Radiation Effects and Dosimetry

Radiation Protection and Dosimetry: Radiation Effects

• Jonathan Grant is the author of these notes as part of DRAD401 Radiographic Science and Radiation Protection

Learning Objectives

• Define and understand the effects of ionizing and non-ionizing radiation
• Understand 'radiosensitivity' of tissues and organs
• Differentiate between deterministic, stochastic, genetic, and somatic effects of radiation

Key Definitions

• One should be able to define the following terms:
  • Radiosensitivity
  • Stochastic effects
  • Deterministic effects
  • Erythema
  • Hereditary effects
  • Double-strand break
  • Single-strand break
  • Direct action
  • Indirect action
  • Somatic effects

Non-Ionizing vs. Ionizing Radiation

• Non-ionizing radiation lacks sufficient energy to cause ionization, existing within a specific part of the electromagnetic spectrum
• Ionizing radiation contains sufficient energy to remove an electron from its orbit around a nucleus, leading to ionization

Exposure to Ionizing Radiation

• Populations are exposed to ionizing radiation daily from both artificial and natural sources
• Artificial sources include radiology/radiotherapy, the nuclear industry, and warfare, such as the atomic bombs in Hiroshima and Nagasaki in 1945, Chernobyl in 1986, and Fukushima in 2011
• Natural sources include certain rocks, radon gas, and cosmic radiation; radiation doses increase with altitude due to a diminished protective atmospheric function

Average UK Radiation Dose

• UK radiation exposure is 84% from natural sources and 16% from artificial sources
• 11% of radiation exposure comes from “intakes,” including radionuclides in food, drink, and air
• Examples of intakes are almonds and bananas
• Terrestrial gamma radiation arises from buildings and the ground from naturally-occurring radioactive materials
• Radon is a colourless, odourless radioactive gas formed from uranium decay in rocks and soils

UK Legislation and Information

• Legislation in the UK mandates providing patients with information about radiation risks
• Access patient dose information on GOV.UK
• Information is also available on the Health and Safety Executive and the World Health Organization's websites under "ionising radiation FAQs" and "what is ionising radiation"

Effects of Ionizing Radiation on Living Things

• Radiobiology is the study of ionizing radiation's effects on living things
• DNA damage is the most significant consequence of ionization in human tissue
• DNA is the 'rulebook of life' controlling cellular processes
• Damage to DNA can lead to cell division loss of control which can lead to cancer
• Cell damage results from direct or indirect radiation action

How Ionizing Radiation Affects DNA

• Direct action occurs when radiation interacts directly with DNA strands
• Single-strand breaks in DNA can be fully repaired
• Double-strand breaks have a higher chance of repair errors.
• Repair errors passed onto new cells are mutations
• Mutations cause the cell to divide uncontrollably, causing exponential cell growth, which can lead to tumors

Indirect Action

• Radiolysis of cell water occurs upon radiation interaction with water molecules, which form roughly 80% of a cell's composition
• Energy causes water molecules to split, resulting in free radicals like hydrogen peroxide
• These free radicals can lead to alterations in the base sequences that code the DNA molecule

Sequelae of Changes to DNA Sequence

• Ionization caused by X-rays can result in:
  • Complete cell repair
  • Fatal damage and cell death, resulting in impaired organ function in serial or parallel organs
  • Loss of cell division control, causing formation of tumours

Radiosensitivity Explained

• The relative susceptibility of cells, tissues, organs, and organisms to harmful radiation action
• Cell radiosensitivity is directly proportional to the rate of cell division, making actively dividing cells or those not fully mature most at risk from radiation

Radiosensitivity Categorized

• Lymphoid organs, bone marrow, blood, testes, ovaries, and intestines exhibit high radiosensitivity
• Optic lens, stomach, growing cartilage, fine vasculature and growing bone exhibit moderate radiosensitivity
• Muscle, brain, and spinal cord exhibit low radiosensitivity

Factors Influencing Adverse Health Effects

• Health effects are a complicated function of many factors like:
  • Type of radiation
  • Dose
  • Dose rate
  • Exposed body part
  • Individual biological differences and age

Type of Radiation and its Harmfulness

• Alpha particles are much more harmful than X, gamma, or beta particles for a unit of absorbed dose

Dose Size and the Effects that Follow

• The risk of stochastic effects increases with dose size
• Deterministic effects increase with dose until a threshold is reached

Rate the Dose is Received

• When radiation is received affects the outcome on tissue
• Effects are often not as serious if the dose occurs over days or weeks, instead of within a matter of minutes

Exposed Body Part and Radiosensitivity

• Radiosensitivity is also greater in rapidly dividing cells

Individual Age and Cell Division

• Younger patients will live longer, so age is an important factor when considering radiation impact
• When cell division slows, radiation effects are less damaging

Biological Differences

• Some individuals are more sensitive than others, like in instances of sunburn

Radiation Effects Categorization

• Somatic effects occur in the individual who receives the radiation
• Genetic effects occur in future generations of the person receiving (hereditary) radiation after irradiation of the gonads
• Stochastic effects' probability increases with the dose received
  • They have no dose threshold
  • These effects may have a long latent period such as with cancers
  • Generally, severity is not dose-related in stochastic effects
  • This is a primary concern of the diagnostic radiology department

Deterministic Effects

• Deterministic effects occur when a certain higher dose threshold is reached
• Further dose above the threshold increases the severity of the effect
• They generally have a shorter latent period observed within hours or days, like skin reddening and radiation sickness
• These effects can result from large doses, such as those encountered in radiation therapy

Stochastic and Cancer Risk Relations

• Radiation exposure can cause cancer, however, it is hard to link a cancer to the radiation exposure
• This is because dose information is generally scarce and cancer is so commonly developed in patients
• A radiation-induced breast cancer will cause 3-6 excess cases per 10,000 women per sievert per year
• Thyroid tumors may also develop in groups of patients that were irradiated in childhood

Biological Impacts

• Deterministic effects include necrosis (death of tissue), epilation (hair-loss), erythema, and radiation sickness

Radiation's Effects on the Embryo and Foetus

• Embryonic tissues consisting mostly of rapidly proliferating cells are highly susceptible to the effects of radiation
• Such effects include malformations and CNS disorders, and can induce neonatal death
• Studies of those exposed in utero to radiation from Hiroshima and Nagasaki
• Later exposures are most likely to kill cells
• Leukaemia risk increases by 50% after irradiation in-utero

Radiation Dosimetry

• Radiation dosimetry is divided into Part 2: Radiation Dosimetry and Part 3: Radiation Risk Factors
  • Radiation risk factors include the UK average risk of cancer and perspectives on single chest x-rays

Key Terms for Radiation Dosimetry

• One should be able to define the following terms:
  • Absorbed Dose
  • Equivalent Dose
  • Effective Dose
  • Dose Rate
  • Tissue Weighting Factor
  • Radiation Weighting Factor
  • Dose-Area Product (DAP)

Need for Radiation Measurement and Dosimetry

• Needed to record dose measurements to staff, patients, and the public
• Medical imaging investigations and radiation therapy are procedures required under the law
• There are various quantities available to measure radiation dose
• The three measurable levels are the measure of energy into a patient, organ doses, and full-body doses

Absorbed Dose

• Absorbed dose (D) is the energy (E) absorbed per unit mass (m) of the absorbing material
• D = E/m (joules per kilogram)
• The unit of absorbed dose is the gray (Gy)
• The absorbed dose is 1 gray if joule of energy is absorbed per 1 kg of the object being irradiated

Factors of Tissue Harm

• The risk of harm to tissue from an exposure to ionizing radiation depends on:
  • The absorbed dose (D)
  • The type of radiation that is absorbed (alpha particles, beta particles, x-rays or gamma rays)
  • The body organs or tissue that are exposed to the radiation

Type of Radiation

• When measuring equal absorbed doses
• the biological effects depend on the energy and radiation types
• The impact of radiation can be lessened by using specific radiation levels
• Therefore, it is important to remember to get Equivalent Dose readouts

Equivalent Dose Explained

• Equivalent Dose expresses the radiation's relative biological impact, applying radiation weighting factors (wR)
• It is measured in sieverts (Sv)
• Radiation weighting factor value (wR) will have different radiation
• ICRP 2008 provides a listing of radiation weighting factors

Radiation Type & Weighting Factor

• Alpha particles weighting factor is 20
• Gamma rays weighting factor is 1
• X-rays weighting factor is 1
• Positrons weighting factor is 1

Radiations in Medical Imaging

• Radiations (X-ray, gamma radiation, positron) all have a radiation waiting factor (wR) value of one (1)
• When measured Equivalent Dose (Sv) = Absorbed Dose (Gy) x wR is numerically equivalent to the absorbed dose (Gy)
• So 1 mSv from gamma radiation has the same biological impact as 1 mSv from x-ray radiation

Personnel Dose Measurement

• Values can be taken in personnel monitoring devices (film badges, TLDs etc.) for radiation workers
• Doses are generally very low for radiographers but can be much higher for workers in the nuclear industry
• Dose measurements will use the terms millisievert (mSv) which is 1/1000 of a sievert
• And microsievert (µSv) which is 1/1,000,000 of a sievert

Dose Rate Defined

• Fluoroscopy in diagnostic imaging is used with ionizing radiation
• 100 mSv received in one day is more damaging than the same dose received over the course of one month
• Dose rate = dose/time
• The dose rate can be quoted in grays, sieverts, millisieverts, and microsieverts per second; minute; hour

Effective Dose Explained

• Most treatments will irradiate more than one tissue
• The risk to a patient after irradiation needs to be assessed
• Because it can be different depending on what tissue is exposed

Effective Dose and Weight Factors

• Tissue and organ radiosensitivity varies
• Bone marrow is much more radiosensitive than muscle or nerve tissue
• Effective Dose is a quantity that accounts for specific body tissues irradiated to arrive at a figure of whole body dose, which can assess the patient's risk
• Tissues are assigned weighting factors (wT) to the effective dose
• Effective Dose (Sv) = ∑ Equivalent Dose (Sv) x wT

Example

• A thoracic spine examination records the following values:
  • The breast's measured equivalent dose is 1.3 mSv
  • The tissue weighting factor is at 0.12 meaning the weighted tissue dose = .156mSv
  • Bone marrow is 0.70 for a weighted tissue dose of .084mSv
  • Lungs at 2.8 and Thyroid and Bone both at 1.5 the summation of Tissue Over body is .651mSv, which equates to the same biological reactions

Dose-Area Product (DAP)

• This is commonly displayed on the X-ray console when a diagnostic X-ray examination takes place
• Varies with exposure factors and field size
• It is measured in cGy.cm²
• Comparing doses to the same examination across departments can identify good and poor practices
• Conversions of these values can provide an estimation of the effective dose
• Absorbed dose takes into account the energy absorbed per mass of the absorber
• Equivalent dose takes into account the kind of radiation
• Effective dose takes into account the radiosensitivity irradiated tissue
• Dose-area product is easily incorporated into examinations utilizing effective doses

Radiation Risk

• ICRP's nominal risk coefficient for cancer is 5.5% per Sv
• After receiving exposure of 1 sievert approximately 5 people can develop cancer
• The risk of 1 Sv dose is 1/20 that cancer will develop
• If one is exposed to 1 mSv the risk of this goes down to 1 in 20,000
• Children are more susceptible at 1 in 13,000

X-Ray Risks

• Single chest X-ray risk is 1 in 1 million with fatal cancer
• Mammogram is 1 in 20000 with high sensitivity

UK Radiation Figures

• UK average gives a risk of 2.7 mSv gives an increased annual risk of cancer of 1 in 7400
• More likely to die from smoking however
• 1 mSv Medical procedures include:
  • Lumbar Spine
  • Barium meal
  • CT scans
• Risk-benefit analysis of these procedures must be considered

How to Minimize Radiation Exposure: Time, Distance, and Shielding

• Dose is ∝ to exposure time
• If you double your distance to the source, your exposure becomes a quarter of original level
• Shielding can be placed between the patient and source such as lead, concrete and brick

Radiation Safety in Projection and Fluoroscopy

• When standing in any protective area or behind a lead apron, staff exposure should also be minimized
• Patient positioning can prevent anatomy from being further exposed
• Fluoroscopies should minimize time
• and always ensure staff are wearing led aprons and glasses