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Radiation Safety > Radiation Risk > Flashcards

Radiation Risk Flashcards

1
Q

Equivalent Dose

unit

equation

A

Equivalent dose (HT) = absorbed dose (D) x radiation weighting factor (wR)

H (Sv) = D (Gy) * WR

For x-rays and electrons, wR = 1

Equivalent dose takes into account the harmfulness of different radiations

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2
Q

Radiation effects on humans:

Stochastic:

  • Probability?
  • Evidence?
  • Example?
A
  • Probability of effect occurring is proportional to dose (no threshold)
  • Evidence: Japanese survivors, early radiologists, uranium miners, radium dial painters
  • The stochastic effect of interest is carcinogenesis
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3
Q

Radiation effects on humans:

Deterministic (tissue reactions)

Probability?

Evidence?

Example?

A
  • No effect below a threshold dose
  • Above threshold, severity of effect increases with dose
  • Carcinogenesis & deterministic effects are somatic effects – they affect the irradiated individual
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4
Q

Radiation effects on humans:

Genetic:

How is it caused?

How can we assess risk?

A
  • chromosome damage – breakage followed by faulty repair no convincing evidence
  • Problems of genetic risk assessment:
    • only gonad exposure is relevant
    • mutations may be recessive
    • mutations may be unstable
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5
Q

Problems of estimating risk to an individual (x2)

A
  • Different organs and tissues have different radiosensitivities
  • Dose to organs and tissues are not uniformly distributed
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6
Q

Effective dose

A
  • E (Sv) = SUM( WT * HT)
    • WT = tissue weighting factor for organ sensitivity
    • HT = equivelent dose to organ or tissue
      *
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7
Q

Tissue weighting factor:

  • Skin
  • Lung
  • Breast
  • Bone
A
  • Skin - 0.01
  • Lung - 0.12
  • Breast - 0.12
  • Bone - 0.01
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8
Q

What is detriment?

What 4 components does it have?

A

Detriment is a measure of toal harm arising from an exposure

Components:

  1. P( fatal cancer )
  2. P( severe genetic effects )
  3. Length of life lost
  4. weighted probability of non-fatal cancer
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9
Q

UK average annual dose from all sources

A

2.7 mSv

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10
Q

Per caput dose from medical irradiation

UK

USA

A

UK - 0.4mSv

USA - 0.4mSv

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11
Q

Total risk factors:

Adult

8-15 week conceptus

where do they come from?

A
  • Adult - 5%/Sv
  • 8-15 week conceptus - 30 IQ points / Sv

Numbers come from ICRP 103

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12
Q

What do radiation risk factors depend on?

A

Age - decreasing as you get older, particularly high for developing children

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13
Q

Why is irradiation in utero more risky?

A
  • Rapidly dividing cells = higher radiosensitivty - possibility of death
  • Smaller number of cells have specific functions, posibiliy of abnormality
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14
Q

Radiation effects on embryo & foetus

  • Tissue
    • lethality
    • gross malformations
    • abnormal brain development
  • Stochastic
A
  • Tissue
    • lethality - 0.1 - 1 Gy
    • gross malformations - 0.2Gy (2-5wks) 0.5Gy(5-7 weeks)
    • abnormal brain development - 16-25 (0.6-0.7Gy)
  • Stochastic
    • Heritable - 2.4 %/Gy
    • Cancer induction
      • excess cancer induction risk : 6%/Gy
      • Excess risk of cancer deth = 3%/Gy
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15
Q

What are the 3 highest contributors to the collective dose from medical exams?

A
  1. CT - 7.4% frequency, 68% dose
  2. interventional radiology -1% frequency, 8% dose
  3. Mammo - 5.8% frequency, 5.4% dose
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16
Q

Effective dose for CT

  1. Head
  2. chest
  3. Abdo
  4. Pelvis
A
  • Head - 2
  • chest - 8
  • Abdo - 10
  • Pelvis - 10
17
Q

What would you use to measure surface dose to a patient?

2 positives

2 negatives

A

TLD chip on body in beam

Positives

  • Easy measured
  • Easily calculated

Negatives

  • No indication of volume irradiated
  • Non-additve if beam position changes (e.g. if you open up collimator, surface dose stays the same but you could be including someone’s lungs as well)
18
Q

Surface dose equation

A

Surface dose = tube output x inv. sq. factor x BSF x mAs

(mGy) (mGy / mAs) (mAs)

19
Q

How do you measure organ dose?

A
  • Measure or calculate surface dose
  • calculate attenuation of beam at depth (PDD)
  • tables of normalised organ dose data available
20
Q

DAP

What is it?

Unit

When is it useful?

A

DAP = Dose * Area of beam (Gy cm^2)

  • Proportional to total amount of energy passing through chamber
  • Correlates reasonably well with risk, useful for when x-ray beams vary in size and position
21
Q

CTDI

What is it?

Unit?

A

Computed tomography dose index CDTI

  • CTDIair = 1/s * integral(x) D(x) dx
  • D(x) is the dose profile across a slice
  • s is the slice width
  • Organ doses and effective dose calcualted from monte carlo simulations

Area under dose profie for one rotation, measured with partially irradiated pencil CT chamber. CDTI100 is measured in 100mm chamber.

22
Q

CDTIw

A
  • Weighted CDTI
  • Measured in cylindrical perspex phantom, four measurements at periphery, one at centre
  • Weighted acerage dose within standard phantoms
  • CTDIW = 1/3 CTDI100, centre + 2/3 CTDIW100, periphery
23
Q

CDTI vol

DLP

A

CTDIw corrected for pitch and for the MAs used in the scan

Dose length product = CTDIvol * irradiated length

Radiation Safety (8 decks)

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