Shielding Calculation: Radiographic Room. Jerry Williams

Transcription

1 Shielding Calculation: Radiographic Room Jerry Williams

2 What you need to know Room use and layout DAP workload DAP averaged kv Distance to barrier Construction details Walls Ceilings/ floors Surrounding areas Rooms above or below Occupancy

3 DAP Workload Two approaches: 1. Predict clinical usage and use typical DAP values per exam 2. Assume typical total DAP values

4 Method 1: DAP data UK Dose Surveys HPA-RPD Room doses for kg patient sub-set set Mean Min/ max 25 th / 50 th / 75 th percentile values kv and mas values National Diagnostic Reference Levels Based on 3 rd quartile values Boundary between good and normal practice and bad and abnormal practice

5 Radiography doses (1) Projection No of rooms DAP Gy cm 2 Skull PA/AP Skull Lat Chest PA Chest AP Chest Lat Thoracic spine AP Thoracic spine Lat Abdomen AP Lumbar spine AP Lumbar spine Lat LSJ Pelvis AP IVU Source: HPA-RPD Report 029 (2007)

6 Radiography doses (2) Anatomical region DAP Gy cm 2 Cervical Spine 0.05 Femur Knee 0.04 Lower leg 0.02 Foot/ Ankle 0.02 Toes Shoulder 0.05 Elbow/ forearm/ upper arm 0.02 Hand/ wrist 0.01 Finger Source: Personal data

7 Radiographic workload (UK) Frequency 0% 10% 20% 30% 40% DAP Head Chest Abdomen Pelvis Spine Upper extremity Lower extremity Data from: UK examination frequencies in NRPB Report W4 (2002) DAP data * HPA-RPD Report 029 (2007) * personal data (extremities)

8 Radiographic workload (UK) Frequency 0% 10% 20% 30% 40% DAP Head Chest Abdomen Pelvis Spine Upper extremity Lower extremity Abdo/ pelvis/ spine 23% frequency 87% DAP Chest 35% frequency 8% DAP Extremities 37% frequency 1.5% DAP Conclusion To predict DAP workload estimate number of abdomen/ spine/ pelvis examinations Ignore extremities Chest workload only required for primary beam shielding

9 Method 2: DAP workload in typical rooms Survey data from 10 CR consoles Collection period: 9 to 43 weeks Number/ type of examination Number of patients Assumption Images processed on console correspond to specific X-ray room Average DAP values

10 Workload survey DAP (Gy cm 2 / week) Patient ts/ week Examinations per patient

11 Workload DAP (Gy cm 2 / week) Patien nts/ week DAP per patient Ave: 0.75 Gy cm 2 Range: Gy cm 2 Typical workload No of patients: 180 DAP: 150 Exceptional workload 24/7 room Examinations per patient No of patients: 800 DAP: 500 Examinations per patient Ave: 1.32 Range:

12 DAP averaged kv kv DAP averaged Weighted average: 78.2 kv

13 Construction details Floors/ ceilings Concrete thickness Profile Wall construction Stud partition? Blockwork?

14 Adjacent areas Occupancy New buildings assume 100% Layout

15 Radiographic Room 1.5 m 1 m 3.5 m 3 m 6.3 m 6m

16 Lead shielding vs DAP workload Lead shielding mm d = 1 m d = 1.5 m DAP w orkload Gy cm 2 Occupancy = 100% Typical room (DAP = 150 Gy cm 2 ) 0.45 mm (d = 1.5 m) 0.6 mm (d = 1 m) Exceptional room (DAP = 500 Gy cm 2 ) 0.7 mm (d = 1.5 m) 0.9 mm (d = 1 m)

17 UK/ NCRP Comparison

18 Example NCRP busy room 160 patients/week Wall at 1.5 m 100% occupancy NCRP: 2.5 ma min/ patient UK: 0.75 Gy cm 2 / patient

19 Scatter 1m Side scatter NCRP x10-2 mgy/ patient 5.4 mgy/ week Back/ forward scatter 4.8x10-2 mgy / patient 7.77 mgy/ week UK Max scatter 5.0 µgy (Gy cm 2 ) mgy/ week Factor 10 difference?

20 Shielding calculation l NCRP Dose constraint = 20 µsv B < 0.02/ /54 x = 0.8% workload weighted kv = 0.81 mm Pb UK Dose constraint = 6 µsv B < 0.006/ /0 6 x = 2.3% 80 kv = 0.41 mm Pb

21 Differences Scatter model Field area assumption Patient workload data Dose constraints t Transmission data

22 Scatter model 2 ) -1 µgy (Gy cm S UK Scatter fraction 85 kv NCRP 147 MonteCarlo Angle S max analysis kv UK NCRP

23 Field area NCRP F = 1000 cm 2 UK F incorporated into DAP Plate size F cm 2 8" x 10" 20 x 24 cm " x 12" 24 x 30 cm " x 17" 35 x 43 cm

24 Patient workload data NCRP ma min/ patient UK data HPA-RPD-029 kv/ mas Dundee (Dave Sutton) Downloaded from RIS 8 rooms HPA-RPD Dundee Skull PA/AP Chest PA Chest Lat Thoracic spine AP Thoracic spine Lat Abdomen AP Lumbar spine AP Lumbar Spine Lat Pelvis/ hip AP mas per examination

25 Patient workload data NCRP ma min/ patient UK data HPA-RPD-029 kv/ mas high dose exams Dundee (Dave Sutton) Downloaded from RIS 8 rooms Combined with CR workload data 0.27 ma min/ patient Factor of 10??

26 Patient workload data Examinations per patient US: 3.37 UK: 1.32 US: UK Ratio = 2.5 Types of examination? ESD (normalised to 1986) Period of survey US: early 1990s UK: early 2000s 100% 80% % % % 0% Lat Skull PA Chest AP Lumbar spine Lat Lumbar spine AP Pelvis

27 Dose constraints NCRP 20 µgy (100 µgy Radiology staff) UK 6 µgy

28 Transmission factors NCRP UK Workload weighted transmission factors Workload weighted kv For this analysis 78 kv B NCRP 80 kv

29 WARNING! UK workload data apply only in UK UK workload drivers Legislation Justification/ Optimisation Public Healthcare provision Derive your own workload data

30 Standard specification: 1 mm lead for all radiographic rooms

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