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
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
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~
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
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
,
,
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
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
Chris J. Huyskens @2006
Chris J. Huyskens @2006
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
.
Chris J. Huyskens @2006