Radiation Hazards and Dosimetry

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First FRCR Examination in Clinical Radiology
Diagnostic Radiology Radionuclide Radiology
(4b) Patient Dosimetry John
SaundersonRadiation Protection Adviser
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RCR Syllabus

• Methods
• Diagnostic reference levels (including high dose
  techniques)
• Magnitude and measurements .

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Methods

• General radiology
• Fluoroscopy
• Computed Tomography
• Nuclear Medicine .

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General radiology

• ESD
• Thermoluminescent dosemeter (TLD)
• exposure factors
• Dose-Area Product (DAP)
• Effective dose .

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T.L.D.

• Crystals, e.g. lithium fluoride
• Radiation causes electrons to be caught in
  traps
• At lab. TLDs heated to 240oC
• Electrons released, light emitted
• Amount of light emitted proportional to dose .

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T.L.D.s /-

• Small
• Tissue equivalent
• Read 1 uGy
• Easy for radiographer

• No direct readout
• Sensitive to heat, UV, dirt
• Tricky to calibrate
• Easy to loose
• Special ones needed for low dose (e.g. chest) .

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Using exposure factors

• kV, mAs, field size, FSD
• If D dose at 100 cm for 80 kV, 100 mAs then
• ESD D
• x kV2/802
• x mAs/100
• x 1002/FSD2
• x BSF
• BSF backscatter factor, depends on HVL and
  field size.

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E.g.

• What is ESD for 100 kV, 50mAs, 15 x 15 cm field,
  75 cm FSD for Room 3?
• From annual survey
• _at_ 80 kV, D 9.1 mGy/100mAs _at_ 1 m
• HVL for 100kV was 3 mmAl
• From tables
• BSF for 3mmAl, 15 x 15 field 1.33
• Therefore, entrance surface dose
• 9.1mGy x 1002/802 x 50/100 x 1002/752 x 1.33
• 0.17 mGy .

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Using exposure factors /-

• No fiddly TLD for radiographers!
• No fiddly TLDs for physicist!
• Radiographers can calculate ESDs
• No minimum dose

• No direct readout
• Exposure parameters must be recorded
• Assumes tube output not changed from last survey.

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Dose Area Product

• Because dose falls with 1/d2 and area increases
  with d2, DAP is independent of distance.

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DAP /-

• No fiddly TLD for radiographers!
• Only one number to record
• Instant answer

• Doesnt take into account backscatter
• Initial cost (several thousand )
• Units sometimes cause confusion (cGy.cm2, or
  Gy.cm2) .

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Effective dose

• Complicated to calculate from ESD or DAP
• Can use computer models which make assumptions on
• field size
• patient size
• field position

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Fluoroscopy

• Dose-Area Product (DAP)
• Exposure factors
• Effective dose .

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DAP /- for fluoroscopy

• Instant answer, etc.
• DAP moves with the tube
• Gives good indication of relative risks of
  inducing cancer

• Not directly linked to erythema risk.

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Exposure factors

• Based on assumed FSDs, etc.

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Effective dose

• Can be fudged using radiograph software.

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Computed Tomography

• CT Dose Index (CTDI)
• Dose-Width Product (DWP)
• Effective dose

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CT Dose Index (CTDI)

• Applies to a single slice
• Can be used to compare
• different slice widths
• different physical filter
• different scanners
• etc.

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Dose-Width Product (DWP)

• DWP CTDI x n x T
• Gives an idea of relative dose for a whole scan
• Can be used to compare effect of pitch, etc.

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Effective dose

• NRPB program similar to radiography one

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Nuclear Medicine

• MIRD
• Add up dose to each organ from the dose
  irradiating from each organ.

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Diagnostic Reference Levels

• Early 80s survey
• DRLs today
• IRMER

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Mid-80s surveyMethod

• Survey of twenty UK hospitals
• 13 most common views
• For each 10-20 patients (60-80kg) at
• DAP or ESD by TLD measured.

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Mid-80s survey Results

• E.g. Chest PA
• Median ESD 0.18 mGy
• Minimum ESD 0.03 mGy
• Maximum ESD 1.43 mGy
• Max / min 48 !!.

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Mid-80s surveyRecommendation

• Use 75th percentile as reference value
• i.e. carry out local surveys and take action if
  average dose is greater than ¾ of national survey
  doses
• e.g. for chest PA reference 0.3mGy ESD
• Send results to NRPB to review national reference
  doses every 5 years.

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DRLs today

• A DRL is essentially a guide to the rather
  indistinct border between good and normal
  practice and bad and abnormal practice.
• See handout for values

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IRMER

• National DRLs
• set as 3rd quartile
• average from survey of standard patients
• should be below DRL
• Local DRLs
• ?Average for Trust?
• Most should be below average

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Staff and Environmental Monitoring

• Devices
• TLD
• OSLD
• Film
• Electronic
• Body
• Extremity
• Environment

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Relevant measurement techniques
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Measuring Dose

• Luxel dose badges
• TLD finger rings

• Can be cold sterilized
• Heat sensitive

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Luxel badges

• Wear underneath lead rubber apron
• Assume dose to badge effective dose
• Can be worn for 2 weeks to 3 months (usually 1
  month)
• Must be returned promptly.

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Film Badges

• Can distinguish different types of radiation
• Wide dynamic range
• Permanent record
• Not very accurate below 0.2 mGy
• Filters can fall out
• Sensitive to heat, and abuse!
• Delayed readout

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Electronic Dosemeters

• Used by Radiation Physics staff to test x-ray
  unit and measure environmental doses

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