Biological Effects Of Ionising Radiation Flashcards
Two types of ionising radiation
By-products of radioactive decay
Artificially produced EM radiation
3 by-products of radioactive decay
Alpha particle
Beta particle
Gamma ray
What is the result of ionising radiation
Free electron and positively charged ion (Ion Pair)
3 steps of radiation interaction
- When radiation passes through matter it ionises atoms in path
- Each ion pair deposits around 35ev of energy in air and tissue
- This energy is greater than energy involved in atomic bonds
Most significant effect of ionising radiation
Damage to DNA
E.g. large radiation dose> faulty repair of chromosome breaks>abnormal cells>cancer
Two types of DNA damage
Direct
Indirect
Direct DNA Damage
Radiation interacts with atoms of DNA molecule or another important part of cell
Indirect effect
Radiation interacts with water in cell> H2O becomes ionised> free radical formed> two join forming hydroxyl radical causing DNA damage
DNA damage to one strand of helix vs both strands
One strand reads, held in place by other> easily fixed
Both strands break, two remaining ends will seek to rejoin other free ends, not necessarily right ones (DSB)
4 factors affecting biological effect of radiation
Type of radiation DSB usually alpha
Amount of radiation (dose)
Time over which dos is received (dose rate)
Tissue or cell type irradiated
Dose effect
Low doses>less damage
Linear relationship for alpha particles which kill more cells than a similar dose of X-rays would
N.b WEIGHTING FACTOR
Dose Rate Effect
Low dose rate> less damage
Cells can repair less serious DNA damage before further damage occurs
High dose rate, DNA repair capacity of cell likely to be overwhelmed
Organ cancer risks
After large radiation exposures, higher incidence of cancer in certain tissues
Depends on organ receiving highest dose
What is tissue radiosensitivity dependent on
The function of the cells that make up the cells
If cells are actively dividing
Two types of cells and their radiosensitivity
Stem cells
Produce cells for another cell population
Divide frequently
Very radiosensitive
Differentiated cells
Do not exhibit mitotic behaviour
Less sensitive to radiation damage
High radiosensitive tissues
More rapid cell division>greater sensitivity to radiation
Bone marrow, lymphoid, GI, gonads, embryonic
Moderately radiosensitive tissues
Skin, vascular endothelium, lung, lens of eye
Least radiosensitive tissues
CNS, bone and cartilage, connective tissue
Possible outcomes when radiation hits cell nucleus
No change
- Mutation repaired > viable cell DNA mutation - Cell Death > Unviable cell
- Mutated cell > Cancer?
Three different types of dose quantities
Absorbed Dose (Gy)
Equivalent Dose (Sv)
Effective Dose (Sv)
Absorbed Dose
Measures the energy deposited by radiation
E.g. IO X-ray typical entrance skin dose at colllimator tip is 2mGy
Equivalent dose
Is the absorbed dose multiplied by radiation weighting factor depending on radiation
Beta, gamma and X-rays - weighting factor 1
Alpha particles - 20
Effective Dose
Is the EQUIVALENT DOSE multiplied by tissue weighting factor
Is probability of cancer induction
Typical intra oral X-ray is 5uSv
What does LNT model do
Estimates long term damage from radiation
Assumes always harmful with no safety threshold and several small exposures have same effect as one large
Two types of radiation effects
Deterministic- tissue reactions, only occur above threshold dose, severity of effect related to dose received
Stochastic- probability of occurrence related to dose received, no threshold to the effect and severity of not dependent on dose. Basis of LNT model
Deterministic effects of ionising radiation
Unusual to see in radiology
Effects wont show immediately but several days after
Certain things happen at certain values e.g. hair loss at 3Sv, lethal dose at 6Sv
Typical io xray 5uSv (0.000005Sv)
Stochastic effects of ionising radiation
Have no known thresholds
Cannot predict if they will happen or how severe they will be
Effects can develop years after exposure
Can be somatic (resulting in disease or cancer) or genetic (resulting in abnormalities of descendents)
Effect of radiation during pregnancy
Doses for any abnormalities to occur are 1000 times greater than that of an IO xray
Dose to foetus is so low (0.01uSv and 8uSv which is less than background dose received)
what must not happen when taking radiograph of pregnant woman
Foetus must not be irradiated inadvertently nor should X-ray Bea, be directed towards abdomen
Examples of natural background radiation
Cosmic rays
Radionuclides from diet
Radionuclides in air e.g. radon
External gamma radiation e.g. soil, rocks
Estimated UK background radiation 2.2mSv
Lifetime risk of cancer from IO xray
From one in 10m to one in 100m
Guidance for X-ray use
Guidance notes for dental practitioners on safe use of X-ray equipment by college of general dentistry
IRMER 2017 protecting patient
Justification- must have sufficient benefit to individuals or society in order to offset detriment
Optimisation- individual doses and number of people exposed should be kept As Low As Reasonably Practicable (ALARP)
How do we keep pt dose ALARP
Using E speed film or faster
Using kV rang 60-70kV
Focus to skin distance > 200mm
Diagnostic Reference Levels in patients
Established dose levels for typical exams on standard sized pts
Enables identification of units giving higher doses
Adult: 0.9mGy (digital sensors) and 1.2mGy (phosphor plates and film
Child: 0.6mGy and 0.7mGy
What can we do to improve image quality and therefore reduce pt dose
Reduce damage by inserting CR plates between two plastic sheets
Replace/clean damaged detectors
Save images with minor artefacts, can be referred to and used in training
