Shielding Calculation: Radiographic Room Jerry Williams
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
DAP Workload Two approaches: 1. Predict clinical usage and use typical DAP values per exam 2. Assume typical total DAP values
Method 1: DAP data UK Dose Surveys HPA-RPD-029 http://www.hpa.org.uk/radiation/ Room doses for 50 90 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
Radiography doses (1) Projection No of rooms DAP Gy cm 2 Skull PA/AP 20 0.78 Skull Lat 19 0.49 Chest PA 210 0.11 Chest AP 12 0.12 Chest Lat 23 0.31 Thoracic spine AP 36 0.93 Thoracic spine Lat 27 1.43 Abdomen AP 127 2.58 Lumbar spine AP 118 160 1.60 Lumbar spine Lat 120 2.44 LSJ 25 2.59 Pelvis AP 150 2.12 IVU 35 13.60 Source: HPA-RPD Report 029 (2007)
Radiography doses (2) Anatomical region DAP Gy cm 2 Cervical Spine 0.05 Femur 004 0.04 Knee 0.04 Lower leg 0.02 Foot/ Ankle 0.02 Toes 0.005 Shoulder 0.05 Elbow/ forearm/ upper arm 0.02 Hand/ wrist 0.01 Finger 0.005 Source: Personal data
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)
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
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
Workload survey DAP (Gy cm 2 / week) 600 500 400 300 200 100 0 1 2 3 4 5 6 7 8 9 10 800 700 600 500 400 300 200 100 0 Patient ts/ week 2.0 1.8 1.6 1.4 1.2 1.0 Examinations per patient 1 2 3 4 5 6 7 8 9 10
Workload DAP (Gy cm 2 / week) 600 500 400 300 200 100 0 1 2 3 4 5 6 7 8 9 10 800 700 600 500 400 300 200 100 0 Patien nts/ week DAP per patient Ave: 0.75 Gy cm 2 Range: 0.20 1.42 Gy cm 2 Typical workload No of patients: 180 DAP: 150 Exceptional workload 24/7 room 2.0 1.8 1.6 1.4 1.2 1.0 Examinations per patient 1 2 3 4 5 6 7 8 9 10 No of patients: 800 DAP: 500 Examinations per patient Ave: 1.32 Range: 1.10 1.82
DAP averaged kv kv DAP averaged 90 88 86 84 82 80 78 76 74 72 70 1 2 3 4 5 6 7 8 9 10 Weighted average: 78.2 kv
Construction details Floors/ ceilings Concrete thickness Profile Wall construction Stud partition? Blockwork?
Adjacent areas Occupancy New buildings assume 100% Layout
Radiographic Room 1.5 m 1 m 3.5 m 3 m 6.3 m 6m
Lead shielding vs DAP workload Lead shielding mm 1.2 1 0.8 0.6 0.4 0.2 0 d = 1 m d = 1.5 m 0 100 200 300 400 500 600 700 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)
UK/ NCRP Comparison
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
Scatter dose @ 1m Side scatter NCRP 147 3.4x10-2 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 ) -1 0.6 mgy/ week Factor 10 difference?
Shielding calculation l NCRP Dose constraint = 20 µsv B < 0.02/5.4 002/54 x 1.5 15 2 = 0.8% Shielding @ workload weighted kv = 0.81 mm Pb UK Dose constraint = 6 µsv B < 0.006/0.6 006/0 6 x 1.5 2 = 2.3% Shielding @ 80 kv = 0.41 mm Pb
Differences Scatter model Field area assumption Patient workload data Dose constraints t Transmission data
Scatter model 2 ) -1 µgy (Gy cm S 12 10 8 6 4 2 0 UK Scatter fraction (S) @ 85 kv NCRP 147 MonteCarlo 0 30 60 90 120 150 180 Angle S max analysis kv UK NCRP 50 4.05 4.10 70 4.67 4.41 85 514 5.14 464 4.64 100 5.60 4.87 125 6.38 5.26
Field area NCRP F = 1000 cm 2 UK F incorporated into DAP Plate size F cm 2 8" x 10" 20 x 24 cm 2 480 10" x 12" 24 x 30 cm 2 720 14" x 17" 35 x 43 cm 2 1505 2
Patient workload data NCRP 147 2.5 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 0 10 20 30 40 50 60 70
Patient workload data NCRP 147 2.5 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??
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% 1986 60% 1996 2002 40% 2007 20% 0% Lat Skull PA Chest AP Lumbar spine Lat Lumbar spine AP Pelvis
Dose constraints NCRP 20 µgy (100 µgy Radiology staff) UK 6 µgy
Transmission factors NCRP UK Workload weighted transmission factors Workload weighted kv For this analysis 78 kv B NCRP 80 kv 0.05 0.34 0.27 0.02 0.54 0.43 0.01 0.74 0.56 0.005 0.96 0.71 0.002 1.29 0.92 0.001 1.55 1.08
WARNING! UK workload data apply only in UK UK workload drivers Legislation Justification/ Optimisation Public Healthcare provision Derive your own workload data
Standard specification: 1 mm lead for all radiographic rooms