ICRP Symposium on Radiological Protection Dosimetry
Historical Review and Current Activities
The University of Tokyo
Thursday, February 18, 2016

Abstracts

Organized by
the International Commission on Radiological Protection (ICRP)
with support from Nippon Foundation

Restaurants and shops in the campus

Photograph on the cover: Faculty of Engineering Building 2 (left, symposium venue)
and the Yasuda Auditorium (right) in Hongo Campus, the University of Tokyo.
The photograph is reprinted courtesy of “UT‐Life” of the University of Tokyo.
http://www.ut‐life.net/

BackgroundlCRP Committee 2 is engaged in the development of dose coefficients for the assessmentof internal and external radiation exposures; development of reference biokinetic anddosimetric models, and reference data for workers and members of the public. The
Committee and its Task Groups are currently engaged in a large program of work toprovide revised dose coefficients that implement the 2007 Recommendations andintroduce improvements in methodology.
ICRP Committee 2 works closely with the International Commission on Radiation Units
and Measurements (ICRU). It also works with the other ICRP Committees, for example incurrent work to provide advice on the use of effective dose as a protection quantity.

Objectives of the symposium
ICRP Committee 2 welcomes this opportunity to discuss the current work with Japanesescientists and practitioners, covering topics including: Scientific bases of ICRP dose calculations
 Current status of the development of models and data Application of the models and data to dose assessments for workers and the
public with focus on environmental exposures after accidental releases, risk
assessment and radiation monitoring Use of equivalent and effective dose as protection quantities
 Research needs and future developments

Symposium venue
Room 221, Faculty of Engineering Building 2, Hongo Campus, the University of Tokyo

Coffee break and lunch
Coffee will be served during the morning and afternoon breaks.
Participants could visit for lunch the restaurants and cafeterias of the university campus.
Locations of restaurants and shops are found at the inside of the front cover.

1

Program
Registration starts at 9:30

Opening Address
10:00 – 10:10
Structure and Mission of the International Commission on Radiological Protection
John Harrison, C2 Chair

Session 1 – Introduction
10:10 – 10:30
Overview of the ICRP System of Internal and External Dosimetry

Akira Endo, Session Chair
Wesley Bolch

Session 2 – Models and Data Used for the Calculation of Dose Coefficients
Michael Bellamy, Session Chair
10:30 – 10:55
Computational Phantoms of the Reference Adults: Stylized and Voxel
Maria Zankl
10:55 – 11:20
Polygon Mesh Conversion of the ICRP Reference Phantoms
Chan Hyeong Kim
11:20 – 11:40

Coffee Break

11:40 – 12:05
12:05 – 12:30
12:30 – 13:00

Methods of Dose Assessment to the Skeletal Tissues
Computational Phantoms of Children and Pregnant Females
Biokinetic Models and Dose Coefficients for Internal Exposures

13:00 – 14:00

Lunch

Derek Jokisch
Choonsik Lee
John Harrison

Session 3 – Dose Coefficients Using ICRP Reference Models and Data
14:00 – 14:25
Reference Dose Coefficients for Occupational External Exposure
14:25 – 14:50
Reference Dose Coefficients for Environmental Exposure

Kimiaki Saito, Session Chair
Nina Petoussi‐Henss
Helmut Schlattl

Session 4 – Application of Effective Dose and Radiation Monitoring Practice
14:50 – 15:15
Effective Dose and Risk Assessment
15:15 – 15:40
Revisions to ICRU Operational Quantities

Kwang Pyo Kim, Session ChairJohn Harrison
Nolan Hertel

15:40 – 16:00

Coffee BreakSession 5 – Research Needed for the ICRP System of Radiological Protection Dosimetry
Wesley Bolch, Session Chair
16:00 – 16:30
Issues on the Radiation Weighting Factor
Tatsuhiko Sato
16:30 – 17:00
Dose Assessment of Workers and the Public: Lessons Learned from Fukushima
Osamu Kurihara
17:00 – 17:25

General Discussion of Research Needs

Concluding Remarks
17:25 – 17:30
Concluding Remarks

John Harrison, C2 Chair

2Overview of the ICRP System of Internal and External Dosimetry
W.E. Bolch
University of Florida, Gainesville, FL 32611-6131 USA; e-mail: wbolch@ufl.edu

The system of protection established by the International Commission on RadiologicalProtection for both workers and members of the general public is based primarily upon theprotection quantity effective dose. The effective dose is a doubly-weighted average of theradiation absorbed dose (energy imparted per unit mass) to various individual organs andtissues of the body. The first factor – the radiation weighting factor wR – accounts for theradiobiological effectiveness of different radiation particle types. The second factor – thetissue weighting factor wT – accounts for the relative risk of stochastic biological effect to thevarious exposed organs and tissues of the body. The effective dose is a non-measurable
quantity and is thus must be given using various computational models. For internal
exposures, models are invoked that account for the movement of the inhaled or ingested
radionuclide through the respiratory or alimentary tract, respectively, to circulating blood.
Next, systemic biokinetic models follow the radionuclide in blood as it is taken up bothvarious sources organs of the body, released back to blood, and eventually eliminated via
urinary or faecal excretion. Physics models of radionuclide decay are used to determine, ateach location within the body, what types and energies of radiation particles emitted. Finally,
anatomic and radiation transport models are used to follow these emitted particles as they
travel through the body and deposit energy through both absorption or scattering events. The
ultimate result of these computations is the effective dose coefficient – defined as the effective
dose per activity inhaled or ingested. For external exposures, the very same anatomic andradiation transport models are employed, with the incident radiation coming fromradioactivity or radiation sources found in either the occupational or natural environment. For
external exposures, the effective dose coefficient relates the effective dose to computed or
measured values of air kerma, particle fluence, or radioactivity concentration.
In this presentation, we will review the various dose quantities and computationalmodels needed to determine dose coefficients for both internal and external radiation
exposures. Further details are given the accompanying presentations of this symposium.

3Computational Phantoms of the Reference Adults:
Stylized and Voxel
M. Zankla, J. Beckera, K.F. Eckermanb, W.E. Bolchc, R. BehrensdaResearch Unit Medical Radiation Physics and Diagnostics, Department of Radiation Science,
Helmholtz Zentrum München German Research Center for Environmental Health,Ingolsaedter Landstr. 1, 85764 Neuherberg, Germany; e-mail: zankl@helmholtz-muenchen.de;janine.becker@helmholtz-muenchen.deb
Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA; e-mail: eckermankf@ornl.govc
J. Crayton Pruitt Family Department of Biomedical Engineering, University of Florida,
Gainesville, FL, U.S.A.; e-mail: wbolch@ufl.edudPhysikalisch-Technische Bundesanstalt, Department 6.3 "Radiation protection dosimetry",
Bundesallee 100, 38116 Braunschweig, Germany; e-mail: Rolf.Behrens@ptb.de

Phantoms simulating the human body or parts thereof play a central role in radiation
dosimetry. The first computational body phantoms have been based upon mathematicalexpressions representing planes, cylindrical, conical, elliptical, and spherical surfaces
describing the shape and position of idealized body organs. For this first generation ofcomputational body phantoms, the organ masses and volumes were in accordance with the
ICRP (International Commission on Radiological Protection) data of former Reference Man(ICRP Publication 23, 1975).
With the advent of more powerful computers in the 1980s, various groups havedeveloped voxel phantoms as an extension and improvement to these earlier models. Being
based on three-dimensional images of single individuals, they offer a more realistic
replication of human anatomy. Various authors have shown that the organ shapes of theearlier mathematical phantoms present an over-simplification that has an influence on theenergy distribution which may deviate for some cases systematically from that calculated forvoxel phantoms. However, despite their obvious advantages compared to the previously usedstylized phantom type, most of these phantoms do not represent the average Caucasian man or
woman, due to being derived from a specific individual. Hence, the ICRP decided to constructvoxel phantoms being representative of the adult Reference Male and Reference Female(ICRP Publication 89, 2002) with respect to their external dimensions, their organ topology,
and their organ masses (ICRP Publication 110, 2009), for the update of organ doseconversion coefficients following the recent ICRP Recommendations (ICRP Publication 103,
2007).
Since the reference computational phantoms have a limited voxel resolution in the
millimetre range, several organs and tissues having much smaller dimensions cannot be
represented in detail. Hence, for selected calculations, additional stylized phantoms have beenused. For the external irradiations of ICRP Publication 116 (2010), stylized models of the eye
and lens were used for estimating selected dose conversion coefficients. Similarly, for internal
dosimetry, the fine structure of some of the target regions in the human respiratory tract andhuman alimentary tract could not be described by the voxel geometry of the reference
phantoms, and thus stylized models of the airways and of individual segments of thealimentary tract were employed for the calculation of selected Specific Absorbed Fractions.

4Polygon Mesh Conversion of the ICRP Reference Phantoms
C.H. Kima, Y.S. Yeoma, T.T. Nguyena, Z.J. Wanga, H.S. Kima, M.C. Hana,
J.K. Leea, M. Zanklb, N. Petoussi-Henssb, W.E. Bolchc, C. Leed, B.S. ChungeaDepartment of Nuclear Engineering, Hanyang University, 133-791, Seoul, 222 Wangsimni-ro,
Republic of Korea; e-mail: chkim@hanyang.ac.kr, ysyeom@hanyang.ac.kr, ntthang@hanyang.ac.kr,
wzj1987@hanyang.ac.kr, hskim87@hanyang.ac.kr, mchan@hanyang.ac.kr, jakilee@hanyang.ac.krbResearch Unit Medical Radiation Physics and Diagnostics, Department of Radiation Sciences,
Helmholtz Zentrum München Deutsches Forschungszentrum für Gesundheit und Umwelt (GmbH),
Ingolstädter Landstraße 1, D-85764 Neuherberg, Germany; e-mail: zankl@helmholtz-muenchen.de,petoussi@helmholtz-muenchen.dec
J. Crayton Pruitt Family Department of Biomedical Engineering, University of Florida,
P.O. Box 116131, Gainesville, FL 32611-6131, USA; e-mail: wbolch@ufl.edu
dDivision of Cancer Epidemiology & Genetics, National Cancer Institute, 9609 Medical Center Drive,
Bethesda, MD 20892-9760, USA; e-mail: leechoonsik@mail.nih.gov
eDepartment of Anatomy, Ajou University School of Medicine, 443-380, Suwon, 164 Worldcup-ro,
Republic of Korea; e-mail: bschung@ajou.ac.kr

To overcome the limitations of the current voxel-type ICRP Publication 110 reference
phantoms, the ICRP Committee 2 is currently developing a set of mesh-type ICRP reference
phantoms, by converting the current voxel-type Publication 110 reference phantoms into ahigh-quality mesh format. The Publication 110 reference phantoms are voxel phantoms basedon whole-body computed tomography scans of adult male and female patients. The voxel
resolutions are on the order of a few mm; therefore, smaller tissues such as the eye lens, skinor the walls of some organs cannot be properly defined in the phantoms. In addition, the voxel
phantoms cannot represent the thin target tissues of the respiratory or alimentary tract organs,requiring additional simplified mathematical phantoms for dose coefficient calculations. The
objective of the research in the ICRP Committee 2 is to produce a set of replica of Publication
110 reference phantoms in a high-quality mesh-format to address the aforementioned
limitations. The developed phantoms are expected to include continuous and fully-enclosed
surfaces for all the organs and tissues in the phantoms, include the thin target layers in thealimentary and respiratory tract organs, and also include detailed and more accurate models
for skeletal system, eyes, lymphatic nodes, blood vessels, and hands. The developed
phantoms will be also deformable, which functionality will be useful when the ICRP
Committee 2 calculates the dose coefficients for emergency exposure situations in the nextterm of ICRP (2017-2021).
The presentation will first introduce the research project in general, explaining the
motivation and objective, and then explain the current progress and preliminary results, andfinally conclude with the implication of the research outcomes in the ICRP and radiation
protection society in general.

5Methods of Dose Assessment to the Skeletal Tissues
D.W. JokischFrancis Marion University, PO Box 100547, Florence, SC 29502-0547 andCenter for Radiation Protection Knowledge, Oak Ridge National Laboratory,
PO Box 2008, MS-6335, Oak Ridge, TN 37381-6335, USA;e-mail: djokisch@fmarion.edu or jokischdw@ornl.gov

Estimating the amount of energy absorbed in skeletal tissues is complicated due to therelatively small sizes and interlacing structure of two contrasting media: bone and marrow.
The two radiosensitive tissues of interest in the skeleton are the cells responsible for blood
cell formation and the osteoprogenitor tissue. The former is called active (or red) marrow andresides in varying concentrations in the marrow spaces throughout the body. The sensitive
osteoprogenitor tissue is considered to reside on the soft tissue side of the bone-marrowinterface.
Charged particles originating within the skeleton have ranges such that they can
traverse multiple segments of bone and marrow while depositing energy to the targets ofinterest, thus making explicit consideration of the skeletal microstructure desirable. Three
dimensional imaging of human skeletal samples has been performed and used as a basis forcomputational dosimetry models for electrons, protons, and alpha particles. These models
produce specific absorbed fractions which take into account the complex microstructure of theskeleton as well as the varied components of the marrow space (active marrow, adipose tissue,
and the osteoprogenitor tissue). This presentation will briefly review a history of skeletal
dosimetry before describing the latest methodologies and their application to effective dosecoefficient calculations.

6Computational Phantoms of Children and Pregnant Females
C. Leea, W.E. BolchbaDivision of Cancer Epidemiology and Genetics, National Cancer Institute,
National Institutes of Health, 9609 Medical Center Dr. Rockville, MD 20850, USA;e-mail: leechoonsik@mail.nih.govb
Department of Biomedical Engineering, University of Florida,
Gainesville, FL 32611-6131 USA; e-mail: wbolch@ufl.edu

In its Publication 110, the International Commission on Radiological Protection(ICRP) formally adopted two voxel-based computational reference phantoms representing theReference Adult Male and Reference Adult Female. These models were subsequently used toestablish external reference dose coefficients for worker occupational fields, aircrew
exposures, and space radiation environments. They are presently being used by Committee 2
for establishing dose coefficients for internal exposures following radionuclide inhalation andingestions, as well as external exposures to contaminated soil, water, and air. Moving forward,
the ICRP is updating reference dose coefficients to members of the general public including
infants, children, adolescents, and pregnant females. In keeping with the framework of
Publication 110, a new series of reference voxel phantoms have been developed for the
newborn, 1-year-old, 5-year-old, 10-year-old, and 15-year-old male and female. These
phantoms are derived from a series of hybrid phantoms established jointly with the University
of Florida (UF) and the National Cancer Institute (NCI). The UF/NCI pediatric hybrid
phantoms are based on computed tomography (CT) images of patients and then adjusted tomatch the reference organ masses and body dimensions as reported in the ICRP Publication
89. Major changes to the UF/NCI hybrid phantoms include an adjustment of the organ
identification system to mirror that for the adult phantoms of Publication 110. Additional
tissues and substructures were modeled to include organ mucosa/lips/cheeks, glandular andadipose regions of the female breast, major blood vessels of the lungs, and the ureters. New
tissue models were further developed for the lymphatic nodes and skeletal muscle. Detailed
skeletal models are based upon microCT imaging of cored samples of newborn and 15-year
cadaver spongiosa, coupled with pathlength distributions from the University of Leeds at agesof 1.7 and 9 years. The second phantoms series recently established is that of the adultpregnant female, developed at the University of Florida to support radiation epidemiology
studies of in-utero exposures at the Techa River villages and Mayak Plutonium Production
Facility. This series of phantoms include detailed fetal models based upon CT and MR
imaging of preserved fetal specimens, and segmented CT-base anatomy of adult pregnant
females. The final reference series include models at 8, 10, 15, 20, 25, 30, 35, and 38 weeks
post-conception.

7

Biokinetic Models and Dose Coefficients for Internal Exposures
J.D. Harrisona, F. PaquetbaOxford Brookes University, Faculty of Health and Life Sciences, Oxford, OX3 0BP, UK;e-mail: john.harrison@phe.gov.uk
bInstitut de Radioprotection et de Sûreté Nucléaire (IRSN), PRP ENV/SERIS,Saint Paul Lez Durance 13115, France

Since the Publication 30 series on occupational intakes of radionuclides, which
followed from the Publication 26 Recommendations (1977), ICRP has provided
comprehensive information in support of the calculation of organ and effective doses forintakes of radionuclides by workers and members of the public. Dose coefficients (Sv Bq-1
intake) have been provided for workers, adult members of the public and children of different
ages, considering intakes by ingestion and inhalation. Dose coefficients have also been
provided for secondary exposures of the fetus and newborn child following intakes by themother, considering placental transfer of radionuclides and transfer in breast milk.
The calculation of organ and effective doses following intakes of radionuclides into
the body require biokinetic as well as dosimetric models. Biokinetic models describe theuptake and retention of radionuclides in body organs and tissues and are used to calculate thenumber of nuclear transformations occurring in each region. Dosimetric models are then used
to calculate energy deposition in and dose to these organs and tissues and associated energy
deposition and dose to other organs and tissues.
The construction of biokinetic models makes use of human data where these areavailable, supported by animal data and knowledge of physiological processes andbiochemical interactions. Committee 2 is currently in the process of updating biokinetic
models for the calculation of dose coefficients that are compliant with the latest ICRP
Recommendations (Publication 103; 2007). Substantial revisions are being made to the model
used for the human respiratory tract which describes the deposition of inhaled materials andtheir subsequent clearance from the lungs and absorption to blood. An updated model of thealimentary tract will also be implemented. Substantial efforts have also been made to improve
the physiological realism of models used to describe the distribution, retention and excretionof radionuclides absorbed to blood, referred to as systemic models. Age-dependence of model
parameter values is considered in the calculation of dose coefficients for children. Special
models are required for the transfer of radionuclides to the fetus and breast-fed infant.
While the biokinetic models developed by ICRP are primarily for use in thecalculation of reference dose coefficients in support of the system of protection, the models
are also used for scientific purposes, including the calculation of organ doses in
epidemiological studies of health effects. For protection purposes, ICRP provides dosecoefficients for different particle sizes of inhaled materials and different chemical forms ofinhaled and ingested radionuclides but otherwise specifies that no changes should be made toreference model parameter values. In other applications, it may be appropriate to makechanges to adapt models to the characteristics of particular exposed population groups andalso to consider uncertainties and variability between individuals.
The replacement of all reference dose coefficients with Publication 103 compliant
values is a substantial scientific effort that will be completed over the next few years.
Meanwhile, ICRP has provided, as Publication 119 (ICRP, 2012), a compilation of dosecoefficients based on the 1990 Recommendations of Publication 60 (ICRP, 1991) for use
while new values based on the 2007 Recommendations are being calculated.
8

Reference Dose Coefficients for Occupational External Exposure
N. Petoussi-Henss
Helmholtz Zentrum München, German Research Center for Environmental Health,Research Unit Medical Radiation Physics and Diagnostics,
Ingolstaedter Landstr. 1, 85764 Neuherberg, Germany;e-mail: petoussi@helmholtz-muenchen.de

Practical radiological protection for workers uses dosimetric quantities whichappropriately quantify the exposure of humans to ionising radiation for the implementation of
the fundamental principles of limitation and optimization. Protection quantities, equivalentdose and effective dose, are not measurable and their values are assessed using theirrelationship to either physical radiation field quantities, e. g. air kerma free in air, Ka, orparticle fluence, , or operational dose quantities. Coefficients defined for a reference person
provide numerical links between these quantities and it is very important that aninternationally agreed set of coefficients is available for general use in radiological protection
practice. One of the central focus of ICRP Committee 2 is the development of reference dosecoefficients for radiation exposure to workers considering irradiation scenarios under
occupational settings.
Revisions of ICRP recommendations, like the latest of Publication 103 (ICRP, 2007),
invariably require recalculation of dose coefficients because changes are made to the radiationand tissue weighting factors and to the phantoms used in the calculation of equivalent andeffective dose. Dose coefficients for external radiation exposure of adults calculated using
new reference computational phantoms were issued as Publication 116 (ICRP, 2010) jointlywith the International Commission on Radiation Units and Measurements (ICRU).
Differences in the photon and electron dose coefficients from those of Publication 74 (ICRP,
1996), still in use by several countries, do generally not exceed 20-30% and are largely
attributable to specific aspects of the computational phantoms. Somewhat greater differences
seen for low- and high-energy neutrons are attributable to the changes in their energydependent radiation weighting factors. Comparison of the effective doses with operational
quantities revealed that the latter continue to provide a good approximation for photons,
electrons and neutrons and for the energy ranges considered in ICRP Publication 74 but not at
the higher energies considered in ICRP Publication 116.
The presentation will highlight the computational framework, some of the main
findings for photons and neutrons and the impact of new phantoms and updated weighting
factors.

ICRP, 1996. Conversion coefficients for use in radiological protection against external radiation. ICRP
Publication 74, Pergamon Press, Oxford, UK. Ann. ICRP 26(3/4)
ICRP, 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP
Publication ICRP Publication 103, Elsevier, Oxford, UK. Ann. ICRP 37(2-4)
ICRP, 2010. Conversion Coefficients for Radiological Protection Quantities for External Radiation Exposures.
ICRP Publication 116, International Commission of Radiological Protection, Oxford, UK. Ann. ICRP
40(2-5) (Japanese translation of ICRP Publication 116 is also available)

9Reference Dose Coefficients for Environmental Exposure
H. Schlattla, D. SatohbaHelmholtz Zentrum München, Research Unit Medical Radiation Physics and Diagnostics,
Ingolstädter Landstr.1, 85764 Neuherberg, Germany;e-mail: helmut.schlattl@helmholtz-muenchen.deb
Nuclear Science and Engineering Center, Japan Atomic Energy Agency,
Tokai-mura, Naka-gun, Ibaraki 319-1196, Japan; e-mail: satoh.daiki@jaea.go.jp

In the mid- and long-term period after nuclear accidents, and in large distances to theevent, the external exposure is the major contribution to the radiation dose of the public. Thus,
effective and organ dose conversion coefficients for adults and children exposed by aradioactive cloud (volume source in air) or to ground contamination after fall-out (planesource in ground) have been computed for electron and photon sources. In addition, theconversion coefficients for being in contaminated aquatic environment have been calculated.
The calculations were performed with the Monte Carlo code PHITS using the ICRP reference
adult models and forthcoming ICRP reference paediatric models representing newborns and 1,
5, 10 and 15 year old children. We also performed spot-check calculations using the Monte
Carlo code EGSnrc to confirm the reliability of the calculation results of PHITS. The findings
confirm the well-known fact that a smaller body mass yields to higher doses. Despite themoderately larger doses for children, it was found that, e.g., for 137Cs in ground, the effective
dose rate is still lower than the ambient-dose rate. Thus, the ambient-dose monitoring
provides a conservative dose estimate not only for adults but also for children.

10Effective Dose and Risk Assessment
J.D. Harrison
Oxford Brookes University, Faculty of Health and Life Sciences, Oxford, OX3 0BP, UK;e-mail: john.harrison@phe.gov.uk

Effective dose (E) was originally introduced in the Publication 26 Recommendations(ICRP, 1977) of the International Commission on Radiological Protection for the control ofoccupational and public exposures to external and internal sources of radiation at moderate
and low levels of exposure. While the concept has remained essentially unchanged throughthe Publication 60 (ICRP, 1991) and Publication 103 Recommendations (ICRP, 2007), its use
has been extended to members of the public of all ages, including in utero exposures of theembryo and fetus. E is accepted internationally as the central radiological protection quantity
and has proved to be a valuable and robust quantity for use in the optimisation of protection
and setting of control criteria: limits, constraints and reference levels. However, confusion has
arisen in its practical application and communication of dose information to non-experts,
particularly because of the number of different dose quantities. In addition, effective dose isincreasingly used in medical applications, including a problematic and growing application to
the assessment of risks to individuals. A task group is currently preparing advice on the use of
the protection quantities; the presentation will summarise aspects of this advice, focusing on
two key proposals that are under consideration by ICRP.
Confusion can arise in the use of the protection quantities, equivalent and effective
dose (both in Sv), when they are not sufficiently well distinguished, and between equivalentdose and the operational quantity, dose equivalent (Sv), used in measurements of exposures toexternal radiation for the assessment of effective dose. Such difficulties would be avoided if
organ and tissue doses are referred to in terms of mean absorbed dose (Gy), specifying low
and high LET components as necessary. The unit Sv would then apply to the protection
quantity, effective dose, and the corresponding operational quantity, dose equivalent. Limits
to prevent deterministic effects to the lens of the eye, skin, and hands and feet would more
appropriately be set in absorbed dose rather than equivalent dose. It is proposed, therefore,that consideration should be given to discontinuation of the use of equivalent dose as aseparate protection quantity.
E is in widespread use in medical practice as a measure of risk, going beyond its
intended purpose. While doses incurred at low levels of exposure may be measured orassessed with reasonable reliability, health effects have not been reliably demonstrated at suchlevels but are inferred. However, bearing in mind the uncertainties associated with risk
projection to low doses or low dose-rates, it may be considered reasonable to use E as a roughindicator of possible risk, with the additional consideration of variation in risk with age, sex
and population group.

11Revisions to ICRU Operational Quantities
N.E. Hertela, D. BartlettbaGeorgia Institute of Technology, Atlanta, Georgia 30332-0745 and Center for Radiation Protection
Knowledge, Oak Ridge National Laboratory, Box 2008, MS-6153, Oak Ridge, TN 37381-6153, USA;e-mail: nolan.hertel@me.gatech.edu or hertelne@ornl.govbAbingdon, Oxfordshire OX14 1EJ, UK; e-mail: davidtbartlett47@btinternet.com

The International Commission on Radiological Units and Measurements (ICRU)
defines a set of operational dose quantities for the determination of external radiation
exposure by measurement. The protection quantities recommended by the International
Commission on Radiological Protection (ICRP) for the limitation of radiation exposure are
generally not measurable. The ICRU operational quantities are used in practice toapproximate the values of the ICRP protection quantities. The ICRU operational dosequantities for external irradiation in current use were defined 30 years ago.
The rationale for operational quantities has been examined in light of changes in thedefinitions of the ICRP protection quantities and changes in the fields of application of theoperational and protection quantities. Current applications must cover a wider range ofparticle types and energies contributing to doses resulting from external irradiation of worker
and members of the public. The relationship of the new operational quantities
recommendations to the protection quantities has been investigated and will be covered in the
proposed report. In addition the impact of changes on routine measurement practice, including
instrument design and calibration, has been considered. The existing operational quantities
were based on dose equivalent at a point in the ICRU sphere and on dose equivalent at a pointin the body. The proposed operational quantities are particle fluence at a point in space
multiplied by sets of values of conversion coefficients to the protection quantities, effective
dose, absorbed dose to the lens of the eye, and absorbed dose to local skin.

12Issues on the Radiation Weighting Factor
T. SatoNuclear Science and Engineering Center, Japan Atomic Energy Agency,
Shirakata 2-4, Tokai, Ibaraki, 319-1195, Japan; e-mail: sato.tatsuhiko@jaea.go.jp

It is well known that the risk of radiation exposure depends not only on dose and dose
rate but also characteristics of radiation causing the dose. In the radiological protection system,
this dependence is generally called “radiation quality”, and historically represented by linear
energy transfer (LET) in water. The radiation quality factor as a continuous function of LET,
Q(L), was defined in ICRP Publication 26, and its form was revised in ICRP Publication 60
by taking the reduced biological effectiveness of high-LET radiations into consideration. A
similar concept, the radiation weighting factor, wR, was also introduced in ICRP Publication
60, which simply expresses the radiation quality of organ doses by the type and energy of theradiation incident on the body. One reason to introduce this simplified concept was that Q(L)was often interpreted to imply a spurious precision in spite of rather weak biological evidence
to support its justification. However, the concept of radiation weighting factor is considered to
be oversimplified in some situations such as radiation exposure in space as discussed in ICRP
Publication 123.
At the symposium, I would like to discuss the possibility to introduce the radiation
quality factor as a function of specific energy, z, which represents absorbed dose in
microscopic sites such as cells and intracellular structures. A similar concept has already been
proposed by a few decades ago when ICRU defined the quality factor as a function of lineal
energy, y, in ICRU Report 40. The microdosimetry-based quality factor is advantageous incomparison to Q(L) and wR because it can consider:
1. difference in RBE among photons of different energies,
2. difference in RBE among ion species at the same LET,
3. dose effect due to stochastic variation of absorbed doses in each cell, which is particularly
important at low dose, and
4. recent radiobiological findings such as non-targeted effect.
The last 2 items are feasible only when z instead of y is adopted as the index. On the other
hand, its disadvantages are that:
1. the concept as well as the numerical relation of the radiation quality factor becomescomplicated,
2. definitions of z and y are hard to understand for non-specialist of microdosimetry,
3. appropriate site sizes related to the radiation exposure risk must be determined, and4. evaluation of dose distributions as a function of z or y, d(z) or d(y), for various exposure
situation is difficult.
The 1st item seems to be crucial, but it does not directly result in abandoning the simplicity of
the radiological protection system itself. This is because the radiation quality factor is mainly
used for calculating the fluence to effective dose conversion coefficients, and once they areevaluated, people can use the values without considering the difficulty of their evaluation
process. Of course, these disadvantages cannot be overcame within a few years, but I believethat it is worthwhile to try to establish a new microdosimetry-based radiation quality factor asa long term project. The more details of each advantage and disadvantage will be presented at
the symposium.
13

Dose Assessment of Workers and the Public:
Lessons Learned from Fukushima
O. Kurihara
Research Program for Radiation Dosimetry, Research Center for Radiation Emergency Medicine,National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba-city, Chiba, 263-8555,
Japan; e-mail: osakuri@nirs.go.jp

Almost five years have passed since an unprecedented nuclear accident at the Tokyo
Electric Power Company’s (TEPCO’s) Fukushima Daiichi Nuclear Power Plant (FDNPP). An
enormous amount of radionuclides was released into the environment as a consequence of this
accident, which caused radiation exposure to workers in charge of emergency operations at
the site of the accident and the general public. The dose assessment of the exposed individuals
is indispensable although the doses are generally low considering the published information.
However, there exist many difficulties in this, in particular in estimating internal doses due tothe intake of short-lived radionuclides represented by 131I with a physical half-life of 8 days.
Regarding the workers, the individual exposure monitoring could not be sufficiently
performed shortly after the accident. Personal dosimeters were mostly lost by tsunamis andwhole-body counters (WBCs) were disabled because of the elevated radiation levels at the siteof the accident. The monitoring for internal exposure was finally started after the Japan
Atomic Energy Agency (JAEA) sent their mobile WBC unit to Onahama town located about
60 km south of the FDNPP, but its monitoring capability was not enough for a large numberof the workers to be measured. As a result, the largest contributing radionuclide to the internaldose, 131I, could not be detected in most subjects although only the workers whose internal
doses were relatively high were examined in detail at the JAEA’s center located in Ibaraki
Prefecture. According to TEPCO, six workers received the doses over 250 mSv in effective
dose (the maximum: 680 mSv), the temporal dose limit set for the emergency operations at
the FDNPP.
The same problem has also existed in the dose assessment of residents of Fukushima
Prefecture. It was also difficult to perform direct measurements of the residents shortly after
the accident. As a result, there have been only 1,300 data on which the estimation of thyroid
doses due to intake of 131I is directly based. Most of these data were from a screening survey
for examining thyroid exposure to 1,080 children at three places: Kawamata town, Iwaki cityand Iitate village. The National Institute of Radiological Sciences (NIRS) thus estimated theinternal thyroid doses to the residents in the whole area of Fukushima Prefecture using a
combination of the results of the screening survey, late WBC measurements of ~3,000 adults
from various places, and atmospheric transport and dispersion model (ATDM) simulations of
the radionuclides. As a result, the highest internal thyroid equivalent doses were expected toaffect residents of Futaba town, Iitate village and Iwaki city, and their doses were estimated to
be around 30 mSv at most. A large discrepancy between the estimations by the NIRS and
UNSCEAR occurred because the UNSCEAR method included the ingestion dose that shouldbe trivial for most residents, whereas the NIRS method did not.
The presentation provides an overview of the dose assessment related to the FDNPP
accident, and also the remaining issues to be solved in the further studies.

14