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623

ComputationalHuman

Phantoms

and Their

Applications

toRadiation

Dosimetry

Kimiaki Saito

Japan Atomic Energy Agency

Tokai,

Japan

23.1 introduCtion

Various computational human phantoms modeling human bodies have been developed for radiation

dosimetry. The most important eld of application of computational human phantoms is in radiation

protection. The purpose of radiation protection is to protect people and the environment against the

detrimental effects of radiation exposure without unduly limiting the desirable human actions that

may be associated with such exposure (ICRP, 2007). In radiation protection, exposures to ionizing

radiation are managed and controlled so that deterministic effects are prevented, and the risks of

Contents

23.1 Introduction ..........................................................................................................................623

23.2 Dosimetry

in Radiation Protection.......................................................................................624

23.3

Computational

Human Phantoms......................................................................................... 627

23.3.1

Stylized

Phantoms ....................................................................................................627

23.3.2

Voxel

Phantoms ........................................................................................................628

23.3.3

ICRP Reference Phantoms .......................................................................................630

23.4 Dose

Calculation for Radiation Protection Purposes ........................................................... 630

23.4.1

Monte

Carlo Radiation Transport Calculation ......................................................... 630

23.4.2

External

Dose Calculation........................................................................................ 631

23.4.2.1

Photon ........................................................................................................ 631

23.4.2.2 Electron...................................................................................................... 633

23.4.3 Internal

Dose Calculation.........................................................................................634

23.4.3.1

Absorption

Fraction and Specic Absorption Fraction.............................634

23.4.3.2

S-Value....................................................................................................... 636

23.4.4 Environmental

Exposure .......................................................................................... 637

23.5

Application to Other Fields.................................................................................................... 639

23.5.1 Application

to Radiation Therapy ............................................................................ 639

23.5.2

Dosimetry

in Accidents ............................................................................................640

23.5.3

Animal

Phantoms ..................................................................................................... 641

23.6

Summary ..............................................................................................................................642

Acknowledgments..........................................................................................................................643

References......................................................................................................................................643

624 Charged Particle and Photon Interactions with Matter

stochastic effects are reduced to the extent that can be reasonably achieved. For these purposes,

radiation dose limits are set for workers and the public, and the so-called optimization of protec-

tion is performed to reduce exposure as low as can be reasonably achieved. To control and optimize

doses, the effective dose dened as the sum of risk-weighted organ doses has been used. Various

procedures to estimate radiation doses using experimental, theoretical, and simulation approaches

are designated as radiation dosimetry.

It is difcult to measure the organ dose that is the total energy deposited in an organ or tissue

divided by its mass; therefore, organ doses have been evaluated on computational human models

called phantoms with simulation. Exposure conditions are classied into two dominant categories

of external exposure and internal exposure. External exposure is the case where radiation sources

exist outside of a human body, while internal exposure is the case where a human body is exposed

to radiation sources inside the body due to the intake or inhalation of radionuclides. In both cases,

dose coefcients that convert measurable or easily estimable quantities to organ doses and effective

doses

are prepared from simulations assuming typical exposure conditions.

computational human phantom needs to have a body structure properly modeling a human

body: the shapes and weights of dominant organs and tissues need to be modeled properly, and real-

istic elemental compositions and densities need to be assigned to the organs and tissues to ensure

that radiation interactions inside the body and consequent energy depositions be simulated with

demanded accuracy. In radiation protection, stylized phantoms in which the shapes of the body,

organs, and tissues are expressed by combinations of mathematical equations have been widely used

for a long time and provided many essential data. On the other hand, in recent years, voxel phantoms

in which the organs and tissues are expressed as assemblies of small rectangular units called voxels

have come to be developed and used in many ways. Voxel phantoms are usually constructed on the

bases of images from computed tomography (CT) or magnetic resonance imaging (MRI); thus,

realistic

modeling can be attained.

radiation protection, basic dosimetric data have been prepared for the reference Caucasian

persons dened on the statistical analysis of comprehensive anatomical and physiological data

for Caucasians (ICRP, 1975, 2003). Consequently, phantoms have been developed mostly for

Caucasians. This motivated some researchers to investigate difference in doses due to race by devel-

oping Asian phantoms. It would be anticipated that external doses for Japanese are greater than

those for Caucasians generally, since the Asian body size is smaller than that of Caucasians and the

self-shielding effect is smaller for Asians. The Japan Atomic Energy Agency (JAEA) has developed

several Japanese voxel phantoms (Saito et al., 2001, 2008; Sato et al., 2007a,b, 2009) and has investi-

gated dose variation according to races and individuals. Korean (Lee et al., 2006) and Chinese voxel

phantoms (Zhang et al., 2007) have also been developed in recent years. Phantoms developed in the

world so far are reviewed in several publications (ICRU, 1992; Petoussi-Henss, 2002; Lemosquet

etal.,

2003; Zaidi and Xu, 2007).

The

main target of phantoms has been to obtain reference data used in radiation protection;

further, the utility of phantoms has been extended to a variety of research elds. These include

medical applications both in diagnostics and therapy, dosimetry in radiation accidents, dosimetry

for animals aimed at development of radiotherapeutics or environmental protection, evaluation of

exposure

to electromagnetic elds, etc.

This

chapter will describe various phantoms utilized for radiation protection and other purposes,

present examples of basic data obtained from simulation using phantoms, and discuss the present

features

and prospects.

23.2 dosimetry in radiation proteCtion

In radiation protection, a specic dosimetric system has been developed on the basis of the absorbed

dose averaged over an organ or tissue (organ dose). In this dosimetry system, the organ dose is modi-

ed considering biological effectiveness of radiations and organ-specic sensitivity so as to relate

ComputationalHuman Phantoms and Their Applications toRadiation Dosimetry 625

the radiation dose to the radiation risk (detriment) concerning stochastic radiation effects such as

cancer and hereditary effects. The effective dose E, which was introduced in ICRP Publication 60

(ICRP,

1991) and in the succeeding Publication 103 (ICRP, 2007) is dened as:

E w H w w D= =

∑ ∑ ∑

T T

R T,R

(23.1)

where

T,R

is the average absorbed dose in the volume of a specic organ or tissue T due to radiation

type R

is the radiation weighting factor for radiation R

is the tissue weighting factor for tissue T

Σw

= 1

is the equivalent dose

The sum is performed over all organs and tissues of the human body considered in the effective

dose. The unit of effective dose is J kg

−1

with the special name sievert (Sv). The basic concept on

this kind of risk-weighted dose was already accomplished in Publication 26 (ICRP, 1977) as the

effective

dose equivalent H

and was revised as the effective dose E.

The values of w

shown in Table 23.1, taken from Publication 103 (ICRP, 2007), are based on the

relative biological effectiveness (RBE) of the different radiations. However, basic data concerning

RBE are not absolutely sufcient to determine reliable w

especially for charged particles; further

studies

would be desirable.

Ideally,

the radiation weighting to consider differences in biological effects due to radiation

quality should be made at interaction points. In fact, in Publication 26 (ICRP, 1977) radiation

weighting was recommended to be performed at the interaction point using the Q(L) function,

indicating relative biological effectiveness as a function of linear energy transfer (LET). However,

in order to avoid the misunderstanding that the Q(L) function does exactly reect the biological

radiation effects, a macroscopic manner was instead developed in Publication 60 (ICRP, 1991) and

inherited.

Presently, in dose calculations, values of radiation weighting factors are determined at positions

where the radiations enter a human body in external exposures, or at positions of emissions from

radionuclides in internal exposures. This method is somewhat controversial, a radiation is considered

to exert a different effect on a human body according to the initial position and direction, because

the ways to deliver its energy to important organs are different according to the initial conditions.

The radiation weighting factor for photons and electrons is assumed to be 1 regardless of

energy. It is recognized that biological effectiveness for low-energy electrons is higher than that

table 23.1

recommended

adiation

eighting

Factors

radiation type

radiation weighting

Factor, w

Photons 1

Electrons

and muons 1

Protons

and charged pions 2

Alpha particles, ssion

fragments,

heavy ions

Neutrons A continuous function

ofneutron

energy

626 Charged Particle and Photon Interactions with Matter

for high-energy electrons because of substantially higher LET. Figure 23.1 gives the yield of DNA

double strand break (DSB) for electrons obtained from microscopic simulation on track structures

(Watanabe and Saito, 2002) in a manner similar to those described in the references (Nikjoo and

Uehara, 2004; Pimblott and Mozumder, 2004). Complex DNA damage represented by DSB is con-

sidered to be important in terms of biological effects, and the yield obviously changes with electron

energy. The DSB yield for photons also changes according to energy because of the alteration in

secondary electron energy spectrum. Further, it is reported that signicant DNA damage tends to

arise in case that Auger electrons are emitted after photoelectric absorption by elements constituting

DNA (Watanabe et al., 2000; Kobayashi, 2004). Nevertheless, in radiation protection, the radiation

weighting

factor for photons and electrons is taken to be constant as 1 for simplicity.

The

tissue weighing factors shown in Table 23.2 (ICRP, 2007) were determined to represent the

contributions of individual organs and tissues to overall radiation detriment from stochastic effects.

On the basis of epidemiological studies on cancer induction in exposed populations and risk assess-

ments for heritable effects, a set of w

values was chosen considering radiation detriment. They

represent mean values for humans averaged over both sexes and a wide range of ages. To obtain

effective doses, organ doses are calculated using computational human phantoms and weighted

according

to Equation 23.1.

Here,

the basic dose quantities used in general prospective radiation protection are introduced.

However, required dose quantities are different according to purpose. For example, in retrospective

dosimetry for evaluating consequences of accidents, absorbed doses for specic persons should be

DSB

SSB

Electron energy (eV)

DSB×10

–11

(Gy Da

–1

)

SSB× 10

–9

(Gy Da

–1

)

Figure 23.1 Yield of double strand breaks (DSBs) as a function of electron energy.

table 23.2

recommended

issue

eighting

Factors

tissue w

Σw

Bone marrow (red), colon, lung, stomach,

breast,

remainder tissues

0.12 0.72

Gonads 0.08 0.08

Bladder,

esophagus, liver, thyroid 0.04 0.16

Bone surface, brain, salivary glands, skin 0.01 0.04

Total 1.00

Remainder tissues: Adrenals, extrathoracic (ET) region,

gall bladder, heart, kidneys, lymphatic nodes, muscle, oral

mucosa, pancreas, prostate (♂), small intestine, spleen,

thymus,

uterus/cervix (♀).

ComputationalHuman Phantoms and Their Applications toRadiation Dosimetry 627

evaluated but not effective doses for representative humans. In radiation therapy, dose distribution

and around a tumor is critical and organ doses are not always necessary.

23.3 Computational human phantoms

Computational human phantoms are required to ll certain conditions. First, the body structures

need to properly model human anatomies, considering required dose specicity and accuracy.

Generally, in radiation protection, effective doses and organ doses are calculated for the reference

Caucasian male and female that have representative body sizes and organ masses dened by ICRP

Publication 89 (ICRP, 2003). In this publication, a comprehensive amount of anatomical data for

the Caucasian population was accumulated and analyzed, and the reference values of body height,

weight, organ, and tissue masses were dened on the basis of these analyses. Phantoms used for

radiation

protection purposes basically need to have the reference anatomy.

Another

important requirement is that the elemental compositions and densities of organs and

tissues be appropriately considered. It depends on the type and energy of radiation considered in

simulation and how precise the data must be given on elemental compositions. Generally speak-

ing, skeletal parts need to be given their specic elemental compositions because they are obvi-

ously different from other soft tissues; they contain a relatively large number of heavy atoms that

cause radiation interactions that are different from those caused by light atoms. The precise ele-

mental compositions of organs and tissues are tabulated in ICRP Publication 89 (ICRP, 2003).

Compositions of important tissues are tabulated in Table 23.3.

Computational phantoms are classied into two categories: one is stylized phantoms and

theother is voxel phantoms. In this chapter, each type of phantom will be introduced with several

typical

examples.

23.3.1 Stylized phantoMS

In a stylized phantom, the outer shapes of the body, organs, and tissues are expressed by a combi-

nation of mathematical equations showing a plane, sphere, ellipsoid, cylinder, elliptical cylinder,

cone, etc. For example, in Adam and Eva developed at GSF (Kramer et al., 1986) (Figure 23.2), the

trunks and arms are represented by elliptical cylinders, and the legs by truncated circular cones.

The special features of these kinds of phantoms are that the sizes of the body and organs can be

easily changed by adjusting parameters used in the mathematical equations, and that organs can

be located as desired provided they do not overlap. These features enable one to easily construct

phantoms that have reference Caucasian anatomical properties dened by ICRP. Several stylized

phantoms of the reference Caucasian have been constructed. This kind of phantom was developed

as the MIRD-5 phantom at ORNL for internal dosimetry (Snyder et al., 1969), and since then many

stylized phantoms have been constructed and employed in different ways.

table 23.3

Composition

of h

uman

issues

organ/tissue

elemental Composition (% by mass)

h C n o n

p s Cl k Ca Fe

Fat 11.4 59.8 0.7 27.8 0.1 0.1 0.1

Skin 10.0 20.4 4.2 64.5 0.2 0.1 0.2 0.3 0.1

Muscle 10.2 14.3 3.4 71.0 0.1 0.2 0.3 0.1 0.4

Lung 10.3 10.5 3.1 74.9 0.2 0.2 0.3 0.3 0.2

Active

marrow 10.5 41.4 3.4 43.9 0.1 0.2 0.2 0.2 0.1

Bone mineral 3.5 16.0 4.2 44.5 0.3 0.2 9.5 0.3 21.5

628 Charged Particle and Photon Interactions with Matter

Relations among some typical stylized phantoms are shown in Figure

23.3. The MIRD-5 phantom is a hermaphrodite (Snyder et al., 1969): it

has the body size of the reference male and testis, ovaries, uterus, but

not female breast. Cristy (1976) developed several phantoms with dif-

ferent sizes representing different ages for internal dosimetry; they were

modied by Yamaguchi (1992) for external dosimetry. Japanese phan-

toms were developed by Kerr et al. (1976) to evaluate external organ

doses in atomic bombardments at Hiroshima and Nagasaki. Sex-specic

phantoms were constructed by Kramer et al. (1986) for external photon

dosimetry and by Stewart et al. (1993) for external neutron dosimetry.

A pregnant woman was modeled by Kai (1985) intended for emergency

dosimetry in the environment. These stylized phantoms have provided

precious

data from the viewpoint of radiation protection.

23.3.2 voxel phantoMS

According to the method employed, stylized phantoms have limits in mod-

eling human anatomy: it is difcult to model realisticshapes and positional

relations of organs and tissues in the body. While three-dimensionally

precise CT or MRI medical images have become easily available, voxel

phantoms based on the medical images for specic persons have come to

be developed. Voxel phantoms consist of small rectangular units called

voxels (volume pixels). The identication (ID) number for an organ or

tissue is assigned to each voxel, and the assembly of voxels having the

same ID number represent the organ or tissue. An assembly of voxels

Adam

Figure 23.2 Stylized

phantoms Adam developed

at GSF. (Courtesy of Maria

Zankl, Helmholtz Zentrum

Munich, Germany.)

MIRD-5 phantom

Snyder et al. (ORNL, 1969)

Internal

Hermaphrodite

Reference Caucasian adult

Age-specific phantoms

Cristy (ORNL, 1976)

Internal

Hermaphrodite

Reference Caucasian

Age-specific phantoms

Yamaguchi (JAERI, 1992)

External

Hermaphrodite

Reference Caucasian

Sex-specific phantoms

Kramer (GSF, 1986)

External photons

Reference Caucasian adult

Sex-specific phantoms

Stewart et al. (PNL, 1993)

External neutrons

Reference Caucasian adult

Pregnant phantom

Kai (JAERI, 1985)

External photons

Japanese pregnant woman

Japanese phantoms

Kerr et al. (ORNL, 1976)

External(Hiroshima, Nagasaki)

Hermaphrodite

Japanese

Figure 23.3 Typical stylized phantoms with different characteristics.

ComputationalHuman Phantoms and Their Applications toRadiation Dosimetry 629

can model any structure within the resolution of the voxel size, and the voxel phantom can express

human anatomy realistically because it is based on medical images of a real human.

The rst voxel phantoms intended for radiation protection uses were Baby and Child developed

at GSF (Zankl et al., 1988; Veit et al., 1989). Baby was constructed from the CT data of a dead

8-week-old baby; Child was constructed from the CT data of a 7-year-old child who suffered from

leukemia. They have been used for external exposure evaluation in fundamental conditions (Zankl

et al., 1988; Veit et al., 1989) and also in environmental conditions (Saito et al., 1990). After 2000,

the number of voxel phantoms has increased gradually, especially in recent years, many voxel phan-

toms have been constructed. The main voxel phantoms developed so far are tabulated on the Web

site

of the Consortium of Computational Human Phantoms (Xu, 2008).

Most

of the phantoms for radiation protection purposes, both stylized phantoms and voxel phan-

toms, have been constructed for Caucasians. Researchers at JAEA desired to clarify the difference

in doses due to the anatomical differences between Caucasians and Asians. From this viewpoint,

JAEA constructed the rst Asian voxel model, Otoko (Saito et al., 2001). Since then, ve Japanese

voxel phantoms in total have been completed from CT data: three male adult phantoms and two

female adult phantoms (Saito et al., 2001, 2008; Sato et al., 2007a,b, 2009). All the CT data were

taken for healthy volunteers at the Fijita Health University Hospital after receiving the Ethics

Committee’s

approval.

Pictures

of voxel phantoms that have been developed are shown in Figure 23.4, and the physi-

cal characteristics are listed in Table 23.4. In these phantoms, compact bone and bone marrow are

separately modeled in each skeletal voxel according to the CT value after the GSF method (Zankl

et al., 1988; Veit et al., 1989), enabling users to calculate doses taking the bone marrow distribution

the body into account.

The

rst generation phantoms Otoko (Saito et al., 2001) and Onago (Saito et al., 2008) have a

voxel size of 0.98 × 0.98 × 10 mm

, while the second generation phantoms JM (Sato et al., 2007a)

Otoko Onago JM JF

Figure 23.4 Japanese voxel phantoms developed at JAEA.

table 23.4

physical

Characteristics of v

oxel

hantoms

eveloped

at J

aea

gender

otoko onago Jm JF asian reference man

male Female male Female male Female

Weight

(kg) 65 57 66 44 64 46

Height

(cm) 170 162 171 152 170 155

Slice thickness (mm) 10 10 1 1 — —

Pixel side length (mm) 0.98 0.98 0.98 0.98 — —