Lecture 5: radiation sources, measurement & radiobiology Flashcards
What % of UK deaths per year are due to radiation sources?
1%
What % of cancer deaths are due to radiation sources?
4%
Of the UK population dose, what % of radiation is man-made and what % is environmental?
man-made: 17%
environmental: 83%
Of the UK population dose, what % of the man-made radiation is medical/dental?
medical/dental: 90%
Of the medical/dental exposures, what % is dental?
dental: 9%
What % of all x-ray examinations are dental?
dental: 30%
medical: 70%
Give 4 reasons why someone may receive an unnecessary x-ray dose?
- no valid justification for it
- incorrect radiation (60kV instead of 70kV)
- using a round collimator instead of a rectangular one
- having to repeat it
What 2 properties of x-rays can be used to measure the dose?
- ionisation
- excitation
What are the 6 types of dose meters?
How do they each work?
1- Free air ionisation chamber (collects electrons and measure charge)
2-Thimble ionisation chamber (measures discharge due to ionisation)
3-Chemical conversion: Frickes’ Dose Meter (measures chemical change due to ionisation
4-Photographic density (assessed through level of black caused by ionisation of silver bromide)
5- Calorimetry (measures heating effect due to excitation)
6- thermoluminescent dose meter (assess dose by measuring released stored energy)
How does free air ionisation chamber work?
- Two plates are parallel to each other (one single plate above -ve the other broken plate below +ve)
- The arrangement of the charged plates produces a measurable volume between the plates. .
- X rays hit 1kg of free air and electrons (carrying their negative charge) will move towards the +ve bottom plate
- The electrons move through the bottom plate to a meter. Measuring the charge will indicate how many electrons and therefore, ionisation events took place.
What unit measures the amount of electrons/charge released?
Coulombs. It measures the charge transported by a constant current of one ampere in one second
How does a Thermoluminescence dose meter work?
- It operates between the conduction band and valency band of atoms, known as the Forbidden zone
- Exposure to radiation raises the energy level of the electrons and they are trapped in this Forbidden Zone, by ‘cups/traps’ known as energy anomalies
- The energy given to the electrons is the exact same energy given to the patient
- The electrons remain here until released by heating. As the electrons drop down from the ‘traps’ they release energy as light.
- This light is picked up by a metre and can measure the dose. The intensity of the light released is proportional to the dose received
What are the advantages of Thermoluminescence dose meter?
- can be reused
- relatively cheap
- wide range: capable of measuring very large & very small doses
What are the disadvantages of Thermoluminescence dose meter?
- no permanent dose record
- sensitive to extreme environments (heat in particular)
What is Radiation Exposure?
unit?
- Number of ionisation events
- C/kG
What is the absorbed dose?
unit?
- total energy absorbed
- J/kg (Grays)
What is equivalent dose?
unit?
- represents the stochastic health effects of low levels of ionizing radiation on the human body
- Sieverts (Sv)
What is the effective dose?
unit?
- sum of the equivalent doses in all specified tissues and organs of the human body and represents the stochastic health risk to the whole body.
- Sieverts (Sv)
What is the equation to work out the effective dose?
absorbed dose (Gy) X radiation weighting factor (Wr) X Tissue weighting factor (ΣWt)
Gy X Wr X ΣWt
Gy X 1 X 15
Which is the most important measurement?
why?
- Effective dose
- Because it is the only measurement that gives a realistic assessment of the potential harm given to the patient
How many individual weighting factors are there for tissue?
What are the top 3 most affected tissues?
- 15
- ovaries/testes, stomach, intestines
What are the 2 ways photon interactions can damage human tissue?
- direct effects
- indirect effects
How does direct human tissue damage occur?
When a photon interacts with a biological molecule
How does indirect human tissue damage occur?
When a photon interacts with a water molecule
What is the chemical process of radiation damage?
-Once the photon has interacted either directly/indirectly with a target molecule the chemical process is identical:
1- The molecule will become ionised
2-Ionisation causes dissociation of the molecule and the formation of free radicals
3-These free radicals produce abnormal chemical reactions
What is more common in humans, indirect or direct damage?
Why?
- Indirect damage is more common
- because we contain a lot of water (60-65%)
What 3 effects can radiation damage have on the cells?
How likely is it to occur at a dental dose?
1- Cell death: not at dental dose
2- Prevention or delay in cell division: unlikely at dental dose
3- mutation: will occur at dental dose.
Is there a threshold dose at which mutation occurs?
No, any radiation dose can cause mutation
Within the cell, what can be affected?
- DNA
- cytoplasmic organelles (mitochondria, centrioles, Golgi apparatus, smooth/rough ER)
What are the 2 major effects of radiation exposure?
- stochastic (random)
- deterministic
What are the 7 features of stochastic effects of radiation exposure?
1- it is random/unpredictable
2-probability increases as dose increases
3-there is no tissue repair for damaged tissue
4-these effects are cumulative (since they don’t repair, the damage accumulates with every dose added)
5- severity is not dose related (related to which parts of the body becomes abnormal)
6-there is no threshold at which someone will definitely develop a tumour, and there is no threshold to guarantee no cancer
7-no safe waiting period between doses
-will arise from chronic exposure and can occur anywhere in the body at any time (months, years after)
What are the features of deterministic effects of radiation exposure?
1-does have a degree of predictability
2-some tissue repair is possible between exposures
3-tissue damage does not accumulate
4-severity is dose related. The bigger the dose the worse the damage
5-there is some threshold to determine the extent of damage
- will arise from acute exposure and will occur at localised areas where the tissue was exposed.
When receiving an x-ray, what radiation effects are given?
both stochastic and deterministic effects
What is a somatic effect?
- the radiation effects that occur in the individual that has received the x-rays.
- Can be stochastic or deterministic
What is a genetic effect?
- the radiation effects that occur in the offspring of the individual that received x-rays
- only genetic effect if the exposure occurred BEFORE conception of the individual
- stochastic
Does the risk of genetic effects remain the same in all offspring of the affected individual?
No, the further genealogically from the affected individual the less likely the offspring are to present with genetic effects
If a pregnant mother receives an x-ray and the baby develops cancer due to the exposure, is this somatic or genetic effects?
somatic effect. The baby was already conceived and had received their own dose of x-rays.
Who is more at risk of radiation damage, children or adults?
children
Under 10 years the radiation is 2X more dangerous
Why are children more at risk of radiation damage?
1- size: they are a lot smaller and will have less interstitial fluid
2- water content: children contain more water, so more likely to receive indirect effects
3- immune system: children don’t have a fully developed immune system
What causes the severity of x-ray effects?
It is WHERE the abnormal effects are developed. (somewhere harmful: brain, lungs).
