Chapter 10Basic sciencesLast reviewed: · What's new
Radiation Dosimetry and Safety
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Learning objectives and modules3 modules · about 2 h 25 min
Learning objectives
After this chapter you will be able to:
Distinguish absorbed dose, equivalent dose, effective dose and the operational quantities with their units.
Calculate absorbed dose with the MIRD formalism: cumulated activity, effective half-life and S value.
State occupational and public dose limits, worker categories and area classification.
Explain personal dosimetry methods and correct use of the dosimeter.
List the steps to follow after a spill or contamination.
Apply the criteria for radiopharmaceutical administration in pregnancy and breastfeeding and for patient release after radioiodine therapy.
Modules
The chapter is divided into study modules without changing the order of the text: read, watch the animations, then answer the questions linked to those headings.
Category A: likely to receive >6 mSv/year effective, >15 mSv/year eye lens, >150 mSv/year skin/extremity; Category B: those who may exceed the public limit
✓
Absorbed dose: Gray (Gy) = 100 rad → a physical quantity, does not take radiation type into account
✓
Equivalent dose: Sievert (Sv) = 100 rem → corrected with the radiation weighting factor (wR)
✓
Effective dose: Sievert (Sv) → corrected with the tissue weighting factor (wT) → indicator of stochastic risk
✓
For photons and beta wR = 1 → absorbed dose (Gy) = equivalent dose (Sv) numerically
10.1 Dosimetric Quantities
Radiation dosimetry covers the quantities used to measure and evaluate the effect of radiation on biological tissue. In nuclear medicine, a thorough understanding of these concepts is critical for both patient dosimetry and staff dosimetry.
10.1.1 Absorbed Dose
Definition: The amount of energy imparted to tissue by radiation per unit mass.
Formula:
D = dE / dm
•
D: Absorbed dose
•
dE: Energy imparted (joule)
•
dm: Mass (kg)
Unit:
•
SI unit: Gray (Gy) → 1 Gy = 1 J/kg
•
Old unit: rad → 1 rad = 0.01 Gy = 1 cGy
•
Conversion: 1 Gy = 100 rad
Properties:
•
A purely physical quantity → does not take radiation type or biological effect into account
•
Applies to all types of radiation (alpha, beta, gamma, neutron)
•
Independent of tissue type
•
The basic building block of dosimetric calculations
BOARD TIP
▸
"1 Gy = ? rad"
→ 100 rad.
▸
"1 rad = ? Gy"
→ 0.01 Gy = 1 cGy. These conversions are asked frequently in the exam. Remember the 100-fold difference between Gy and rad!
Definition: The absorbed dose corrected for the biological effectiveness of the radiation type. It reflects the fact that different radiation types cause different biological damage at the same absorbed dose.
Formula:
HT = ΣR wR × DT,R
•
HT: Equivalent dose in organ T
•
wR: Radiation weighting factor
•
DT,R: Absorbed dose to organ T from radiation type R
wR values are derived from RBE data for stochastic effects at low doses; the quality factor Q (formerly QF) used in operational quantities is defined as a function of LET
Radiation Weighting Factors (wR) – ICRP 103:
Radiation Type
wR
Photons (X-rays, gamma)
1
Electrons (beta)
1
Positrons
1
Protons
2
Alpha particles
20
Neutrons
Energy-dependent (2.5–20)
Heavy ions
20
BOARD TIP
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Memorise the radiation weighting factors: Photon/beta/positron = 1, Proton = 2, Alpha = 20, Neutron = energy-dependent (2.5–20). In the exam, "wR of an alpha particle?" → 20.
▸
"wR of gamma rays?"
→ 1. Because wR = 1 for photons and beta, for these radiations the absorbed dose (Gy) = equivalent dose (Sv) numerically.
Definition: The total body dose calculated by taking into account the radiation sensitivity of the different organs and tissues of the body. It is a general indicator of stochastic risk (cancer, hereditary effects).
Formula:
E = ΣT wT × HT
•
E: Effective dose
•
wT: Tissue weighting factor
•
HT: Equivalent dose in organ T
Unit: Sievert (Sv) – the same unit as equivalent dose but with a different meaning
Tissue Weighting Factors (wT) – ICRP 103:
Tissue/Organ
wT
Bone marrow (red)
0.12
Colon
0.12
Lung
0.12
Stomach
0.12
Breast
0.12
Gonads
0.08
Bladder
0.04
Oesophagus
0.04
Liver
0.04
Thyroid
0.04
Bone surface
0.01
Brain
0.01
Salivary glands
0.01
Skin
0.01
Remainder tissues
0.12
TOTAL
1.00
Key points:
•
The sum of the wT values must always be 1.00
•
With ICRP 103 (2007), the gonad wT value was reduced from 0.20 to 0.08 (compared with ICRP 60)
•
The breast wT value was raised from 0.05 to 0.12
•
Tissues with the highest wT value (0.12 each): bone marrow, colon, lung, stomach, breast, remainder tissues
•
Collective effective dose: The sum of the effective doses of individuals in a population (unit person·Sv / man·Sv). It is used to compare the burden of medical exposure on society; it should not be used to estimate individual risk
•
Effective dose per unit activity in adults: Tl-201 ~0.14-0.22, Ga-67 ~0.10, In-111 DTPA ~0.02, most Tc-99m agents ~0.005-0.01 mSv/MBq. Single-photon agents with long half-lives give a much higher dose per MBq
BOARD TIP
▸
"Organs with the highest wT value in ICRP 103?"
→ Bone marrow, colon, lung, stomach, breast (0.12 each).
▸
"How did the gonad wT value change from ICRP 60 to 103?"
→ 0.20 → 0.08 (decreased).
▸
"Breast wT value?"
→ 0.12 (in ICRP 60 it was 0.05 → increased). These changes are asked frequently in the exam!
10.1.4 Exposure
Definition: The amount of ionisation produced in air by X-rays or gamma rays. It is defined only for photons and measured only in air.
Unit:
•
SI unit: C/kg (coulomb/kilogram)
•
Old unit: Roentgen (R) → 1 R = 2.58 × 10⁻⁴ C/kg
•
In air 1 R ≈ 0.87 rad; in tissue, absorbed dose = f × exposure. The f factor (rad/R) is ~0.9-0.97 in soft tissue, whereas in bone it rises markedly at low photon energies because of the Z³ dependence of the photoelectric effect
Limitations:
•
Valid only for photons (X and gamma) → not used for particles
•
Defined only in air → not a tissue dose
•
Defined only for photons below 3 MeV
•
Its use has declined in modern dosimetry → absorbed dose and effective dose are preferred
10.1.5 Kerma (Kinetic Energy Released per unit Mass)
Definition: The kinetic energy of charged particles released per unit mass.
Formula:
K = dEtr / dm
Unit: Gray (Gy)
Relationship to absorbed dose:
•
Under conditions of electronic equilibrium: Kerma ≈ Absorbed dose
•
Without electronic equilibrium (e.g. at interfaces): Kerma ≠ Absorbed dose
•
Kerma measures the energy transferred by photons to the medium; absorbed dose measures the energy deposited in the medium
Effective dose cannot be measured directly, even in a uniform external field. For this reason, "operational" quantities are used to calibrate radiation protection instruments:
•
Ambient dose equivalent H*(10): The equivalent dose at a depth of 10 mm in a tissue-equivalent material (unit Sv); area monitors and survey meters work with this quantity
•
Personal dose equivalent Hp(d): The equivalent dose in soft tissue at depth d below a specified point on the body (unit Sv); personal dosimeters work with this quantity
•
Hp(10) is used to estimate effective dose, Hp(0.07) to assess skin (and extremity) dose, and Hp(3) (d = 0.3 cm) to assess eye lens dose
BOARD TIP
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Effective dose is calculated, not measured. Measuring instruments are calibrated in operational quantities: H*(10) → ambient/area; Hp(10) → whole body (estimate of effective dose); Hp(0.07) → skin; Hp(3) → eye lens.
Figure 10.1: Measurement depths of the operational dose quantities: Hp(0.07) skin, Hp(3) eye lens, Hp(10) whole body (schematic).
Did this figure help you understand the topic?
10.1.6 Summary Table of Dosimetric Quantities
Quantity
SI Unit
Old Unit
Conversion
What It Measures
Absorbed Dose
Gray (Gy)
rad
1 Gy = 100 rad
Physical energy transfer
Equivalent Dose
Sievert (Sv)
rem
1 Sv = 100 rem
Biological effectiveness (radiation type)
Effective Dose
Sievert (Sv)
rem
1 Sv = 100 rem
Stochastic risk (organ sensitivity)
Exposure
C/kg
Roentgen (R)
1 R = 2.58×10⁻⁴ C/kg
Ionisation in air (photons only)
Kerma
Gray (Gy)
rad
1 Gy = 100 rad
Kinetic energy released
BOARD TIP
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To remember the unit conversions: the "Gy-rad" and "Sv-rem" pairs → both differ by a factor of 100 (1 Gy = 100 rad, 1 Sv = 100 rem). The exposure unit roentgen is valid only for photons and in air → not used for particles or tissue!
Figure 10.2: From absorbed dose to equivalent and effective dose: radiation (wR) and tissue (wT) weighting factors.
Did this figure help you understand the topic?
10.2 The MIRD Formalism
The MIRD (Medical Internal Radiation Dose) formalism is the standard method used to calculate the internal radiation dose from radiopharmaceuticals administered to the body. It forms the basis of patient dosimetry in nuclear medicine.
"Is the effective half-life closer to the physical or the biological half-life?"
→ Closer to the shorter one. Example: Tphys = 6 hours, Tbiol = 24 hours → Teff = (6×24)/(6+24) = 144/30 = 4.8 hours (closer to the physical half-life). Teff calculations are asked frequently in the exam!
Atoms in an organ are lost by physical decay or biological excretion; physical, biological and effective retention curves and the area under the activity–time curve (cumulated activity) are shown.
Continuous stream · no seeking
Key point: the effective half-life is shorter than both the physical and biological half-lives (1/Teff = 1/Tphys + 1/Tbiol); cumulated activity is the area under the curve, Ã = 1.443 × A₀ × Teff.
Source: book (example Tphys 6 h, Tbiol 24 h → Teff 4.8 h); A₀ = 100 MBq and f = 1 schematic; Tc-99m 6.01 h, I-123 13.2 h, I-131 8.02 d.
How to use: one click on the organ scene pauses and resumes; move over or drag along the graph; in the free step ↑ ↓ change the biological half-life smoothly; ← → change steps.
Speed
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Figure 10.3: Cumulated activity as the area under the curve (A) and the effective half-life being shorter than the physical and biological half-lives (B).
Definition: The absorbed dose delivered to the target organ per unit cumulated activity in the source organ.
Factors determining the S value:
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Emission characteristics of the radionuclide (energy, type, frequency)
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Geometric relationship between source and target organs (distance, size)
•
Masses of the source and target organs
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Attenuation properties of the intervening tissues
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S = Σ Δᵢ·φᵢ / m(target): Δ denotes the mean emitted energy and φ the absorbed fraction; φ depends on photon energy, the size and shape of the organ, and the distance between source and target. For the self-dose of non-penetrating β radiation φ ≈ 1
S value tables:
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Calculated and published by the MIRD committee for standard phantoms (male, female, paediatric)
•
Separate S values are available for different radionuclides and source–target organ pairs
•
Can be calculated with OLINDA/EXM software
Key points:
•
Self-dose: When source and target organ are the same → S(rT ← rT)
•
Cross-dose: When source and target organ are different → S(rT ← rS)
•
Beta particles have a short range → self-dose dominates, cross-dose is negligible
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Gamma rays have a long range → cross-dose is important
10.2.4 Residence Time (τ)
Definition: The ratio of cumulated activity to injected activity.
Formula:
τ = Ã / A₀
•
τ: Residence time (hours or seconds)
•
Ã: Cumulated activity
•
A₀: Injected activity
Clinical importance:
•
Residence time reflects how long the radiopharmaceutical stays in the organ
•
Long residence time → high organ dose
•
In dosimetric calculations τ can be used instead of Ã: D = A₀ × τ × S
10.2.5 Steps of Dose Calculation with the MIRD Formalism
1. Determine the biodistribution of the radiopharmaceutical: How much activity goes to which organs (uptake fraction)
2. Construct the activity–time curve for each organ: With serial imaging or blood/urine samples 3. Calculate cumulated activity (Ã): Area under the curve (integral)
4. Select the appropriate S values: From MIRD tables for the radionuclide and organ pair 5. Calculate the dose for each target organ: D = Σ Ã × S (sum of contributions from all source organs)
6. Calculate the effective dose: E = Σ wT × HT
10.2.6 OLINDA/EXM Software
•
Internal dosimetry calculation software based on the MIRD formalism
•
Includes standard phantoms (adult male/female, paediatric age groups)
•
S values are available for more than 800 radionuclides
•
Organ doses and effective dose are calculated by entering residence times
•
The standard tool in the dosimetric evaluation of new FDA-approved radiopharmaceuticals
BOARD TIP
▸
Remember the MIRD formula simply: Dose = Cumulated activity × S value. Cumulated activity answers "how much radioactivity stayed in the organ, and for how long". The S value answers "how much dose per unit activity". Their product gives the total dose.
Figure 10.4: The MIRD formalism: self-dose and cross-dose from source to target organ and the steps of dose calculation (schematic).
Did this figure help you understand the topic?
10.3 Dose Limits (ICRP Recommendations)
In radiation protection, dose limits are set to keep the stochastic and deterministic risks of individuals exposed occupationally or as members of the public at an acceptable level.
The ICRP (International Commission on Radiological Protection) defines three basic principles of radiation protection:
1. Justification:
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A radiation practice should be carried out only if it provides a net benefit
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The benefit must outweigh the risk
–
Each practice should be assessed individually
2. Optimisation (ALARA):
–
Doses should be kept as low as reasonably achievable
–
ALARA: As Low As Reasonably Achievable
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Optimisation takes economic and social factors into account
3. Dose Limitation:
–
Individual doses should not exceed the set limits
–
Dose limits are defined separately for occupational and public exposure
–
Dose limits do not apply to medical exposure (patient dose)
–
Comparison: The world average from natural radiation is ~2.4 mSv/year (UNSCEAR). The largest share is radon (~1.3), followed by terrestrial (~0.5), cosmic (~0.4) and internal (~0.3 mSv) sources. Natural and medical exposures are not included in the dose limits
BOARD TIP
▸
"To whom do dose limits apply?"
→ Radiation workers and the public.
▸
"Do dose limits apply to patients?"
→ NO → patient doses are managed by the principles of justification and optimisation; there is no fixed dose limit. This distinction is asked frequently in the exam!
10.3.4 Dose Limits for the Pregnant Radiation Worker
•
Fetal dose during pregnancy (for the remainder of the pregnancy): ≤1 mSv (ICRP recommendation)
•
NRC (USA): Fetal dose during pregnancy ≤5 mSv (500 mrem)
•
Monthly fetal dose after declaration of pregnancy: ≤0.5 mSv (NCRP recommendation)
•
A pregnant worker may continue to work with radiation, but the dose limits must be observed
•
Declaration of pregnancy is voluntary (not mandatory)
BOARD TIP
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Memorise the dose limits side by side: Occupational = 20 mSv/year (5-year average, single year ≤50), Public = 1 mSv/year, Fetal = 1 mSv (ICRP) or 5 mSv (NRC) for the whole pregnancy. The occupational limit is 20 times the public limit. The eye lens limit was reduced by ICRP 118 from 150 → 20 mSv/year → this update may be asked in the exam!
Figure 10.5: Comparison of ICRP 103 occupational and public dose limits.
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10.3.5 Embryo/Fetus Dose Limits and Threshold Values
Effect
Threshold Dose
Gestational Period
Lethal effect (death)
~100 mGy
Preimplantation (0-2 weeks)
Major congenital anomaly
~100-200 mGy
Organogenesis (2-8 weeks)
Mental retardation (severe)
~300 mGy
8-15 weeks (most sensitive)
Mental retardation (severe)
≥300 mGy, probably higher (~500-700 mGy); risk ~4 times lower than at 8-15 weeks
16-25 weeks (less sensitive)
IQ loss
~25 IQ points/Gy
8-15 weeks
Microcephaly
~200 mGy
8-15 weeks
Childhood cancer
No threshold (stochastic)
All periods
(For details see Section 4.7 – Radiation Effects in Pregnancy)
Figure 10.6: Fetal radiation effects and threshold doses by gestational period.
If no lead apron is used: On the front of the trunk (chest or waist level)
•
If a lead apron is used: Outside the apron, at collar (neck) level
Use of two dosimeters (recommended):
•
Collar dosimeter: Outside the apron, at neck level → reflects eye lens and thyroid dose
•
Waist dosimeter: Under the apron, at waist level → reflects trunk dose
•
Effective dose equivalent (HE) estimate, two-dosimeter formula (NCRP 122; the method accepted by the NRC in the USA):
HE ≈ 1.5 × Hwaist + 0.04 × Hcollar
For effective dose (E), same report: E ≈ 0.5 × Hwaist + 0.025 × Hcollar
ICRP 139 (2018), simple algorithm:
E ≈ Hwaist + 0.1 × Hcollar (0.05 × Hcollar with a thyroid shield)
WHY IT MATTERS · Effective dose equivalent (HE) versus effective dose (E)
•
What it is: The effective dose equivalent (HE) is the quantity defined in ICRP 26 (1977); the effective dose (E) replaced it in ICRP 60 (1991) and was updated in ICRP 103 (2007). The logic is the same (organ equivalent doses summed with tissue weighting factors), but the list of organs and the wT values differ.
•
In numbers: wT: gonads 0.25 (ICRP 26) → 0.20 (ICRP 60) → 0.08 (ICRP 103); breast 0.15 → 0.05 → 0.12; thyroid 0.03 → 0.05 → 0.04. From the same dosemeter readings, the NCRP 122 formulas give 1.5 × Hwaist + 0.04 × Hcollar for HE and 0.5 × Hwaist + 0.025 × Hcollar for E.
•
Why it matters clinically: Older radiopharmaceutical dose tables (ICRP 53, 1987) give HE; ICRP 80 (1998) and ICRP 128 (2015) give E. Values of the two quantities, or values calculated with different ICRP versions, are not interchangeable; when comparing, state which weights were used. US NRC regulations (10 CFR 20) still use the ICRP 26-based total effective dose equivalent (TEDE); the EU Basic Safety Standards use the effective dose based on ICRP 103.
•
In the exam: "The difference between HE and E?" → Same logic, different ICRP recommendations (26 versus 60/103) and different wT; not interchangeable.
When physical dosimeters cannot be used or are insufficient (e.g. a radiation accident), dose can be estimated by biological methods:
1. Dicentric Chromosome Analysis (Gold Standard):
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Counting of dicentric chromosomes in peripheral blood lymphocytes
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A radiation-specific chromosome aberration → high specificity
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Irradiation in G1 phase leads to chromosome-type aberrations, in S/G2 phase to chromatid-type aberrations. Dicentrics and ring chromosomes are unstable (lost as cells divide), whereas translocations are stable (past exposures are detected by FISH). At low LET the dicentric frequency follows αD + βD²
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The dose–response curve is well defined
–
Disadvantage: Time-consuming (48-72 hours of culture), requires an expert
–
Dose range: 0.1 – 5 Gy
2. Lymphocyte Count (Rapid Triage):
–
After acute radiation exposure, the rate of fall in lymphocyte count is proportional to dose
–
Absolute lymphocyte count within 48 hours:
–
>1500/µL → <1 Gy (mild)
–
1000-1500/µL → 1-2 Gy (moderate)
–
500-1000/µL → 2-4 Gy (severe)
–
<500/µL → >4 Gy (very severe/lethal)
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Advantage: Fast, easily available
–
Disadvantage: Low sensitivity; lymphopenia may have other causes
3. Micronucleus Test:
–
Faster and easier than dicentric analysis
–
Can be automated
–
Lower specificity
4. Electron Spin Resonance (ESR) – Tooth Enamel:
–
Measures radiation-induced free radicals in tooth enamel
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Used to estimate chronic/cumulative dose
–
Retrospective dosimetry (can be measured even years later)
10.5 Radiation Safety Practice
10.5.1 The Three Basic Principles of Radiation Protection2 questions
1. Time:
–
Minimise the time spent near the radiation source
–
Dose = Dose rate × Time → if the time is halved, the dose is halved
–
Practical applications: Planning procedures in advance, practising, working quickly
2. Distance:
–
Reduce dose by moving away from the source
–
Inverse square law: Dose rate is inversely proportional to the square of the distance
Ḋ₁ × d₁² = Ḋ₂ × d₂²
•
If the distance is increased 2-fold → dose decreases 4-fold
•
If the distance is increased 3-fold → dose decreases 9-fold
•
Practical applications: Long forceps/tongs, remote injection systems, syringe shields
3. Shielding:
–
Place suitable shielding material between the source and the person
–
Gamma/X-rays: Lead (Pb), concrete, steel
–
PET (511 keV): Syringe shields are mostly made of tungsten; because of its high density, effective protection is achieved with a thinner shield
–
Beta: Low-Z materials (Perspex, acrylic) → first low Z, then high Z (to prevent bremsstrahlung)
–
Alpha: Paper, skin → no shielding needed (no external exposure risk)
"Why is Perspex used first and lead second to shield a beta source?"
→ When beta particles are stopped in a high-Z material, bremsstrahlung (braking radiation) is produced → beta is first stopped with low Z, and the resulting bremsstrahlung is then shielded with lead. In the exam, "Shielding for P-32?" → Perspex (NOT lead!).
How many photons from a source reach a worker depends on distance, time and lead shielding; the shielding order and bremsstrahlung for a beta source are shown.
Continuous stream · no seeking
Key point: dose rate falls with the square of distance, dose rises in proportion to time, and each HVL of lead halves the photons passing; a beta source is shielded first with low-Z material.
Source: book (Γ Tc-99m 0.78, I-131 2.17, F-18 5.7 R·cm²/mCi·h; lead HVL 0.3 / 3 / 4 mm); example 20 mCi (740 MBq) point source, body attenuation and scatter not included. Beta step: P-32 Emax 1.711 MeV (ENSDF), range NIST ESTAR, bremsstrahlung fraction Cherry, Sorenson & Phelps (Physics in Nuclear Medicine). Magnifier: Rutherford scattering; bremsstrahlung spectrum and angular distribution after Bethe–Heitler (Attix, Introduction to Radiological Physics and Radiation Dosimetry); atomic radii not to scale.
How to use: press and drag the worker (release and it moves back and forth on its own); hold ← → to change the distance smoothly; one click on the drawing pauses and resumes, double-click enlarges.
Speed
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Figure 10.8: The three principles of radiation protection: time, distance (inverse square law) and the correct shielding order for a beta source.
10.5.2 Components of a Radiation Safety Programme
Radiation Safety Committee (RSC):
•
Sets institutional radiation safety policies
•
Members: Radiation safety officer (RSO), management representative, representatives of departments using radiation
•
Meets at least once a year
Radiation Safety Officer (RSO):
•
Responsible for day-to-day radiation safety operations
•
Oversees compliance with licence conditions
•
Staff training, dosimetry follow-up, contamination control
•
Radioactive material inventory and waste management
Staff Training:
•
Initial training on starting work and annual refresher training
•
Basics of radiation physics, protection principles, emergency procedures
•
Information on the rights and risks of pregnant workers
10.5.3 Security of Radioactive Material2 questions
Receipt and recording of radioactive material:
•
Wipe test on receipt of a package (contamination check)
•
Dose rate measurement at the package surface and at 1 metre
•
Inventory record: Radionuclide, activity, date, intended use
Transport classification (DOT/IAEA):
Category
Package Surface
At 1 m
Label Colour
White-I
≤0.005 mSv/h
–
White
Yellow-II
≤0.5 mSv/h
≤0.01 mSv/h
Yellow
Yellow-III
≤2 mSv/h
≤0.1 mSv/h
Yellow
Transport Index (TI):
•
The dose rate at 1 metre expressed in mrem/h
•
White-I: TI = 0
•
Yellow-II: TI ≤ 1.0
•
Yellow-III: TI ≤ 10.0
Storage of radioactive material:
•
In a locked, labelled, ventilated area
•
Radiation warning sign (trefoil symbol) in a visible place
•
Different radionuclides should be appropriately segregated
•
Volatile materials should be stored in a fume hood
•
Radioiodine solutions are volatile (I₂ vapour): Oxidation and acidic pH increase volatility. NaI solution is therefore stored at alkaline pH with a reducing agent added (e.g. thiosulfate) and opened in a fume hood; capsule form is preferred where possible
BOARD TIP
▸
"0.3 mSv/h at the package surface, 0.005 mSv/h at 1 m → which category?"
→ Yellow-II (surface ≤0.5, 1m ≤0.01). Transport Index = dose rate at 1 m (mrem/h) → 0.005 mSv/h = 0.5 mrem/h → TI = 0.5. Package classification questions come up frequently in the exam!
High Radiation Area: Access control is required (lock, alarm or continuous surveillance)
•
Very High Radiation Area: A lethal dose may be received within minutes → additional security measures are mandatory
•
Areas where radioactive material is used/stored: "Caution: Radioactive Materials" sign
The European Basic Safety Standards (BSS) approach:
•
International (IAEA) and regional (EU) Basic Safety Standards have been published on the basis of ICRP recommendations (the most recent from 2007, ICRP 103); the EU BSS Directive has been transposed into the legislation of all member states
•
Work areas: Controlled areas, where access and supervision are stricter, and supervised areas, where the risk is lower
•
Worker categories: Category A workers are those likely to receive an effective dose above 6 mSv per year, or an equivalent dose above 15 mSv/year to the eye lens or 150 mSv/year to the skin/extremities. Category B workers are those likely to exceed the dose limits set for the public but who do not fall into Category A
•
Institutional local rules may be stricter than national and international regulations; local rules must always be followed
•
Contamination and intake of radioactive material must be prevented in particular, because they can render time–distance–shielding measures ineffective
10.5.5 Specific Gamma-Ray Constant (Γ – Exposure Rate Constant)2 questions
Definition: Gives the exposure rate of a radionuclide per unit activity at unit distance.
Formula:
Ẋ = Γ × A / d²
•
Ẋ: Exposure rate (R/h or mR/h)
•
Γ: Specific gamma-ray constant (R·cm²/mCi·h)
•
A: Activity (mCi)
•
d: Distance (cm)
Γ values of common radionuclides:
Radionuclide
Γ (R·cm²/mCi·h)
Tc-99m
0.78
I-131
2.17
I-123
1.55
Ga-67
0.76
Tl-201
0.45
F-18
5.7
Co-60
13.2
Cs-137
3.26
Ra-226
8.25
BOARD TIP
▸
The Γ value of F-18 (5.7) is much higher than that of Tc-99m (0.78) → more shielding is needed when preparing PET radiopharmaceuticals. The Γ value of I-131 (2.17) is also high → patients receiving RAI therapy emit significant radiation into their surroundings. For exam questions such as "which radionuclide poses a greater external radiation risk?", compare the Γ values.
10.6 Contamination and Decontamination
10.6.1 Types of Contamination
Fixed Contamination:
•
Radioactive material firmly bound to a surface
•
Cannot be removed by a wipe test
•
Poses an external radiation risk but a low risk of spread
Removable Contamination:
•
Radioactive material that is easily detached from a surface
•
Detected by a wipe test
•
High risk of spread and internal contamination → more dangerous
1. Gently wash the contaminated area with warm water and soap (3-4 minutes) 2. Check with a survey meter after washing 3. Repeat if necessary (maximum 3-4 times) 4. DO NOT scrub hard → if skin integrity is broken, radioactive material is absorbed 5. For resistant contamination: Potassium permanganate or DTPA solution 6. For wound contamination: Copious irrigation with water → surgical debridement as a last resort
Surface Decontamination:
1. Mark and confine the contaminated area (surround with absorbent paper) 2. Wipe from the outside inwards (to prevent spread of contamination) 3. Clean with a suitable detergent/solvent 4. Check with a survey meter after each cleaning 5. If background level cannot be reached → mark the area and leave it to decay (for short-lived isotopes)
Procedure for a Major Contamination Event (Spill):
Minor spill:
1. Put on gloves, cover the contaminated area with absorbent paper 2. Clean from the outside inwards 3. Check with a survey meter 4. Report to the RSO
Major spill:
1. Leave the area immediately (do not spread the contamination) 2. Lock/mark the room → prevent others from entering 3. Report to the RSO immediately 4. Decontaminate contaminated persons 5. Clean-up by a professional decontamination team
On-Site and Hospital Response in a Radiation Accident
•
Triage has two steps: first, casualties are sorted by the severity of their medical condition using conventional triage systems (not by radiation exposure or contamination criteria); in the second step, people who may have been exposed or contaminated are identified. Life-threatening conditions always take priority over radiological concerns
•
Radiation does not cause an immediately life-threatening condition; unconsciousness or immediately visible signs of injury are rarely explained by radiation, and if present another cause is sought
•
At the scene: a patient with life-threatening injuries is transported to hospital without waiting for a contamination survey; the hospital is told about the conventional injuries and any possible exposure or contamination. Wounds are covered with sterile dressings. The radiological survey must not delay medical care or transport
•
Clothing suspected to be contaminated is removed (unless medically contraindicated), placed in plastic bags and labelled; clothing can serve as a sample for identifying the radionuclide
•
Radiological triage: areas measuring more than 2-3 times background are considered contaminated. The alpha background is taken as zero; if an alpha emitter is detected at any level, the person is considered contaminated. When internal contamination is suspected, nasal swabs are taken within about 60 minutes of intake; a negative oral swab means little
•
At the hospital: the hospital emergency plan is activated; an ambulance reception area and a treatment area are set up, access is restricted, survey instruments are checked and background levels recorded; if time permits the floor is covered (emergency care is not delayed for this). The ambulance crew stays with the vehicle until they, the vehicle and the equipment have been surveyed and released
•
A contaminated patient is not a significant source of external exposure for the team; the main risk is the spread of contamination to the premises, equipment and other people
•
Order of decontamination: wounds → body orifices (eyes, ears, mouth, nose) → highly contaminated skin → less contaminated skin. Wounds are irrigated with saline or water and monitored after each irrigation; irrigation with DTPA may be considered for Am, Pu and Cm; embedded radioactive foreign bodies are removed and kept for analysis. Internal contamination is assumed in a contaminated wound. Hair is washed with shampoo; the skin is not shaved
•
Uninjured people with external contamination only are decontaminated at a facility other than the hospital to conserve hospital resources; in an event affecting many people, radiological triage of the worried well is performed away from the treatment area
•
When high-dose whole-body irradiation is suspected: an immediate full blood count with differential, followed by an absolute lymphocyte count every 6 hours for 48 hours (see 10.4.3)
BOARD TIP
▸
"The most important rule in skin decontamination?"
→ DO NOT scrub hard → skin damage increases absorption of radioactive material.
▸
"Direction of wiping in surface decontamination?"
→ From the outside inwards (to prevent spread of contamination).
▸
"First step in a major spill?"
→ Leave the area and report to the RSO.
BOARD TIP
▸
"Priority in a radiation accident patient who is both seriously injured and contaminated?"
→ Treatment of the life-threatening condition and stabilisation; decontamination and radiological survey must not delay it.
▸
"Order of decontamination in a patient?"
→ Wounds → body orifices → highly contaminated skin → less contaminated skin.
▸
"Criterion for 'contaminated' in radiological triage?"
→ A reading above 2-3 times background; any level of an alpha emitter.
Figure 10.9: What to do in a minor and a major radioactive spill.
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10.6.4 Internal Contamination and Its Treatment2 questions
Internal contamination occurs when radioactive material is taken into the body (inhalation, ingestion, through a wound).
Treatment approaches:
Radionuclide
Treatment
Mechanism
I-131
Potassium iodide (KI)
Thyroid blockade (dilution with stable iodine)
Cs-137
Prussian blue
Binds in the GI tract and increases faecal excretion
Plutonium, Americium
DTPA (Ca-DTPA or Zn-DTPA)
Chelation → renal excretion
Tritium (H-3)
Plenty of water (forced diuresis)
Shortens the biological half-life
Strontium (Sr-90)
Aluminium phosphate, barium sulfate
Reduces GI absorption
Uranium
Sodium bicarbonate
Reduces renal tubular damage
BOARD TIP
▸
Match the treatments for internal contamination: I-131 → KI, Cs-137 → Prussian blue, Plutonium → DTPA, Tritium → plenty of water. In the exam, "treatment of Cs-137 contamination after a nuclear accident?" → Prussian blue.
▸
"When should KI be given for thyroid protection?"
→ Before exposure or within 4 hours at the latest (effectiveness decreases with time).
10.7 Radiation Safety in Pregnancy and Breastfeeding
10.7.1 Diagnostic Nuclear Medicine in Pregnancy2 questions
General principles:
•
Pregnancy is not an absolute contraindication to nuclear medicine studies
•
A risk–benefit analysis should be performed
•
If possible, the study should be postponed until after delivery
•
If postponement is not possible, the protocol that gives the most diagnostic information at the lowest dose should be chosen
Estimated fetal doses in common studies:
Study
Radiopharmaceutical
Typical Activity
Estimated Fetal Dose
Lung perfusion
Tc-99m MAA
2-4 mCi
0.2-0.6 mGy
Bone scintigraphy
Tc-99m MDP
20-25 mCi
4-5 mGy
Renal scintigraphy
Tc-99m MAG3
10 mCi
~5-7 mGy
Thyroid scintigraphy
Tc-99m pertechnetate
5-10 mCi
~2-4 mGy
V/Q scintigraphy
Tc-99m MAA + Xe-133
Combined
0.5-1.0 mGy
FDG PET/CT
F-18 FDG
10-15 mCi
10-20 mGy
Important rules:
•
In diagnostic nuclear medicine studies the fetal dose is usually <50 mGy → below the threshold for deterministic effects
•
Termination is not recommended below 50 mGy (ACR); ICRP 84: it cannot be justified at <100 mGy
•
Above 100 mGy, detailed risk assessment and counselling are needed
•
Hydration and frequent bladder emptying → reduce fetal dose (especially the uterine dose from the adjacent bladder)
•
I-131 is absolutely contraindicated in pregnancy → it crosses the placenta, and the fetal thyroid concentrates iodine from week 10-12 onwards → fetal hypothyroidism/thyroid ablation
BOARD TIP
▸
"Radiopharmaceutical absolutely contraindicated in pregnancy?"
→ I-131 (and all forms of radioiodine → fetal thyroid damage).
▸
"The safest lung perfusion agent in pregnancy?"
→ Tc-99m MAA (can be performed at a low dose, fetal dose <1 mGy).
▸
"Dose threshold for termination of pregnancy?"
→ Not recommended below 50 mGy.
10.7.2 Radioiodine Exposure in Pregnancy
Fetal thyroid development and iodine uptake:
•
The fetal thyroid begins to develop at ~week 3-4
•
It begins to concentrate iodine from week 10-12 onwards
•
In the 2nd and 3rd trimesters the fetal thyroid concentrates iodine at a higher rate than the adult thyroid
Consequences of I-131 exposure:
•
1st trimester (<10 weeks): The fetal thyroid does not yet take up iodine → low risk of thyroid damage, but there is a whole-body dose risk
At high doses: Risk of fetal thyroid cancer (theoretical)
If I-131 is given inadvertently:
•
Give KI (potassium iodide) immediately → fetal thyroid blockade
•
Effectiveness depends on the time since exposure (the earlier the better)
•
Hydration and frequent voiding → reduce maternal and fetal dose
•
Dosimetric assessment and genetic counselling
10.7.3 Radiation Safety in Breastfeeding3 questions
Radiopharmaceuticals can pass into breast milk → they may deliver a radiation dose to the breastfed infant. The duration of breastfeeding interruption varies with the radiopharmaceutical.
Durations of breastfeeding interruption:
Radiopharmaceutical
Breastfeeding Interruption
Tc-99m pertechnetate
12 hours
Tc-99m MAA
12 hours
Tc-99m MDP
Not needed (EANM: ≥4 hours)
Tc-99m MIBI
Not needed (EANM: ≥4 hours)
Tc-99m DTPA
Not needed (EANM: ≥4 hours)
Tc-99m MAG3
Not needed (EANM: ≥4 hours)
Tc-99m DMSA
Not needed (EANM: ≥4 hours)
Tc-99m sulphur colloid
Not needed (EANM: ≥4 hours)
Tc-99m RBC (labelled red cells)
12 hours (in vivo labelling)
I-123 (NaI)
3-4 days (older recommendation: ≥3 weeks)
I-123 MIBG
≥24 hours (EANM)
I-131 (any dose)
Stop completely (permanent)
Ga-67 citrate
≥3-4 weeks (stopped in practice)
Tl-201
48 hours
F-18 FDG
Not needed (very little passes into milk); close contact with the infant is limited for the first ~4 hours (EANM/SNMMI 2024). The US ACMUI recommends a 4-hour interruption
I-131: Breastfeeding must be stopped permanently (it may be resumed in a subsequent pregnancy) → passes into milk at a high rate and concentrates in the infant's thyroid
•
During the interruption, milk should be expressed and discarded (to maintain milk production – except for I-131)
•
The duration of close contact with the infant should also be limited (external radiation)
•
Most Tc-99m compounds do not require interruption; EANM recommends 12 hours for pertechnetate and MAA and ≥4 hours for the others
•
Ga-67 requires a long interruption because of its long half-life and high transfer into milk
BOARD TIP
▸
"When can breastfeeding be resumed after I-131?"
→ IT CANNOT → permanent interruption (breastfeeding of that infant is stopped completely).
▸
"Breastfeeding interruption after Tc-99m pertechnetate?"
→ 12 hours.
▸
"Diagnostic agent requiring the longest breastfeeding interruption?"
→ Apart from I-131, Ga-67 (≥3-4 weeks; stopped in practice). The I-131 and breastfeeding question is a classic in the exam!
Figure 10.10: Durations of breastfeeding interruption by radiopharmaceutical.
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10.7.4 Pregnancy Screening Before a Nuclear Medicine Study1 questions
Indications for a pregnancy test:
•
All women of childbearing age (usually 12-50 years)
•
Regardless of the last menstrual period (early pregnancy may be missed)
•
The old '10-day rule', which restricted non-urgent studies in women of childbearing age to the first 10 days of the menstrual cycle, is no longer applied strictly; what matters is asking about pregnancy and checking β-hCG when necessary
•
Mandatory especially before I-131 therapy
Practice:
•
Serum or urine β-hCG test
•
If the result is negative, the study can be performed
•
If the result is positive → risk–benefit analysis, consideration of an alternative study
•
I-131 therapy → absolutely not given if the pregnancy test is positive
→ 3 options: ≤33 mCi retained activity OR ≤7 mR/h @ 1 m OR ≤5 mSv by patient-specific calculation. The values asked most often in the exam: 33 mCi and 7 mR/h.
▸
"Dose limit to others for a child/pregnant woman?"
→ 1 mSv (5 mSv for an adult).
Figure 10.11: NRC release criteria after I-131 therapy.
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10.8.2 Patient Instructions After Release
Patients released after I-131 therapy should be given written instructions to reduce the radiation exposure of people around them:
General precautions:
•
Limit the time of close contact with other people (especially for the first 2-5 days)
•
Avoid contact with children and pregnant women in particular
•
1 mSv for a child/pregnant woman: ICRP 94/IAEA dose constraint (threshold for written instructions under the NRC)
•
Sleep in a separate bed (usually for the first 3-7 days or depending on the dose)
•
Minimise close contact such as kissing and hugging
Hygiene precautions:
•
Use a separate bathroom (if possible) or clean it after use
•
When using the toilet: Urinate sitting down (including men), flush twice, wash hands afterwards
•
Use separate cutlery, cups and towels (for the first few days)
•
Radioiodine is also excreted in saliva and sweat → keep personal items separate
Distance and time recommendations (for a typical I-131 dose of 100-200 mCi):
Contact Person
Distance
Time Restriction
Adult (spouse/partner)
≥1 metre
First 3-5 days
Children (<18 years)
≥2 metres
First 5-7 days
Pregnant women
≥2 metres
First 7 days or longer
Work colleagues
≥1 metre
Return to work usually after 2-5 days
Travel restrictions:
•
Airport security detectors can detect radioactivity → the patient should carry an information card
•
Alarm period: May range from a few weeks to 3 months after I-131 therapy
•
Problems may arise at border crossings during international travel
The NRC requires certain errors in radiopharmaceutical administration to be reported:
Events that must be reported (Medical Event – 10 CFR 35.3045):
•
Administration to the wrong patient
•
Administration of the wrong radiopharmaceutical
•
Wrong route of administration (oral instead of IV, etc.)
•
Deviation of more than ±20% from the prescribed dose (for therapeutic doses)
•
Dose delivered to an organ other than the intended target organ
•
Additional condition (not a criterion on its own; it must be present together with one of the errors above): an effective dose difference of >50 mSv from that prescribed OR an equivalent dose difference of >500 mSv in any organ or tissue
Reporting process:
1. Immediately: Report to the RSO 2. By the next calendar day: Report to the NRC (or the relevant regulatory authority) by telephone/fax 3. Within 15 days: Submit a written report 4. Patient notification: The patient (or legal representative) should be informed within 24 hours 5. Referring physician: The referring physician should be informed
BOARD TIP
▸
"Reporting time for a medical event?"
→ to the NRC by the next calendar day, written report within 15 days.
▸
"Acceptable deviation in a therapeutic dose?"
→ ±20% (outside this is a medical event).
▸
"Medical event criterion in diagnostic studies?"
→ Wrong patient, radiopharmaceutical or route BUT only if there is >50 mSv effective or >500 mSv organ dose → rare in diagnostic studies because of the low doses.
Key Points for the Board
Operational Quantities and EU BSS – Additional Points:
Category A: likely to receive >6 mSv/year effective, >15 mSv/year eye lens, >150 mSv/year skin/extremity; Category B: those who may exceed the public limit
Dosimetric Quantities:
•
Absorbed dose: Gray (Gy) = 100 rad → a physical quantity, does not take radiation type into account
•
Equivalent dose: Sievert (Sv) = 100 rem → corrected with the radiation weighting factor (wR)
•
Effective dose: Sievert (Sv) → corrected with the tissue weighting factor (wT) → indicator of stochastic risk
•
For photons and beta wR = 1 → absorbed dose (Gy) = equivalent dose (Sv) numerically
•
For alpha wR = 20 → 1 Gy of alpha = 20 Sv equivalent dose
•
Exposure (roentgen) is valid only for photons and in air
•
In air 1 R ≈ 0.87 rad; in bone the f factor (rad/R) rises markedly at low photon energies (photoelectric, Z³)
•
Effective dose per MBq: Tl-201 (~0.14-0.22) and Ga-67 (~0.10 mSv/MBq) are much higher than most Tc-99m agents (~0.005-0.01)
Wrong patient, wrong radiopharmaceutical, wrong route of administration
•
Reporting: to the NRC by the next calendar day, written report within 15 days
•
The patient should be informed within 24 hours
Current guidelines on this chapter's topics
The latest guidelines and consensus documents on the procedures covered in this chapter (official EANM index and primary sources, November 2026). Comparison of the text with these versions is in progress; consult the guideline itself for details.
The radiopharmaceutical in the organ decreases in two ways: physical decay (the atom flashes and fades in place) and biological excretion (the atom leaves the organ). Because both act together, the organ activity falls faster than with either alone: 1/Teff = 1/Tphys + 1/Tbiol. The effective half-life is always shorter than both and closer to the shorter one (6 h and 24 h → 4.8 h).
PhysicalBiologicalEffective
Example from the book: Tphys = 6 h, Tbiol = 24 h → Teff = (6 × 24) / (6 + 24) = 4.8 h. In the cumulated activity step A₀ = 100 MBq and organ fraction f = 1 (schematic). Atoms are not to scale; each atom either decays physically or is excreted from the organ.
Step 1/5
Distance: inverse square law
Photons spread out from the source in all directions; with distance the same number of photons covers a larger area. Dose rate is inversely proportional to the square of distance: Ḋ₁·d₁² = Ḋ₂·d₂². Doubling the distance cuts dose 4-fold, tripling it 9-fold. That is why long tongs and remote injection work.
Example: 20 mCi (740 MBq) source. Γ (R·cm²/mCi·h) and lead HVL from the book: Tc-99m 0.78 / 0.3 mm, I-131 2.17 / 3 mm, F-18 5.7 / 4 mm. Ẋ = Γ·A/d² applies to a point source; body attenuation and scatter are not included. Photons are generated in random 3D directions; hits on the body fall as 1/d².