Shielding Calculation: Radiographic Room

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