Biological Effects of Ionising Radiation Flashcards
What can ionizing radiation be divided into
bi products of radioactive decay
artificially produced electromagnetic radiation
What are byproducts of radioactive decay split into
§ Alpha particle
§ Beta particle
§ Gamma rays (ER) (identical to x rays but source is diff)
What is the result of ionization
Result of ionisation is a free electron and a positively charged atom
What is the effect of ionizing radiation on the body
• Ionising radiation can disrupt structural organic molecules and cells of building elements of our body and the DNA in the cell nucleus
Where can evidence of DNA damage be seen
in faulty repair of chromosome breaks leading to development of abnormal cell populations and the development of cancer
What is seen in individuals exposed to large doses of radiation
- Faulty repair of breaks is seen in individuals who are exposed to large radiation doses
- Majority of damage is easily repaired depending on category of damage
What is the direct effect of radiation
○ Direct effect - radiation interacts directly with the atoms of a DNA molecules or another important part of a cell
What is the indirect effect of radiation
○ indirect effect - radiation interacts with water in the cell. When water molecules becomes ionised a highly reactive free radical ion is formed, two of these can combine to form a hydroxyl radical which can diffuse short distances and cause DNA damage
What is the advantage of DNA being double helix
advantage of this is that if only one strand of the helix breaks the DNA is held in place by the other strand so is easily fixed
What happens if both strands of double helix break
it becomes far more difficult to piece the DNA back together and the two remaining ends will seek to re-join with other free ends, not necessarily the correct matching en
What happens if the repair isn’t done properly
can lead to mutations which can affect cell function
What does the biological effect depend on
○ Type of radiation
○ Amount of radiaiton / dose
○ Time over which dose is received / dose rate
○ Type of tissue irradiated
What is the radiosensitivtiy of tissues depend on
○ The function of the cells that make up the tissues
If cells are actively dividing
What are stem cells
exist to produce cells for another cell population
○ Divide frequently
Very radiosensitive
What are differentiated cells
○ Do not exhibit mitotic (dividing) behaviour
○ Less sensitive to radiation damage
What are highly radiosensitive tissue
○ Skin, vascular endothelium, lung, lens of the eye
What are the least radiosensitive tissues
○ CNS, bone and cartilage and connective tissue
What does the tissue weight factor do
decide the most sensitive tissues
• Bone marrow colon etc have higher tissue weighting factors compared to other tissues
What are the several possible outcomes after radiation hits a cell nucleus
○ No change ○ DNA mutation § Mutation repaired □ Viable cell § Cell death □ Unviable cell § Cell survives but is mutated □ Cancer
What is absorbed dose
○ Can be measured
○ Measures energy deposited by radiation
○ For intra oral x ray the typical entrance skin dose at the collimator tip is around 2 mgy
What is equivalent dose
○ We have seen that diff types of radiation can cause diff levels of damage to tissue
○ Equivalent dose is the absorbed dose multiplied by radiation weighting factors depending on the type of radiation
○ For beta, gamma and X-rays the weighting factor is 1
○ For alpha particles it is 20
What is effective dose
○ Represents sarcastic health risk to whole body
What is the risk of cancer from intra oral x ray
1 in 10 mil
What are the two types of biological radiation effects
deterministic effect
stochastic effect
What is the deterministic effect
§ Tissue reactions
§ Only occur above a certain threshold dose
§ The severity of the effect is related to the dose received
• Unusual to see in radiology although possible in high dose areas such as interventional radiology
• Often the effects will not show immediately but rather several days after the exposure
• Can get skin damage/erythrema
What is the stochastic effect
§ Probability of occurrence is related to the dose received. Basis of LNT model
§ No threshold to the effect of severity of the effect is not dependend on dose
○ No dose below which effect will not occur
• Cant predict if they will occur in exposed individual
• Likelihood increases as dose increases
• Can develop years after exposure
• Effect can be divided into somatic or genetic
What is the effect of radiation during pregnancy
- Pregnancy does not need to be taken into account for dental radiology
- Fetus must not be radiated inadvertently or beams directed at it
- In early pregnancy, radiation can damage or kill enough cells for the emrbyo to undergo resorption
What are sources of natural background radiation
○ Cosmic rays
○ Radionuclide from diet
Radionuclides in the air
What is the effective dose form examinations
• Intra-oral x ray ○ Negligible risk • Lumbar spine x ray ○ V low risk • Abdominal ct ○ Low risk • Cumulative exposures can be bad
What legislation makes up radiation protection
- IRMER17 (px)
- IRR18 (staff and public)
- QA programme for x ray equipment, imaging processing, viewing, image quality
What is IRR17
- IRR17
- Controlled areas should extend at least 1.5m from x ray tube and px
- X ray beam should be directed away from staff
What is IRMER 17
• Justification, optimisation (ALARP)
○ Dose optimisation is a legal requirement
○ Make sure dose to px is ALARP
§ Still maintain adequate image quality
§ Circular collimators increase dose by 40% so use rectangular collimator
○ Px dose can be reduced using a variety of methods
§ E speed film or faster
§ Kv range 60-70kv
§ Fsd should be >200mm
What are diagnostic reference levels
- Not appropriate to apply this to medical exposures since there is by definition direct benefit to px
- These provide clinical audit tool that can help identify poor ?
- Legislation requires employers to have established dose levels for typical examinations for standard sized px
Why is image quality important
- Keep dose ALARP but enough for image quality as without image quality the exposure is not justified
- CR are prone to damage by teeth marks
What is a radiographic risk assessment
- Employer must look at radiation risk assessments
- They should evaluate the risks arising form them
- Staff scatter exposure is when exposure does not terminate and staff did not follow the procedure of turning off the x ray machine after removing the patient
