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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

– @ 80 kV, D = 9.1 mGy/100mAs @ 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

- Doesn’t 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.

dzTNzDCTDI

mm

mm

50

50100

)(

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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 80’s survey• DRLs today• IRMER

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Mid-80’s 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-80’s 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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Entrance Surface Dose (mGy) Radiograph

Min. Median 3rd Quartile Max. Lumbar spine AP 0.8 7.7 10 59

Lat 2.4 20 30 108

LSJ 7.4 35 40 131

Abdomen AP 0.7 6.7 10 62

Pelvis AP 0.9 5.7 10 32

Chest PA 0.03 0.2 0.3 1.4

Lat 0.14 1.0 1.5 11

Skull AP 0.7 4.0 5 14

PA 1.8 4.3 5 13

Lat. 0.4 2.2 3 9

Dose-area product (Gy.cm2) Radiograph

Min. Median 3rd Quartile Max.

Lumbar spine

(3.4 films)

2.0 12 15 93

Barium enema

(8.5 films, 224 s fluoro)

6.2 41 60 272

Barium meal

(7.8 films, 193 s fluoro)

0.5 17 25 163

IVU

(8.2 films)

3.3 29 40 251

Abdomen

(1.4 films)

0.7 5 8 30

Pelvis

(1.1 films)

0.49 4 5 19

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Mid-80’s 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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