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
Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.
Measuring radiation dose
| 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
| 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.
| Examination | Typical fetal dose (mGy) | Risk of childhood cancer per examination |
X-ray skull; teeth; chest; thoracic spine; breast CT head +/- neck 51Cr GFR measurement 81mKr lung ventilation scan | 0.001 – 0.1 | <1 in 1,000,000 |
CTPA 99mTc lung ventilation scan | 0.01 – 0.1 | 1 in 1,000,000 to 1 in 100,000 |
X-ray abdomen; pelvis; hip Barium enema CT pelvis, chest and liver 99mTc lung perfusion scan; thyroid scan; DTPA; MAG3; DMSA; white cell scan | 0.1 – 1.0 | 1 in 100,000 to 1 in 10,000 |
X-ray lumbar spine Intravenous urography Barium enema CT lumbar spine; abdomen 99mTc bone scan; cardiac blood pool scan; myocardial scan; cerebral blood flow scan 18F PET tumour scan | 1.0 – 10 | 1 in 10,000 to 1 in 1,000 |
CT pelvis; pelvis and abdomen; pelvis, abdomen and chest 99mTc myocardial scan 18F PET/CT whole body scan | 10 – 50 | 1 in 1,000 to 1 in 200 |
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.
