Internal dosimetry:Concepts and MIRD formalism

MIRD schema General formalism for calculation of absorbed

doses from internal radio nuclides– Loevinger & Berman 1976

Medical Internal Radiation Dose Committee

of the Society of Nuclear Medicine– MIRD pamphlets

Anthropomorphic phantom / Reference man 70kg– Snyder et al. 1975 – Loevinger et al.1991

ICRU report 67, 2002

Chris J. Huyskens @2006

MIRD schema

Basic concepts for dosimetry

Source Target model

absorbed fraction in target

conventions for notations

multiple source single target

application for internal dosimetry– in nuclear medicine– radiological protection

strengths and limitations


MIRD basic concepts

Absorbed dose rate to target ‘tissue’

from nuclear transformations in a ‘source’ tissue

Absorbed fraction of the energy emitted by radioactivity in the source region that is absorbed in the target region

Mean energy emitted per nuclear transition

= N .E

D N E Am


MIRD basic concepts

The specific absorbed fraction is defined as the absorbed fraction per unit mass of the target region

Cumulated activity represents the total number of radioactive transformations in the source region over time of interest

time integral of the activity

m

~ t d tt

t

1

2


MIRD mean absorbed dose

the mean absorbed dose to the target volume from nuclear transitions in the source region results from integration of the absorbed dose rate over the time interval

DA t

md t

t

t

( )

1

2

DA

m

~

D ~ D A S~


MIRD basic assumptions for

for non penetrating radiations / beta– if source and target is the same then = 1– if source and target is different then = 0

for penetrating radiations / photons– for all source – target combinations 0 < < 1


MIRD (specific) absorbed fraction

Absorbed fraction of the energy emitted by radioactivity in the source region that is absorbed in the target region

The specific absorbed fraction is defined as the absorbed fraction per unit mass of the target region

Reciprocity theorem : for any pair of regions, the specific absorbed fraction is independent of which region is designated as source or as target region

this implies

i k h k h i h kr r r r r r 1

m


MIRD specific absorbed dose in target

absorbed dose in target region per unit cumulated activity in source region

absorbed dose in target region per transformation in source region

mean absorbed dose in target region

mean absorbed dose per unit administered activity Ao D

AS

0

D A S~

Sm


MIRD cumulated activity

the activity in the source region is represented by the sum of exponentials for each biological process j that contributes to deposit and/or clearance of radioactive material in source region.

the cumulated activity follows from integrating A(t) over time interval t1 –t2

A t e A etj

t

j

j( )

~( )( ) ( )A

Ae ej

j

t

j

tj j

1 2


MIRD residence time in source region

residence time in source region is defined as

encompasses the uptake of radioactivity in the source region relative to the administered activity Ao

not to be confused with the mean lifetime of the radioactivity

Residence time for radio nuclides– Loevinger et al.1991

~A

A 0


MIRD half-time / half live

physical half - life / physical decay constant

biologic half- time of biologic component j

effective half - time for biologic component j

T (ln 2 )

Tb j j ln / .2

Te j j, ln / 2 1 1 1

T T Te j j

j e j

,

,


MIRD multiple source h / single target k

mean absorbed dose in target region

specific absorbed dose in target region– mean absorbed dose per transformation in

source region– mean absorbed dose per unit cumulated

activity in source region

D A S r rk h k hh

~

S r r r r

r r

mk h i ii

k hi i k h

ki


MIRD anthropomorphic phantom

Assumptions in constructing the MIRD phantom

major organs are taken as source and target regions

target region is radio sensitive (part of) organ

radioactivity uniformly distributed in the source organs

source and target regions homogeneous in composition

a single 70-kg phantom to represent all persons– Snyder phantom


MIRD symbols & conventions ICRP- mean absorbed dose (Dk) in target organ (k)

- committed equivalent dose (HT) in target organ (T)

- committed effective dose

source region (h) & target region (k)

- source organ (S) & target organ (T)

absorbed fraction Φ(rk rh)

- absorbed fraction AF(TS)

mean absorbed dose per unit cumulated activity S(rkrh)

- specific effective energy SEE(TS)

cumulated activity in source region

- committed number of transformations in source organ Chris J. Huyskens @2006

MIRD source target model ICRP


MIRD symbols & conventions ICRP

MIRD : mean absorbed dose in target region

ICRP : committed equivalent dose in target organ

D A S r rk h k hh

~

S r r r r

r r

mk h i ii

k hi i k h

ki

H U SE E T sT Si

ii


MIRD formalism strengths

The utility of the MIRD formalism lies in its simplicity and generality

clear separation of physics and biology:– physical aspects: embedded in S values– biologic aspects: embedded in cumulated

activity & residence time

the organ S-values are published in MIRD pamphlets

ICRP: SEE values are based on revised values for the absorbed fraction AF

– Cristy & Eckerman, 1987, 1993


MIRD strengths

advanced -dedicated- internal dosimetry based on MIRD formalism for:

– complex composition and geometry of the source & target regions

– non uniform distribution of radioactive material in source region

– temporal dependence of the mass of organs

The MIRD schema can accommodate a wide variety of radio nuclide dosimetry applications

– nuclear medicine diagnostics & therapy– internal contamination in radiation protection



MIRD mean absorbed dose

m

DA t

md t

t

t

( )

1

2

DA

m

~D ~

D A S~ .

= N .E ~ t d tt

t

1

2

DA

S0

.


Education

Internal dosimetry:Concepts and MIRD formalism