ICRP Dosimetry Models

Learning Objectives

• Describe the concepts used in different types of dosimetry models

• Describe the structure of the ICRP Bone Dosimetry Model

Reference Man • We almost never compute the internal dose to a real

individual; instead, we compute the dose to Reference Man (or Woman or Child)

• ICRP Publication 89 (2003) gives the anatomic and physiological parameters for the reference individuals

• Reference Man is 170 cm tall, has a mass of 70 kg, and excretes 1.6 L of urine per day

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Anatomical Model • Divide body into two sets of regions

– Source regions are regions containing radioactive

– Target regions are those tissues at risk

• Region may be a member of both sets

• Some source regions are not living tissue; contents of GI-tract

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Phantoms Currently Used by ICRP Series Special

• Newborn

• 1 y old

• 5

• 10

• 15

• Adult (hermaphrodite)

• Adult Female

• 3 mo pregnant

• 6 mo pregnant

• 9 mo pregnant

• Fetus (8 - 38 wk) – 7 fetal ages

Phantoms defined in terms of conic sections,i.e., closed form mathematical expressions.

Mathematical dosimetry model

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ORNL Family

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Models: Mathematical vs. Voxel

Mathematical and Voxel Models

Progression of Phantom Formats

Stylized Phantoms (Flexible , yet anatomically unrealistic)

Voxel Phantoms (Anatomically realistic, yet inflexible)

Hybrid Phantoms (Both anatomically realistic and flexible)

UF Family of Hybrid Phantoms

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Anatomical Model • Distribution of absorbed energy in

the body assessed using Monte Carlo radiation transport calculations

• Absorbed fraction quantity is the fraction of the energy emitted in source region that is absorbed in target region.

• Source is assumed to be uniformly distributed by volume

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Anatomical Model • Specific absorbed fraction is

absorbed fraction per unit mass of target

• Alpha particles and beta particles are typically absorbed in source regions – AF(T<-S) = 1, T = S

– AF(T<-S) = 0, T ≠ S

• Photon and neutron detailed calculations

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Absorbed Fraction

• Typical photon SAF pattern with source = target region.

• At low photon energy SAF approaches 1/mT

• Soft tissues of similar composition

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Absorbed Fraction

• Typical SAF pattern with source and target regions are distinct.

• Magnitude depends on spatial relationship of the regions and relative size of the regions.

• Soft tissues differ little in their composition

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Electrons: Bronchial Cells at Risk

• Electron AF for secretory and basal cells in bronchial wall (BB region)

Mucus (Gel Layer)

10 microns0

30 microns35 microns

50 microns

Macrophage Layer 500 microns thick

Nuclei ofSecretory

Cells

Nuclei ofBasal Cells

- 6 microns

-11 microns

Mucus (Sol Layer)Electron Energy (MeV)

10-2 10-1 100 101

AF

(Bas

al C

ells

)

0.000

0.050

0.100

Sol Layer Gel Layer

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Bone Dosimetry Model

• Two types of bone: – Cortical bone: hard, hollow cylinder, mostly

appendicular skeleton, 4 kg – Trabecular bone: spongy, honeycomb, mostly

axial skeleton, 1 kg • Each type has one-half total bone surface • Two target tissues:

– Bone surface cells – Red marrow cells (only in trabecular bone)

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Bone Dosimetry Model, (Con’t.) • Six types of radiation sources:

– all photon emitters in body

– alpha emitters in bone volume

– alpha emitters on bone surface

– beta emitters in bone volume

– beta emitters on bone surface (E<0.2 MeV)

– beta emitters on bone surface (E>0.2 MeV)

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Bone Dosimetry Model, (Con’t.) • Six types of radiation sources:

– all photon emitters in body

– alpha emitters in bone volume

– alpha emitters on bone surface

– beta emitters in bone volume

– beta emitters on bone surface (E<0.2 MeV)

– beta emitters on bone surface (E>0.2 MeV)

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Assumed Distribution of Activity

• Activity initially deposits on bone surfaces

• Models for alkaline-earth and actinides consider burial by apposition of new bone

• All other elements – Biokinetic model specified volume or surface seeker

– If physical half-lives < 15 days then decays taken to occur on bone surfaces

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Absorbed Fractions for Bone Seekers