2. Brief Overview
Definition
MIRD Internal Dosimetry Method
Biokinetic Model for Radiopharmaceutical uptake and
elimination
S-factor
Dose to Target
Sample Calculation
Biokinetic Model for Embryo and Foetus
Dose to infant via breast milk
Example
Summary
References
3. Internal Radiation Dosimetry
Radiation that happen inside the body due to uptake of radiopharmaceuticals
cannot be measured directly.
Therefore biokinetic and dosimetric model are needed in order to calculate
radiation doses received by a person.
Relevant Organization
MIRD – Medical Internal Radiation Dose Committee of the Society of
Nuclear Medicine
Standard methods to estimates internal doses
ICRP –International Committee on Radiological Protection
Calculate doses for many radiopharmaceuticals based on best available data
Local Organization e.g. AELB (Malaysia), ARSAC (UK), NUREG (US) etc
Guidelines on radiopharmaceuticals limit, dose to children, etc
4. Definition
Absorbed Dose
Absorbed Dose (Gray)
Energy deposited per unit mass
Dm = dε
dm
Joules/kg = Gray (Gy)
*Medium should always be specified
Old unit - rad, 100rad = 1 Gy
5. Dose Equivalent = Sievert (Sv)
To reflect biological effect
Absorbed Dose x Radiation Weighting Factor (WR)
*was known as effective dose equivalent
Effective Dose = Sievert (Sv)
• Uniform dose to the whole body that would have the same
risk
- Dose X Tissue Weighting Factors
• Unit sieverts, Sv
6. MIRD Method – 5 steps
Consider uptake organ as source organ
Part that absorb the radiation as target organ (source and
target can be the same organ)
Step 1 : Cumulated Activity Target (e.g. lung)
Step 2 : S – Factor
Radiation rays
Step 3 : Dose to Target Organ
Step 3 : Effective Dose to Whole body Source
(e.g. heart)
Step 5 : Total dose for administered activity
7. Activity
Activity – rate of disintegration (1 Bq = 1 disintegration per
second)
= physical decay constant
This represent exponential decay with physical half life
8. Activity
Fraction of Pharmaceutical (Fs) = rate of biological uptake and
elimination in source organ ‘S’.
Activity time curve = how activity in source organ change with
time.
activity is different to fraction of pharmaceutical because it takes into
consideration radioactive decay
Activity in source organ = Administered activity x Fraction of
pharmaceutical x Decay factor
9. Cumulated Activity
Cumulated activity in source organ is defined as area under
activity time curve (Bq.sec or MBq.hr)
Residence time is defined as accumulated activity divided by
administered activity
or using effective half-life
10. Residence time is more practical than cumulated activity
because it is independent from administered activity.
Use of residence time (in hr) instead of cumulated activity
(MBq.hr) allows for calculation radiation dose per
administered activity.
11. Effective Half -Life
λeffective = λ physical + λbiological
Biological can be either uptake or elimination
In terms of half-life,
12. Biokinetic Models of Radiopharmaceutical
Uptake for Dosimetry Calculation
Step 1: Calculation of Residence Time
5 Basic Biokinetic Model
13. Model 1 – Instantaneous Uptake with No
elimination
Fs
Pharmaceutical e.g. I-131
λp
Activity
Fraction
Time
23. Step 2: The ‘S-Factor’
S-factor is considered to be a calculation of energy emitted by
radiation of certain type of isotope and fraction of that energy
absorbed by organ.
So, S-factor can be define as absorbed dose per unit cumulated
activity (Gy/Bq.sec or μGy/MBq.hr)
MIRD pamphlet 11 tabulated ‘S’ factor to target organ for
large selection of radiopharmaceuticals based on Monte Carlo
simulation with ‘70kg mean man’phantom (given in rad/μCi.hr)
25. Step 3: Dose to Target Organ
Absorbed Dose to target organ ‘t’ from all source organs ‘s’
Unit μGy/MBq
Equivalent Dose
= Absorbed Dose * Radiation Weighting Factor (or Quality
factor)
Unit (μ Sv/MBq)
Its a measure of biological effectiveness of different type of radiation
energy.
In nuclear medicine the quality factor is 1.
In nuclear medicine, absorbed dose = equivalent dose
26. Radiation Radiation Weighting Factor (Quality Factor )
X-rays, gamma rays, beta rays 1
Alpha rays, heavy nuclei 20
Proton 2
* Source ICRP Report 103
Step 4: Effective Dose
Effective Dose is the weighted sum of all target organ
doses (μSv/MBq)
28. Step 5: Total dose for Administered Activity
Result * Administered Dose
- Absorbed Dose (mGy)
- Equivalent Dose (mSv)
- Effective Dose (mSv)
Dose to Children
29. Final Result
MIRD published methods on how to calculated absorbed dose,
equivalent dose and effective dose based on several model of
radiopharmaceuticals uptake.
However, absorbed dose value for a lot of radiopharmaceutical
used in nuclear medicine can also be found in ICRP report 53
and 80. This report calculated the data based on best available
data on radiopharmaceutical with ‘70 kg mean man’ phantom.
30. So what’s the use of MIRD?
MIRD method is useful when we want to do specific calculation or
custom calculation for patient based on patient’s individual uptake of
radiopharmaceuticals.
Example, we want to know dose to uterus for a patient who has a
tumour near kidney or adrenal gland which has a high
radiopharmaceutical uptake.
Usually we take dose to uterus as an estimation for dose to foetus
from the ICRP publication. But ICRP result only take into account
contribution from standard source organs to uterus.
In this case, we can assume that there is an addition source organ,
the tumour which will contribute a significant dose to the uterus.
31. Solution
Combine dose value from tumour and absorbed dose to
uterus from ICRP Report.
Example: A patient was given 200MBq of Tc-99m DTPA and a
tumour was found near adrenal gland with high
radiopharmaceutical uptake. What is the absorbed dose to
uterus?
From ICRP Report 53 (Tc-DTPA, bi-exponential elimination,
normal renal function)
Residence time = 1.97hr, Fs = 1.0
From MIRD 11, ‘S’ factor for adrenal gland to uterus
= 1.1E-6 rad/μCi.hr = 2.97E-01 μGy/MBq.hr
*We assume ‘S’ factor for tumor is similar to adrenal gland because of the
anatomical position.
33. Calculation
Additional absorbed dose to uterus from tumour
=
From ICRP 53
Absorbed Dose to Uterus = 7.9E-03 mGy/MBq.
Total absorbed dose to fetus from administered activity
=
34. Biokinetic Model for Embryo and Foetus
ICRP Publication 88 published biokinetic and dosimetric model
also dose coefficient for embryo and foetus due to
radiopharmaceutical uptake by mother
First 8 weeks of pregnancy (mass < 10g)
Dose rate = dose rate to uterus
More than 8 weeks
Dose rate = maternal activity + activity which has cross the placenta
and has accumulated into the foetus tissue.
Some radioisotope like iodine can cross the placenta.
At birth
There might be some activity left in infant. This is use to calculate
committed effective dose equivalent until the age of 70 years old
35. Dose to infant via breast milk
ICRP 95 gives dose coefficient for ingestion of breast milk
by infant after activity uptake by mother.
A different biokinetic model for radiation pathway was
used in the calculation of the coefficient.
An Annex of ICRP Publication 95 also examines the
external dose to infants by contact with its mother who
has radioactivity uptake.
In some case e.g. mother’s uptake of insoluble gamma-emitters.
External dose to infant might be higher than internal dose.
36. Example
A female patient fell pregnant after 58 days of receiving
15mCi of I-131 for treatment for Thyrotoxicosis.
Calculate dose to foetus due to activity administered.
Solution:
There are 3 ways to solve this.
1. Start from scratch using MIRD method.
2. Using value from ICRP 53
3. Using dose coefficient of biokinetic modelling from
ICRP 88.
37. Using value from ICRP 53
We calculate from from 58 days onwards.
At 58 days, activity remaining = 0.1010 mCi = 3.7370MBq
Dose to fetus = dose to uterus.
Days/ Thyroid Thyroid Thyroid Thyroid Thyroid Thyroid Thyroid
Uptake Uptake Uptake Uptake Uptake Uptake Uptake
Activity (MBq) 0% 5% 15% 25% 35% 45% 55%
(mGy) (mGy) (mGy) (mGy) (mGy) (mGy) (mGy)
Absorbed dose at organ 5.40E-02 5.50E-02 5.40E-02 5.20E-02 5.00E-02 4.80E-02 4.60E-02
(uterus)/ Adult per unit
activity administered (mGy/MBq)
Day one (Administered)(8 May 2012)
562.4 30.37 30.93 30.37 29.24 28.12 27.00 25.87
Day five (Discharge) (13 May 2012)
342.06 18.47 18.81 18.47 17.79 17.10 16.42 15.73
Day 40 (18 June2012)
16.73 0.90 0.92 0.90 0.87 0.84 0.80 0.77
Day 58 (Might be pregnant) (
01/07/12)
3.73 0.20 0.20 0.20 0.19 0.19 0.18 0.17
39. Result
Dose to fetus is around 0.19 – 0.16 mGy.
Using biokinetic modeling of ICRP 88,
< 8 weeks = dose to uterus
From 8 weeks until birth at 38 weeks, the dose is estimated
using element specific tissue activities and retention times.
40. Solution
At conception effective dose coefficient = 7. 8E-11 Sv/Bq
Activity, 3.73MBq
So, 7.8E-11*3.54MBq = 0.27 mSV
Using ICRP 53, the value was 0.17 - 0.20 mGy, depending on
iodine uptake( = equivalent dose of 0.17 - 0.20mSv)
From ICRP 84 (later adapted by IAEA) recommendation,
there is no justification for termination of pregnancy as
the dose received by foetus <100mGy. There is no
evidence of detrimental effects to foetus.
41. Foetus Risk
Mentrual / Conception age <0.01 Gy 0.05 – 0.1 Gy > 0.1 Gy
gestational age (weeks)
(weeks)
0-2 Prior to conception None None None
3–4 1–2 None Probably None Possible spontaneous abortion
5 – 10 3–8 None Potential effect uncertain Possible malformation, increase with
dose
11 – 17 9 – 15 None Potential effect uncertain Increased risk mental retardation of
deficit in IQ
18 – 27 16 – 25 None None IQ deficits not detectable at diagnosis
dose
> 27 > 25 None None None application to diagnostic
medicine
* Taken from ICRP 84 and 90
42. Pregnancy and breastfeeding following
treatment
ICRP / IAEA recommends women do not become pregnant
until estimated foetal dose falls below 1mGy (100mrem)
(diagnostic application)
For therapeutic treatment – 6 months after treatment. Not
because of radiation dose risk, more to make sure that
treatment was effective and follow-up treatment can be
carried out without obstruction.
Some organization (e.g. ARSAC) published elapsed time
between treatment and breastfeeding after taking into account
the activity that might transfer to infant for selected
radiopharmaceuticals
44. Why we need to know all this?
How Bad is Bad?
Dosimetry calculation allows us to quantify the doses received
by patient and used that as a measurement of radiation risk
Sievert was design to represent stochastic biological effects of ionizing
radiation.
1 Sv = 5.5% probability of developing cancer (ICRP103)
Organization which actively involve in radiation protection use
specific dose value as guideline.
AELB 2010 guideline :
Public <1mSv/yr
Radiation worker <20 mSv/ yr
Foetus < 1mSv for the duration of pregnancy
45. Dose value can be use as a reference across all radiation
related exposure.
Effective dose from CT, X-Ray, radiotherapy, dental radiograph, airport
security screening can all be sum up together
Result from dose calculation can be use as benchmark on
whether a certain procedure is worth it or not.
46. References
[1] Dr Richard Lawson, Notes Radiation Dosimetry [Lecture Notes], Manchester Royal Infirmary, (March 2011)
[2] MIRD Pamphlet no 5, Estimates of Absorbed Fractions for Monoenergetic Photon Sources Uniformly
Distributed in Various Organs of a Heterogeneous Phantom; L.T. Dillman and F.C.Van der Lage Littman, Society
of Nuclear Medicine, New York (1969).
[3] MIRD Pamphlet no 10, Radionuclide Decay and nuclear parameters for use in in radiation dose estimation,
L.T. Dillman and F.C.Van der Lage Littman, Society of Nuclear Medicine, New York (1975).
[4] MIRD Pamphlet no 11, ‘S’ Absorbed Dose per unit Cumulated Activity for Selected Radionuclides and
Organs , W.S Synder, M.R. Ford, G.G. Warner and S.B. Watson, Society of Nuclear Medicine, New York (1975).
[5 ICRP Publication 53 Radiation Dose to Patient from Radiopharmaceuticals, Annals of the ICRP, vol 18,no 1-4
(1987)
[6] ICRP Publication 80 Radiation Dose to Patient from Radiopharmaceuticals, Addendum to ICRP 53, Annals of
the ICRP, vol 28,no 3 (1998)
[7] ICRP Publication 84 Pregnancy and Medical Radiation, Annals of the ICRP, vol 30,no 1 (2000)
[8]Notes of Guidance on the Clinical Administration of Radiopharmaceuticals and of sealed Radionuclide
Sources, Administration of Radioactive Substances Advisory Committee, 2006.
47. References
[9] ICRP Publication 88 Doses to the Embryo and Fetus from Intakes of Radionsuclides by the
Mother, Annals of the ICRP, vol 31,no 1-4 (1987)
[10] ICRP Publication 90 Biological Effects after Prenatal Irradiation (Embryo and Fetus),
Annals of the ICRP, vol 33,no 1-2 (2000)
[11] ICRP Publication 95 Radiation Dose to Patient from Radiopharmaceuticals, Annals of the
ICRP, vol 34,no 1-4 (1987)
[12] ICRP Publication 103, The 2007 Recommendations of the International Commission on
Radiological Protection. Annals of the ICRP Vol 37, no 2-4 (2007)
[13] Peraturan-peraturan Perlesenan Tenaga Atom (Perlindung Sinaran Keselamatan Asas)
2010. Lembaga Perlesenan Tenaga Atom, Malaysia (2010)
