Radiation risk and dose calculation Flashcards
What is equivalent dose?
- HT = D*WR [Sv]
- absorbed dose * radiation weighting factor.
- Take into account the harmfulness of different radiations.
Describe what is meant by stochastic effects.
- Probability of effect occurring is proportional to dose (no threshold).
- Stochastic effect of interest is carcinogenesis.
Describe/draw the features of the stochastic effects graph.
- Linear no threshold (LNT).
- Starts at the natural incidence line.
- Probability of effect proportional to dose.
- Curves off at high doses due to cell kill.
Describe what is meant by deterministic effects.
- e.g. erythema, cataracts, sterility etc.
- No effect below a threshold dose
- Above threshold, severity of effect increases with dose.
Describe/draw the features of the deterministic effects graph.
- Well-defined threshold.
- sigmoid-like curve
Describe what is meant by genetic damage.
- Chromosome damage - breakage followed by faulty repair (no convincing evidence).
- Gonad exposure relevant.
What are the problems associated with estimating the risk to an individual?
- Different organs and tissues have different radio-sensitivities.
- Doses to organs and tissues are not uniformly distributed.
- Cannot directly measure, usually rely on simulation.
- Not much data on exposure to radiation and risk.
What is effective dose?
- E = SUM(WTWRDT,R) = SUM(WT*HT) [Sv].
- Sum over exposed tissue (SUM(WT) = 1).
- Tissue weighting factor * Radiation weighting factor * absorbed dose.
What is the effective dose received if the equivalent dose to the: oesophagus is 15mSv, lungs 10mSv, liver 6mSv, stomach 3mSv, breast 15mSv?
- 3.6mSv
- i.e. equivalent to whole body being exposed to 3.6mSv.
What do the tissue weighting factors depend on (and state the four components)?
- Detriment: a measure of total harm arising from an exposure.
- Has four components:
- probability of fatal cancer.
- Probability of severe genetic effects.
- Relative length of life lost.
- Weighted probability of non-fatal cancer.
What is the average annual dose to the public from all sources in the UK?
-2.7mSv.
What fraction of the average annual dose to the public is due to medical exposures in the UK and how many mSv is this?
- 16%.
- Approx 0.4mSv.
What is the annual risk of death from radiation exposure at 1mSv per year?
-1 in 80 000.
What is the lifetime risk of fatal cancer in %/Sv for adult workers and the whole population? What is the risk of mental retardation (8-15 weeks conceptus)?
- 4%/Sv for adult workers.
- 5%/Sv for whole population.
- 30 IQ points/Sv
How and why do risk factors change with age?
- Risk decreases with age.
- There is greater opportunity for expression of induced effects with children (i.e. have more life ahead).
- Children’s cells divide more quickly so greater sensitivity for some forms of cancer.
List and describe the radiation effects on the embryo and foetus.
- Tissue effects:
- (a) Lethality ~ 0.1 - 1Gy (age 0 - term).
- (b) Gross malformations ~0.2Gy (2-5weeks) ~ 0.5Gy (5-7 weeks), very few observed after 7 weeks.
- (c) Abnormal brain development, 8-15 weeks no threshold, 16-25 weeks ~ 0.6-0.7Gy, 25 weeks-term no effects observed.
- Stochastic effects:
- (a) Heritable effects ~ 2.4%/Gy but limited data
- (b) Cancer induction 6%/Gy to age 15 (3%/Gy death).
List patient dose quantities, both measurable and un-measurable.
- Surface dose.
- Organ dose.
- DAP.
- CTDI.
- Effective dose.
Describe how the surface dose can be measured or calculated (give equation).
- Place TLD chip on surface of patient/phantom.
- x-ray beam exposes TLD chip.
- Can also calculate from radiographic factors.
- Surface dose = tube output [mGy/mAs] * inverse square factor * BSF * mAs (ionisation chamber measures tube output).
How can you obtain organ dose from surface dose?
- Calculate from surface dose if position of organ is know and the attenuation of the beam at depth of organ is known (PDD).
- Table of normalized organ dose data available.
Explain what is meant by the DAP.
- Product of dose * area of the beam at specific cross-section of the beam [Gycm^2].
- Proportional to the total amount of energy passing through the chamber (also total energy passing through the patient).
- Total energy imparted correlates well with risk.
- Useful when x-ray beams vary in size and position.
State some pros and cons of direct dose measurement techniques.
Good: -Straightforward. -Confined to surface (usually). Bad: -Not retrospective. -Delay between measurement and readout.
State some pros and cons of indirect dose measurement techniques.
Good:
-Retrospective estimates possible.
Bad:
-Large number of estimates possible for small number of measurements.
-Not all factors may be recorded.
-Calibration needed if AEC is used.
-Difficult in fluoroscopy (use DAP meter).
What is the CTDI and how is it measured?
- CTDI(air) = 1/s * integral[D(x)dx].
- D(x) is the dose profile across a slice.
- s is the nominal slice width.
- Measured using ion chamber aligned with z-axis.
- Relates to machine output in air.
- Area under single or multi-slice dose profile for one rotation.
What is the CTDIw and how is it measured?
- Weighted computed tomography dose index.
- Weighted average dose within the standard phantoms.
- Measured using cylindrical perspex phantom, four measurements at periphery, one at centre.
- Separate head (16cm D) and body phantoms (32cm D).
- CTDIw = 1/3CTDIw100,centre + 2/3CTDIw100,periphery.
State the derived CT dose quantities.
- CTDIvol: CTDIw corrected for pitch or couch increment and for the mAs used in the scan.
- DLP: CTDIvol multiplied by the irradiated length.
- [CTDIvol represents the average radiation dose over the x, y and z directions, taking into account gaps and overlaps between the radiation dose profile from consecutive rotations of the x-ray source]
- [DLP is a measure of the ionising radiation exposure during the entire acquisition of images]
