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Radiation Safety: custom cards > Radiation risk and dose calculation > Flashcards

Radiation risk and dose calculation Flashcards

1
Q

What is equivalent dose?

A
  • HT = D*WR [Sv]
  • absorbed dose * radiation weighting factor.
  • Take into account the harmfulness of different radiations.
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2
Q

Describe what is meant by stochastic effects.

A
  • Probability of effect occurring is proportional to dose (no threshold).
  • Stochastic effect of interest is carcinogenesis.
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3
Q

Describe/draw the features of the stochastic effects graph.

A
  • 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.
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4
Q

Describe what is meant by deterministic effects.

A
  • e.g. erythema, cataracts, sterility etc.
  • No effect below a threshold dose
  • Above threshold, severity of effect increases with dose.
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5
Q

Describe/draw the features of the deterministic effects graph.

A
  • Well-defined threshold.

- sigmoid-like curve

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6
Q

Describe what is meant by genetic damage.

A
  • Chromosome damage - breakage followed by faulty repair (no convincing evidence).
  • Gonad exposure relevant.
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7
Q

What are the problems associated with estimating the risk to an individual?

A
  • 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.
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8
Q

What is effective dose?

A
  • E = SUM(WTWRDT,R) = SUM(WT*HT) [Sv].
  • Sum over exposed tissue (SUM(WT) = 1).
  • Tissue weighting factor * Radiation weighting factor * absorbed dose.
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9
Q

What is the effective dose received if the equivalent dose to the: oesophagus is 15mSv, lungs 10mSv, liver 6mSv, stomach 3mSv, breast 15mSv?

A
  • 3.6mSv

- i.e. equivalent to whole body being exposed to 3.6mSv.

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10
Q

What do the tissue weighting factors depend on (and state the four components)?

A
  • 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.
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11
Q

What is the average annual dose to the public from all sources in the UK?

A

-2.7mSv.

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12
Q

What fraction of the average annual dose to the public is due to medical exposures in the UK and how many mSv is this?

A
  • 16%.

- Approx 0.4mSv.

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13
Q

What is the annual risk of death from radiation exposure at 1mSv per year?

A

-1 in 80 000.

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14
Q

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)?

A
  • 4%/Sv for adult workers.
  • 5%/Sv for whole population.
  • 30 IQ points/Sv
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15
Q

How and why do risk factors change with age?

A
  • 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.
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16
Q

List and describe the radiation effects on the embryo and foetus.

A
  • 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).
17
Q

List patient dose quantities, both measurable and un-measurable.

A
  • Surface dose.
  • Organ dose.
  • DAP.
  • CTDI.
  • Effective dose.
18
Q

Describe how the surface dose can be measured or calculated (give equation).

A
  • 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).
19
Q

How can you obtain organ dose from surface dose?

A
  • 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.
20
Q

Explain what is meant by the DAP.

A
  • 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.
21
Q

State some pros and cons of direct dose measurement techniques.

A 
Good:
-Straightforward.
-Confined to surface (usually).
Bad:
-Not retrospective.
-Delay between measurement and readout.
22
Q

State some pros and cons of indirect dose measurement techniques.

A

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).

23
Q

What is the CTDI and how is it measured?

A
  • 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.
24
Q

What is the CTDIw and how is it measured?

A
  • 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.
25
Q

State the derived CT dose quantities.

A
  • 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]

Radiation Safety: custom cards (5 decks)

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