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

(scroll sideways to view whole table)

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 (Sv)
Effective dose Sum of (equivalent dose x tissue weighting factor) Sensitivity of different tissues to radiation taken into account

Sievert (Sv)

 

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

 

Risk of stochastic effects

The risk of stochastic effects is linked to the effective dose. For adults, the risk of inducing a cancer is approximately 5% per Sv. Therefore, for a 1 mSv effective dose (e.g. an abdominal x-ray), the risk is 1 in 20,000 of inducing a cancer. For comparison, the lifetime natural incidence of cancer is 1 in 2 or 1 in 3.

Children have a higher probability of radiation damage as they are developing and growing and there is more time for them latent effects of radiation to manifest in lateral life.

Deterministic effect thresholds

(scroll sideways to view whole table)

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

Radiation-related risks throughout pregnancy depend upon the stage of the pregnancy and the absorbed dose. The highest risk is during the early fetal period, then the 2nd trimester, and finally the 3rd trimester. Preconception irradiation of either parent's gonads has not been shown to result in a higher risk of cancer or malformations in their children. 

To cause malformations, typically to the central nervous system, the threshold is ≥100-200 mGy. These levels are very rarely reached with CT or conventional x-ray scans but can be reached with fluoroscopically guided interventional procedures of the pelvis or radiotherapy.

In females of child-bearing age there must be an attempt to determine whether the patient is, or could be, pregnant before exposure to radiation. One missed menstruation in a regularly menstruating woman should be considered positive for pregnancy until proven otherwise.

The natural childhood risk of cancer is approximately 1 in 500. From the table below you can see that the risk of childhood cancer is very low for most studies. At the highest doses, however, the childhood cancer risk can be double the natural risk.

Breastfeeding

Some radionuclides are excreted in breast milk. It is recommended to suspend breastfeeding in the following situations:

  • Completely after 131I therapy
  • For 3 weeks after 131I, 125I, 67Ga, 22Na and 201Tl
  • For 12 hours after 131I hippurate and all 99mTc compounds except the below
  • For 4 hours after 99mTc red cells, DTPA and phosphonates
References

ICRP, 2000. Pregnancy and Medical Radiation. ICRP Publication 84. Ann. ICRP 30 (1)

Wall, B. F., Meara, J. R., Muirhead, C. R., Bury, R. F., & Murray, M. (2009). Protection of pregnant patients during diagnostic medical exposures to ionising radiation. London: Royal College of Radiologists.

 

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