Biological effects of Ionising Radiation Flashcards
What two forms can be ionising radiation be divided into?
- By-products of radioactive decay
- Artificially produced electromagnetic radiation
What categories can By-products of radioactive decay by further separated into?
- Alpha particle (2protons/ 2neutrons) Large particle and 20um in water
- Beta particle (electron), v small and less than 1cm in water
- Gamma ray (form of EM radiation), high energy, travels long distances, 10s of cm in water
What is ionising radiation?
- Atoms have equal number of protons and electrons
- Ions are charged
- Ionising radiation has enough energy to turn atoms into ions
- Does this by removing electron orbiting the nucleus of atom and leaves ion with +ve charge
What is an ion pair?
- +vely charged ion and negative electron
What occurs during interaction of radiation?
- When radiation passes through matter it will ionise atoms along its path
- After each ionisation process, each ion pair, will deposit certain amount of energy locally
- Approx 35eV for air and tissue
- This energy greater than energy involved in atomic bonds e.g. ionic and covalent bonds in molecules have approx 4eV
How ionising are gamma and electrons?
- Sparsely ionising
How ionising are alpha particles, protons and neutron?
- Densely ionising
What is the most significant effect of ionising radiation?
- Damage to DNA most significant
What does evidence of DNA damage show us?
- Can be see in faulty repair of chromosome breaks in DNA
- Leads to development of abnormal cell populations and development of cancer
- Faulty repair of breaks seen in individuals exposed to large radiation doses
- Majority of damage is easily repaired, depending on category of damage
How can DNA damage by radiation occur?
- Directly
- Indirectly
What is the Direct effect of radiation to DNA?
- Radiation interacts with atoms of DNA molecule or another important part of cell
What is the Indirect effect of radiation to DNA?
- Radiation interacts with water in cell (75% water)
- When water molecule becomes ionised it forms a highly reactive free radical
- Two free radicals can combine to form hydroxyl radical
- Hydroxyl radical can diffuse short distances and cause DNA damage
What are free radicals?
- Unstable, highly reactive molecules
What factors does the biological effect of DNA damage depend on?
- Type of radiation
- Amount of radiation (dose)
- Time over which dose is received (dose rate)
- Tissue or cell type irradiated
What is the correlation between radiation doses and damage?
- Low doses of radiation produce less damage
- Linear relationship for alpha particles and kills more cells than similar dose of x-rays would
What factors contribute to tissue radiosensitivity?
- Function of cells that make up the tissues
- If cells are actively dividing
What do stem cells do?
- Exist to produce cells for another cell population
- Divide frequently
- Very radiosensitive
What are some highly radiosensitive tissues?
- Bone marrow
- Lymphoid
- Gastrointestinal
- Gonads
- Embryonic
What are some moderately radiosentitive tissues?
- Skin
- Vascualr endothelium
- Lung
- Lens of the eye
What are some least radiosensitive tissues?
- Central nervous system
- Bone and cartilage
- Connective tissue
What is tissue weighting factor?
- The fraction of the overall health risk, resulting from uniform, whole body irradiation, attributable to specific tissue
- T
What possible outcomes are there after radiation hits a cell nucleus?
- Either no change or DNA mutation
- DNA mutation can lead to
1. Mutation repaired and viable cell is left
2. Cell death and unviable cell is left
3. Cell survives but is mutated and can lead to cancer
What is dose a measure of?
- Measure of the amount of energy that has been transferred and deposited in a medium
What is the absorbed dose? (Gy)
- A quantity that can be measured
- Measure energy deposited by radiation
What is the Equivalent dose (Sv)?
- Absorbed dose multiplied by radiation weighting factor depending on type of radiation
- Beta, gamma and x-rays have weighting factor 1
- Alpha particles have 20
What is effective dose (Sv)?
- Equivalent dose to each organ multiplied by tissue weighting factor
- Represents stochastic health risk to whole body, which is probability of cancer induction
What is the effect of radiation during pregnancy in relation to dentistry?
- Would take a dose of 1000X more than intra-oral x-ray to cause abnormalities
- Pregnancy does not need to be taken into account for dental x-rays because dose to foetus is so low and usually less that estimated daily natural background dose received by foetus
What is the effect of radiation during pregnancy?
- Foetus must not be irradiated inadvertently or x-ray beam pointed directly at abdomen
- In early pregnancy, exposure above 100mGy could damage or kill enough cells of embryo to undergo resorption
- During organogenesis (2-8 weeks post conception) when organs not fully formed, doses >250mGy could lead to growth retardation
What are some sources of natural background radiation?
- Cosmic rays
- Internal radionuclide from diet
- Radionuclides in air
- External gamma radiation e.g. soil and rocks
What is effective dose of Intra-oral x-ray?
- 0.005mSv
- Negligible risk or cancer
What is the controlled area in radiation room?
- Controlled area should extend at least 1.5m from x-ray tube and patient
- X-ray beam should be directed away from staff member and stand behind solid wall
- Protection of staff
What is the Radiation Protection Philosophy?
- IRMER 2018 Protecting the patient
- Practices must have sufficient benefit to individuals or society in order to offset the detriment
- Individual doses and number of people exposed should be kept as low as reasonably possible (ALARP)
What is dose optimisation?
- A legal requirement
- Need to make sure dose to patients is ALARP
- Still need to maintain adequate image quality
- Circular collimators shown to increase dose by 40% therefore rectangular collimators should be used
What methods can patient dose be reduced?
- Use E speed film or faster (fewer x-rays photons required)
- Use kV range of 60kV to 70kV
- Focus to skin distance should be >200mm
What are Diagnostic Reference Levels (DRLS’s)?
- Legalisation requires employees have established dose levels for typical examinations for standard sized patients?
- Are a comparative standards used in optimisation
- Compared to national reference levels
- Individual x-ray units compared to DRL’s and national reference levels
- Enables identification of units giving higher doses
What are the current DRL’s for intra-oral examinations?
- Adult is 0.9mGy for digital sensors and 1.2mGy for phosphor plates and film
- Child is 0.6mGy for digital sensory and 0.7mGy for phosphor plates and films
Why is a Radiation risk assessment carried out?
- Identify all hazards with a potential to cause radiation accident
- Evaluate risks of arising from the hazards
- Done before equipment used
What are some scenarios that can occur in radiation risk assessment?
- Staff scatter exposure
- Patient scatter exposure
- Staff accidental exposure
- Non termination of exposure
- Warning light failure
- Emergency stop failure
