Previously, since calculations were carried out with kerma approximation, the 10-mm depth seemed to adequately represent the effective dose. For neutrons, the depth of 10 mm is not appropriate as an estimate of effective dose. At energies greater than 40 MeV, there is underestimation of effective dose. For electrons of energies <2 MeV, the range is <10 mm, and so there is no contribution to H*(10) or Hp(10). With the current definition, such electrons also contribute to effective dose. H*(10) for protons overestimates effective dose for energies <80 MeV and underestimates for energies >100 MeV.
Chapter 5 discusses the practical consequences of introduction of the new quantities. It states that instruments from before the introduction of the ICRU 39 operational quantities continue to be used giving credit to the knowledge of radiation protection professionals. The use of operational quantities is to get an estimate of effective dose. When this value approaches the value of the annual limit, the ensuing investigation will reveal additional knowledge which may be used to either retain the measured value or modify it appropriately. This is done by radiation protection professionals with experience and knowledge.
Due to the current recommendations, there will be a difference in calibration for photons of about 16%, with the current recommendations being lower. At low photon energies which are typically encountered in diagnostic X-ray fields, the difference is much larger with current recommendations being lesser by 60%–70%. However, the energy and angular spectrum in real fields are large which may reduce the impact of change in conversion coefficients, the low doses that are measured on incremental monthly basis can have uncertainties as large as a factor of 2, and dosimeters in general do not have a perfect response for all angles and energies. Due to these reasons, the collective dose may not be impacted to the same extent. Where the measurement is concerned with using an instrument, it may be possible to adjust the instrumental response by a simple change in the calibration factor.
One objection to the implementation of Hp as it is defined is that it relates directly to effective dose. Any future changes in effective dose due to change in tissue weighting factors will therefore affect the value of Hp also. The committee feels that such changes may be minor and not affect Hp significantly. Further, these changes are made only with the general recommendations of ICRP, published at a frequency of about 15 years. Therefore, changes in Hp are expected only in 20 years which may exceed the lifetime of dosimeters in use and preparations can be made for changes in dosimeters.
Chapter 6 concludes with the statement that the system of protection and operational quantities is simplified by the proposed changes. In radiation fields of high-energy photons, neutrons, electrons and for other particle types, these changes are required and will bring consistency in measurement. There will be a change required in either the algorithm or design of existing instruments.
Appendices list the conversion coefficients for theoretical monoenergetic beams in terms of Sv per unit fluence for ambient and personal dose. Since ambient dose is meant to give a maximum value, single values are provided for each energy and radiation type. For personal dose, values are provided for all irradiation angles and beams. Conversion coefficients for directional and personal dose to eye lens are given in terms of Gy per unit fluence. Similarly, conversion coefficients for directional and personal dose to local skin are given in terms of Gy per unit fluence. In all cases, for photons, in addition, the conversion coefficients are also given in terms of unit air kerma, both with and without kerma approximation to simulate charged particle equilibrium. For the eye lens, conversion coefficients are given for both the entire eye lens and the radiation-sensitive cells of the eye lens.
The Monte Carlo radiation transport codes used for calculation are described. PHITS, FLUKA, MCNP, MCNP 6.2, EGSnrc have been used.
In conclusion, it can be stated that the proposed changes definitely constitute a paradigm shift. It will mean either changes in design of existing dosimeters or changes in algorithms etc. Considering the numbers of area and individual monitoring instruments in use, this will mean a large increase in work. However, implementation is still some distance away since the corresponding IAEA safety standards and IEC/ISO documents for practical application are yet to be published. Therefore, there is sufficient time for preparation.
The fact that H*(10) and Hp(10) overestimated effective dose in general for photons of energy less than 3 MeV and by factors of 2-4 for lower energy photons was well known. This was generally considered advisable and in the interest of the radiation worker. Although construction of dosimeters which were sufficiently sensitive at low photon energies was found to be difficult, the overestimation was continued. This seems to be a change in philosophy. The present approach has been considered by several authors in the 70s and 80s but has not been followed by ICRP/ICRU. The absence of internationally agreed phantom is the stated reason which has now been fulfilled by the voxel phantom.
The conversion coefficients for skin and eye lens are given in terms of absorbed dose per unit fluence. In general, for neutrons, the conversion coefficients for whole body are higher than for eye or skin. RBE for deterministic effects is lower than RBE for stochastic effects. Therefore, for uniform fields, the extremity or eye lens dose may be less than the effective dose. Photon conversion coefficients are given for fields with and without electronic equilibrium. This would imply that dosimeters must be capable of accurate measurements in both conditions. The practical implication of conversion coefficients for alpha radiation is not very clear.
The system of protection and operational quantities is much simplified by the new changes. The changes also bring consistency between the protection and the operational quantities.
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Conflicts of interest
There are no conflicts of interest.
| References|| |
ICRU Report 95: Operational quantities for external radiation exposure. J ICRU 2020;20:7.
Petoussi-Henss N, Bolch WE, Eckerman KF, Endo A, Hertel N, Hunt J, et al.
ICRP Publication 116. Conversion coefficients for radiological protection quantities for external radiation exposures. Ann ICRP 2010;40:1-257.