Effects of radiation


Ionising radiation

Ionising radiation is electromagnetic (EM) radiation that causes ionisation of atoms. The minimum energy needed to ionise any atom is approximately 10 eV.

Ionising radiation includes:

  • X-rays
  • Neutrons
  • Beta particles
  • Alpha particles

When the radiation interacts with the body damage is caused to irradiated cells by two mechanisms:

  • Indirectly: ionisation produces free radicals which then damage DNA and cell membranes
  • Directly: release of energy from ionisation event is enough to break molecular bonds directly

It also damages non-irradiated cells via:

  • Genomic instability in progeny of cells: DNA defects passed on
  • Bystander effect: release of chemicals and transmitters affect cells around the irradiated cell

Dividing cells are most sensitive to radiation when in G2 and mitosis. The more rapidly a cell is dividing, the greater its sensitivity

Sources of ionising radiation

Sources of radiation

  • Terrestrial
  • Radon - accounts for about 50% of the average annual dose to people in the UK (some of course will get more, some zero). Emits alpha particles
  • Radionuclides in food - especially potassium-40 (half-life of billions of years)
  • Cosmic rays

Background radiation

Average effective dose in the UK is 2.7 mSv/year (source)

  • 2.3 mSv from natural sources (0.006 mSv/day)
  • 0.4 mSv from medical exposures

Dose at which there is a statistically significant increase in risk of cancer = 100-200 mSv

 

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Measuring radiation dose

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Name Definition / formula Pros and cons Units
Absorbed dose Energy deposited per unit mass of tissue Doesn't take into account effect for different types of radiation or sensitivity of different organs irradiated Gray (Gy)
(1 Gray = 1 joule/kg)
Equivalent dose Absorbed dose to tissue x radiation weighting factor Takes into account effectiveness of different radiation types in producing biological damage Sievert
Effective dose Sum of (equivalent dose x tissue weighting factor) Sensitivity of different tissues to radiation taken into account Sievert

 

Equivalent dose

Equivalent Dose = Absorbed dose to tissue x radiation weighting factor

(summed for all types of radiation)

Different types of ionising radiation deposit different amounts of energy. This is measured by their Linear Energy Transfer (LET) or the density of energy deposition along the track of a photon or particle.

Low LET: x-rays, gamma rays, beta-particles. These are of high energy but pass through material quickly and deeply which leaves less time for energy to be deposited in any one area along its track.

High LET: alpha particles, neutrons. These are heavy and don't travel as far so all their energy is deposited into a small area.

This is then used to calculate the Radiation Weighting Factor (WR), the higher the WR the more energy is deposited and the higher the equivalent dose from that type of radiation.

Radiation Radiation weighting factor (WR)
X-ray and gamma ray (photons) 1
Beta particles and positrons 1
Protons 2
Alpha particles, fission fragments, heavy ions 20
Neutrons A continuous function of neutron energy (see ICRP103 for graph/formula)

 

For x-rays, gamma rays and beta particles the WR is 1 and so the equivalent dose in Sieverts is numerically the same as the mean absorbed dose in gray.

Effective dose

Effective dose = sum of (Equivalent dose x tissue weighting factor)

Each tissue in the body has a different sensitivity to radiation - its Tissue Weighting Factor. The Effective Dose takes this into account. The higher the tissue weighting factor, the higher that tissue's sensitivity to radiation i.e. the gonads have a higher sensitivity to radiation than skin. The below table shows the tissue weighting factors as stated in ICRP (2007).

Organ Tissue weighting factor
Skin, bone, brain, salivary glands 0.01
Bladder, oesophagus, liver, thyroid 0.04
Gonads 0.08
Red bone marrow, colon, lung, stomach, breast, remainder of tissues  0.12

 

Effects of radiation

The effects of radiation are down to how much and where the energy is deposited. A large weighting factor (WR) leads to highly localised damage whereas gamma/x-rays deposit energy/dose over a much greater range. Thus, ingestion of alpha emitters (e.g. Radon) can have a large effect on the sensitive lining of the lung for instance.

Deterministic vs stochastic effects

Effects are either deterministic or stochastic.

Deterministic Stochastic
Appear above a given threshold No threshold to effects
The severity of the effect increases with dose The probability of the effect increases with the dose
Effects occur within days of the exposure Effects may happen years after exposure
Includes tissue effects e.g. erythema Includes only cancer and genetic mutations
Deterministic effect thresholds

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Exposed tissue Net effect Absorbed dose required for effect (Gy) Time for effect to develop
Skin Initial erythema 2 2-24 hours
Erythema 3-6 1-4 weeks
Hair-loss 3-4 2-3 weeks
Lens of eye Cataract 3-5 Years
Bone marrow Depression of blood formation 0.5 3-7 days
Gonads Temporary sterility in males 0.15 3-9 weeks
Permanent sterility 3.5-6 3 weeks

 

Acute whole body exposure

Whole body absorbed dose (Gy) Principle organ involved Time between exposure and death (days)
1-6 Bone marrow 3-60
5-15 GIT and lungs 10-20
>15 CNS 1-5

 

Pregnancy

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Period Effects Dose required to cause effects
Pre-implantation 2 weeks pre-conception and 2 weeks post-conception

Resistant to malforming effects (cells still haven't differentiated so any dysgenesis is corrected).

Highest risk of fatality in this period.

50 mSv
Organogenesis
3rd to 8th week >5th week = gross malformation >250 mSv
3rd-8th week = growth retardation 200-500 mGy
Fetal period 8th to 15th week CNS most sensitive At >300 mSv IQ falls 30 points per Sievert
>9th week Childhood cancer 10-20 mSv
>10th week Uptake of radio-iodine by fetal thyroid significant  

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