Shielding for Diagnostic X-rays: UK Guidance. Jerry Williams

Transcription

1 Shielding for Diagnostic X-rays: UK Guidance Jerry Williams Royal Infirmary of Edinburgh

2 BIR/ IPEM Working Party Report British Institute of Radiology Institute of Physics and Engineering in Medicine Working Party: David Sutton, Jerry Williams, Colin Martin, Don McIntosh, Tony Cotterill, Graham Hart, David Gallacher Publication date: 2000

3 BIR/ IPEM Working Party Report Content: Design criteria and dose constraints Primary & Secondary radiations Building materials X-ray transmission ss factors Assessment of shielding Worked examples Working Party: David Sutton, Jerry Williams, Colin Martin, Don McIntosh, Tony Cotterill, Graham Hart, David Gallacher Publication date: 2000

4 Radiation Sources Radiography (film/ screen) Radiography + Fluoroscopy Angiography CT (single slice) Mammography Dental DEXA

5 This talk Design criteria Transmission factors Shielding materials Pi Primary and Secondary Radiations

6 Design Criteria: Dosimetric Quantities Dose limits Effective dose, E msv Dose monitoring Operational quantity Personal dose equivalent, H p (d) msv Shielding calculations l Air kerma, K mgy

7 Design Criteria Dose constraint (people) Based on public dose limit (1 msv/ year) Applied to everyone (staff + public) 0.3 mgy/ year (6 µgy/ week) Occupancy factor 5% Dose to be < 6 mgy/ year (Controlled area) Dose constraint (film or CR) 0.4 µgy/ day (2 µgy/ week)

8 Occupancy factors Based on individual occupancy Examples: Office 100% Reception area 100% X-ray control room 100% Patient examination room 50% Corridor 20% Toilets/ bathrooms 10% Outdoor area with seating 10% Store rooms 5% Unattended waiting rooms 5%

9 X-ray Transmission factors Adopted Archer s empirical model B + β α = α γ t 1 e β α 1 γ Used Simpkin s α, β, γ values Added data for brick and barium plaster Archer et al (1983) Health Physics, 44, 507. Simpkin (1995) Health Physics, 68, 704

10 Shielding material UK Standard lead thicknesses Code t mm

11 Primary and Secondary Radiations Leakage radiation Primary/ transmitted beams Scatter

12 X-ray Tube Leakage Assumptions: Tube leakage factor: 150 kv/ 3.3 ma Field size: 1000 cm 2 Distance: 1 m from tube/ patient WP Recommendation: Leakage contribution may be ignored

13 Primary & Transmitted Radiation X-ray beam Patient Fluoroscopy Beam restricted to image intensifier/ flat panel detector Detector has > 2mm lead equivalent Mammography Imaging device

14 Radiography: Film dose method Cassette Bucky system Dose to film/ imaging plate 400 speed system Dose 10 µgy Lead equivalence (100 kv) Cassette 0.19 mm Cassette + grid 0.26 mm Cassette/grid/table 0.8 mm Cassette/grid/chest Bucky mm Lead equivalence data derived from Dixon (1994) Med Phys, 21, 1785

15 Radiography: Film dose method Dose to film/ imaging plate 400 speed system Dose 10 µgy Lead equivalence (100 kv) Cassette 0.19 mm Cassette + grid 0.26 mm Cassette/grid/table 0.8 mm Cassette/grid/chest Bucky mm

16 Attenuated primary: limiting HVL 1 Limiting HVL: = ln(2) / α 0.1 α t D = D o e Transmission VL mm Limiting H Lead thickness mm kv

17 Radiography: ESD method Unattenuated primary beam Beam outside patient Entrance Surface Dose (ESD) Situations include Beam not collimated to cassette Beam not directed to Bucky Examples Chest radiography Cross table radiography Out of Bucky radiography (e.g. extremities) Area of wall irradiated Not easy to predict Variable

18 Examples

19 Chest Radiography (film dose) Parameters 100 films/ week 90 kv ESD = 0.15 mgy Film dose = 10 µgy FFD = 1.8 m FSD = 1.4 m Film-wall ll = 0.7 m Best Radiographic Practice

20 Calculation (Film-dose method) Attenuated kerma at wall per week: Film dose x Workload x ISL correction = 0.52 mgy No of HVLs required (6 µgy/ week) = 6.5 Limiting HVL of 90 kv = 0.23 mm Total equivalent lead thickness = 1.5 mm Additional lead shielding = mm

21 Chest Radiography (ESD) Parameters 100 films/ week 90 kv ESD = 0.15 mgy Film dose = 10 µgy FFD = 1.8 m FSD = 1.4 m Film-wall ll = 0.7 m

22 Calculation (ESD method) Worst case assumption: Beam size greater than patient and Bucky system Unattenuated kerma at wall per week: ESD x Workload x ISL correction = 4.7 mgy Maximum transmission permitted = 6x10-3 / 4.7 = Lead 90 kv 1.4 mm

23 Summary Best practice Beam collimated to patient/ image plate/ Bucky 0.8 mm lead Poor practice Beam outside patient and Bucky 1.4 mm lead

24 Scatter

25 NCRP 49 method Scatter Model K s = a K u F 400 K s scatter kerma; K u primary kerma F Field size (cm 2 ); a scatter factor Trout and Kelly (1972), Radiology, 104, 161 K u. F Area-air kerma product (AKP) or Dose area product (DAP) K s = S DAP

26 Why DAP? No assumptions on field size Availability of DAP data Requirement in UK legislation to record patient dose DAP preferred patient dose metric (fluoroscopy and radiography) National surveys of patient dose

27 Scatter factor normalised to DAP (S) Scatter factor, S µgy.(gy.cm 2 ) kvp 100 kvp 85 kvp 70 kvp 50 kvp Angle of scatter S = ( a θ + b θ + c θ + d θ + e ) ( kv 85 ) [ f + 1 ] JR Williams (1996) Br J Radiol, 69, 1032

28 Scatter factor normalised to DAP (S) -1 Scatter factor, S µg Gy.(Gy.cm 2 ) kvp 100 kvp 85 kvp 70 kvp 50 kvp Angle of scatter a -1.04E-07 b 327E E-05 c -2.75E-03 d 8.37E-02 e 158E E+00 f 5.99E-03 S = ( a θ + b θ + c θ + d θ + e ) ( kv 85 ) [ f + 1 ] JR Williams (1996) Br J Radiol, 69, 1032

29 Calculation vs measurement Clinical i l application No of DAP/ week Ave Calc positions Gy cm 2 Dose mgy Measured/ Calc dose Mean Min Max Interventional Radiology (liver disease) Abdominal + lower limb angiograpy Cardiac angiography Cerebral angiography Ba contrast studies (o/c tube) Ba contrast t studies (u/c tube) General Radiography

30 Validation of scatter data by Monte-Carlo S µgy/ Gy cm Scatter fraction 85 kv Measured MC Angle

31 Comparison with NCRP 147 S µgy (G Gy cm 2 ) UK Scatter fraction 85 kv NCRP 147 MonteCarlo Angle

32 Differences in scatter factors? Phantom X-ray spectrum Measurement methods

33 Method in practice Scatter factor, S µgy.(gy.cm 2 ) kvp 100 kvp 85 kvp 70 kvp 50 kvp Angle of scatter S ( aθ + bθ + cθ + d + e) ( kv 85) = θ [ f + 1]

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36 Method in practice (1) 1.0 θ r d dose Relative Scattering angle S max = [( ) ] ( 2 ) kv µgy Gy cm 1 d = 1 m; θ = 117

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38 θ d = 1 m S ave = S( θ) dθ d θ

39 Method in practice (2) S ave over C-arm rotation (30 to 150 ) S max for parallel beam kv S max S ave

40 To be continued.. Application examples Fluoroscopy Radiography Comparisons with NCRP 147