Radioiodine Physics

Types of Nuclear Decay

Alpha decay 2 protons and 2 neutrons From nuclei with excess of protons and neutrons
Beta decay Electrons From nuclei with too many neutrons: neutron splits into proton and electron, the latter is emitted from the nucleus
Gamma rays Photons Carry off excess energy when nucleus falls from excited to ground state after nuclear decay. NB this is usually instantaneous, but some nuclei are metastable - persist in excited state, e.g. 99m-Technetium decays to 99-Technetium with half-life of 6 hours.
Electron capture   Too few neutrons - an inner shell electron falls into the nucleus and binds with a proton.
Positron emission Positron Alternative when too few neutrons - both electron and positron are created, the latter is expelled, and ultimately interacts with an electron somewhere causing mutual annihilation and emission of two gamma rays. The mass energy for P & E derives from the fall in nuclear energy.

Measuring Radiation

Suppose a radiopharmaceutical is taken up by liver, spleen and bone marrow: each becomes a source capable of radiating other organs. We need to calculate:

  • Total no. of disintegrations in source organ (cumulated activity)
  • Type of radiation emitted - e.g. for a radionuclide that emits only α-particles (range <5mm in tissue), the entire dose will be delivered to the source organ
  • Geometric relationship of source and target organs

The last two factors are combined into a single parameter known as an “S factor” – tables are available for different pairs of source and target organs for an average sized adult.


Radiation Weighting factor
X-rays & γ-rays 1
β-particles & positrons 1
Neutrons <10 keV 5
Neutrons 100 keV-2MeV 20
α-particles 20
What New unit Old unit Description
Activity Becquerel Curie 1 Bq = average of one nuclear transformation per second
1 Curie = activity of 1g radium, equivalent to 37 MBq
Absorbed dose Gray Rad 1 Gray = 1 joule per kg. NB this is a very large dose
1 Rad = 0.01 Gray
X-ray and γ-ray dose Gray/hr Roentgen Ionization in air due to X and γ-ray activity
Expressed as tissue dose rate or dose (1 Roentgen causes 1 Rad)
Equivalent dose Sievert Rem Takes account of effects of different kinds of radiation, so should be comparable irrespective of whether α, β or γ radiation
Sievert = absorbed dose in Gray x weighting factor
Rem = absorbed dose in Rad x weighting factor
Effective dose Sievert   Includes sensitivity of different organs to radiation - represents the uniform whole-body dose that would cause comparable harm to a non-uniform dose actually received.


Iodine-131

Energy (keV) % of γ-rays emitted
80 3
284 6
365 81
637 7
131 I → 131 Xe + β (606 keV) + γ
53 54

98% of γ–rays are emitted instantaneously, but about 1.3% of Xe nuclei persist in excited state (metastable) with a half-life of 11.9 days. Thus I-131 is also a source of radioactive Xe gas.

400 MBq I-131 = about 100 ng sodium iodide.

Effective half-life of I-131

  • Biological half-life = excretion of radionuclide from an organ
  • 1/Teffective = 1/Tphysical + 1/Tbiological
  • Effective half-life of I-131 is about 6 days – varies, but must always be less than the physical half-life of 8.04 days.

Interaction of Radiation With Biological Tissue

Thing Energy
Hydrogen bond 0.4 eV
Ionic bond 4 eV
Light photon 2-3 eV
UV photon 3-12 eV
To ionize an atom 12 eV
From ionization of atom 35 eV
Typical X-ray beam 20-100 keV
Typical Tc-99m γ-ray 140 keV
Typical α-particle >1 MeV
Radiation Effects
Ionizing radiation X-rays, β particles, neutrons
12ev is minimum energy to ionize, so UV and below are non-ionizing
Ionizing radiation is most harmful: typical energy involved is 35eV (energy to break covalent DNA bonds is 4eV)
Free radicals Created by ionization (mainly OH- from water) - cause 80% of damage
Alpha particles Interact strongly, leaving many ionized atoms in their wake
High probability of multiple events affecting the same DNA molecule, hence more damaging
Localized energy deposition Localized is the important bit - the amounts of energy are not large; a cup of coffee will introduce more energy into the body than a lethal radiation dose

Biological effects relate to damage to DNA and damage to cell membranes.

Low X-ray doses cause little damage (in terms of clonogenic survival). Single DNA breaks are easily repaired; the probability of two breaks affecting one molecule is very low with low X-ray doses.

External hazard: neutrons > γ/X > β > α. Neutrons are not charged, so can enter far into the body. α-particles interact so strongly that they are absorbed in the outer layer of skin.

Internal hazard: α > neutron > β > γ/X

Typical Doses, Dose Limits and Risk Equivalents

Population annual exposure Occupational annual exposure Medical procedures Medical procedures
UK total 2.7 mSv Health workers 0.1 mSv Dental X-ray 0.001 mSv Lung perfusion scan 1.0 mSv
UK background 2.3 mSv Nuclear power 0.7 mSv Chest X-ray 0.02 mSv Lumbar spine X-ray 1.0 mSv
UK medical 0.4 mSv Aircrew 2.0 mSv H pylori breath test 0.02 mSv CT head 2 mSv
Cornwall background 8 mSv     Skull AP + lat 0.04 mSv Bone scan 3 mSv
Cornwall radon 6 mSv     Lung ventilation 0.4 mSv Ba enema 4-7 mSv
        Pelvis X-ray 0.7 mSv CT chest 8 mSv
        Abdomen AP X-ray 0.7 mSv CT abdomen 10 mSv


Category Dose (mSv) Comparable to Lifetime risk
Negligible < 0.1 Drinking 30 cans Diet Coke 1:1,000,000
Minimal 0.1 - 1.0 Death by lightening 1:300,000
Very low 1-10 1000 hrs commercial aviation 1:30,000
Low 10-100 Being murdered 1:3000
Moderate 100-1000 Road accident 1:500
Annual dose limits for radiation workers Thresholds for harmful effects
Whole body 20 mSv Cancer risk in atomic bomb survivors seen above... 100-200 mSv
Eye (cataract risk) 150 mSv Suppression of blood cell formation 500 mSv
Any specific organ or area of skin 500 mSv Cataracts, skin erythema, minimum lethal dose 3 Sv


Shielding

Stuff TVL
Technetium-99m 0.9mm Pb
Iodine-131 11mm Pb

Linear attenuation coefficient = fractional reduction in intensity per unit thickness of attenuating medium.
Tenth-value layer = thickness required to reduce intensity to 10%.


Pregnancy

When ingested Fetal thyroid dose (mGy) Fetal effective whole body dose (mSv)
Conception 0.08 0.08
5 weeks 0.24 0.08
15 weeks 240 12
35 weeks 1100 55
  • Early days - cells not specialized - only effect of small dose is developmental delay
  • Organogenesis 2-8 weeks after fertilization
  • Neural development is most sensitive to radiation at 8-15 weeks
  • Fetal thyroid established from 8 weeks; increasingly avid fetal thyroid uptake of radioiodine after week 10