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Radiation Hazards and Dosimetry

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Mid-80's survey. Results. E.g. Chest PA. Median ESD = 0.18 mGy. Minimum ... surveys and take action if average dose is greater than of national survey doses ... – PowerPoint PPT presentation

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Title: Radiation Hazards and Dosimetry


1
First FRCR Examination in Clinical Radiology
Diagnostic Radiology Radionuclide Radiology
(4b) Patient Dosimetry John
SaundersonRadiation Protection Adviser
2
RCR Syllabus
  • Methods
  • Diagnostic reference levels (including high dose
    techniques)
  • Magnitude and measurements .

3
Methods
  • General radiology
  • Fluoroscopy
  • Computed Tomography
  • Nuclear Medicine .

4
General radiology
  • ESD
  • Thermoluminescent dosemeter (TLD)
  • exposure factors
  • Dose-Area Product (DAP)
  • Effective dose .

5
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 .

6
T.L.D.s /-
  • Small
  • Tissue equivalent
  • Read 1 ?Gy
  • 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) .

7
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.

8
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 .

9
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.

10
Dose Area Product
  • Because dose falls with 1/d2 and area increases
    with d2, DAP is independent of distance.

11
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, ?Gy.m2) .

12
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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18
Fluoroscopy
  • Dose-Area Product (DAP)
  • Exposure factors
  • Effective dose .

19
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.

20
Exposure factors
  • Based on assumed FSDs, etc.

21
Effective dose
  • Can be fudged using radiograph software.

22
GAFCHROMIC film
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25
GAFCHROMIC film
  • optical density is proportional to the absorbed
    dose
  • 0.01 Gy to 50 Gy
  • Energy independent from 30 keV to 30 MeV
  • 20 per 14 x 17 sheet

26
Computed Tomography
  • CT Dose Index (CTDI)
  • Dose-Width Product (DWP)
  • Effective dose

27
CT Dose Index (CTDI)
  • Applies to a single slice
  • Can be used to compare
  • different slice widths
  • different physical filter
  • different scanners
  • etc.

28
Dose-Length Product (DLP)
  • DLP CTDI x n x T
  • Gives an idea of relative dose for a whole scan
  • Can be used to compare effect of pitch, etc.

29
Effective dose
  • NRPB program similar to radiography one

30
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31
CT dose/risk calculation
  • e.g. wrong patient referred for CT scan
    (laryngectomy)

32
Nuclear Medicine
  • MIRD
  • Add up dose to each organ from the dose
    irradiating from each organ.

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34
Diagnostic Reference Levels
  • Early 80s survey
  • DRLs today
  • IRMER

35
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.

36
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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38
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.

39
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

40
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

41
Staff and Environmental Monitoring
  • Devices
  • TLD
  • OSLD
  • Film
  • Electronic
  • Body
  • Extremity
  • Environment

42
Relevant measurement techniques
43
Measuring Dose
  • Luxel dose badges
  • TLD finger rings
  • Can be cold sterilized
  • Heat sensitive

44
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.

45
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

46
Electronic Dosemeters
  • Used by Radiation Physics staff to test x-ray
    unit and measure environmental doses

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48
f i n
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