Lecture 9 Flashcards
Define equivalent dose
The dose that takes into account the net effect of the radiation on tissue (in the body) by using a radiation weighting factor. It is measured in sieverts (Sv).
What is the equation for equivalent dose?
H = equivalent dose
D = dose
Wᵣ = radiation weighting factor
How can the weighting factor for equivalent dose be found?
It can be found using physical evidence (data) of the effects of different sources of radiation rather than being based on calculations.
What is the radiation weighting factor for beta?
1
What is the radiation weighting factor for alpha?
20
What is the radiation weighting factor for x-rays?
1
What is the radiation weighting factor for gamma rays?
1
Describe how the radiation weighting factor changes for neutrons of different energies
Define effective dose
A description of the biological relevance of radiation exposure when different tissues/organs receive varying absorbed doses potentially from different radiation sources, measured is Sv.
Effective dose is a quantification of _____.
Risk
What is a tissue weighting factor?
A factor that describes the sensitivity of different tissues to radiation.
What is the equation for effective dose in multiple organs?
E = effective dose
Wₜ = tissue weighting factor
H = equivalent dose
D = dose
Wᵣ = radiation weighting factor
What is the total tissue weighting factor for the whole body?
1.00
What is the risk factor per sievert of radiation in the whole body?
5.0%
What sources of radiation have natural origin?
- Cosmic rays
- Terrestrial gamma
- Internal (K-40 and C-14)
- Inhaled (radon Rn-222 and decay products)
What sources of radiation have man-made origin?
- Nuclear weapons, Chernobyl, leaks, etc.
- Average medical exposure
What is the average annual equivalent dose ?
2.3 mSv (in UK)
What is the average annual equivalent dose from cosmic rays?
0.3 mSv
What is the average annual equivalent dose from terrestrial gamma?
0.35 mSv
What is the average annual equivalent dose from internal sources (C-14 and K-40)?
0.38 mSv
What is the average annual equivalent dose from inhalation (radon Rn-222 and decay sources)?
0.97 mSv
What is the average annual equivalent dose from nuclear weapons, Chernobyl, leaks, etc.?
0.03 mSv
What is the average annual equivalent dose from medical exposure?
0.25 mSv
The average annual equivalent dose varies with ________ and ____________.
Geography
Geology
Describe the pie chart for the distribution of annual equivalent dose
What is the dose of a CT scan?
~10 mSv (> ~10 mGy)
What is the lifetime risk of fatal cancer from a CT scan?
1:2000
What is the normal risk of fatal cancer?
~1:5
How can the lifetime dose be used to calculate the chances of fatal cancer in a lifetime?
1) Multiply the average annual equivalent dose by the average lifespan.
2) Find 5% of this value.
Why is it risky to have many scans over a lifetime?
There is a high radiation dose administered to the patient (equal to 4 years worth of annual dose per scan).
Why do children have a higher health risk from getting a CT scan?
- Smaller
- Growing tissue
- More stem cells
- More years left after scan than an adult (younger)
- Only used when the child is very unwell so they’re more likely to need multiple scans
What is the dose of a paediatric CT scan?
5 mSv - 60 mSv
Name 5 types of dosimeter technology
- Ionisation chamber (e.g. farmer-type chamber or parallel plate chamber)
- Geiger Mueller counter
- Film
- Thermoluminescence dosimetry
- Semiconductors
What are parallel plate ionisation chambers used for?
- Low energy x-rays (< 60 kV)
- Electrons of any energy
These are used for radio-therapy and x-ray beam monitoring and calibration.
What is Fricke gel?
A simple and cost-effective gel made of iron that can be used as a phantom tissue for parallel plate ionisation chambers. It changes colour when radiation is incident on it so is good for checking the position of beams and the dose of beams.
What are BANG polymers?
Polymers that show a radiation pattern. The radiation causes polymerisation to take place so solid areas are areas of radiation. It is used as a phantom tissue for parallel plate ionisation chambers.
How does thermoluminescence dosimetry work?
A type of radiation dosimeter, consisting of a piece of a thermoluminescent crystalline material. When the thermoluminescent crystal is exposed to ionising radiation, it absorbs and traps some of the energy in its crystal lattice. The level of this radiation can then be determined by a specialised detector that measures the intensity of the emitted light.
What is a Geiger-Müller counter?
A very sensitive counter of radiation events (rather than a dosimeter) that can give a general alert of a radiation source. They are lightweight and portable.
