An Introduction to
Installation Shielding in
Radiotherapy
Professor P W Horton
Honorary Consultant Physicist
Royal Surrey County Hospital
Guildford, UK
Part 1
Contents
Linear Accelerator Rooms
•
•
•
•
•
•
•
•
Primary radiation shielding
Secondary radiation shielding
Beam orientation factor, occupancy of adjacent areas and IMRT factor
Shielding Materials
Scattered x-radiation down the maze
Neutron scatter down the maze for linacs operating at 10MV or above
Direct Doors
Practical Considerations
Linear Accelerators
Linear Accelerator with On Board Imaging
Adequate protection from the imaging equipment is provided by the protection for the linear accelerator
Bunker for 6 and 10MV Linear Accelerator with Magnetite Walls and No Door
Typical Linear Accelerator Bunker Plan with Maze
Shielding Material
Leakage
Radiation
T
Scattered
Radiation
P
Primary Radiation
T = linear accelerator target
Scatter down
the Maze
P = patient
Typical Linear Accelerator Bunker Elevation with Maze
Scattered
Leakage Radiation
Radiation
Primary Radiation
Shielding Material
Maze
P
T
T = linear accelerator target
P = patient
Basic Principles
The purpose of radiation shielding is to reduce
the equivalent dose from a linear accelerator to
a point outside the bunker to a dose limit or
constraint, set by national standards.
These typically set limits for an uncontrolled or
public area and a controlled area staffed by
radiation workers and can be annual doses and
dose rates over a period of time, e.g. one hour.
Some National Dose Limits
Country
Public Area
UK
0.3mSv/year
Occupational
Exposure
7.5µSv/hour
Ireland
0.3mSv/year
20-30µSv/hour
German
0.3mSv/year
0.12mSv/week
Sweden
0.1mSv/year
0.12mSv/week
USA
1mSv/year
1mSv/week
0.02mSv in any one hour
Shielding Calculations
Iin
Iout
B = Iout/Iin is the barrier transmission factor
The number (n) of tenth value layers (TVLs) of
absorber required to reduce the dose to an accepted
value is given by:
n = -log10B
Primary Shielding Calculation
Annual Dose
Da = Dp x n x 250 x U x T x B x 103/d2
where
Da is the annual dose constraint (mSv)
Dp is the dose per patient (fraction) (Gy)
n is the number of patients/day
250 is the number of working days/year (5 days/week x 50 weeks/year)
U is the use factor in the direction of the barrier
T is the occupancy factor in the area adjacent to the barrier
B is the barrier attenuation required to achieve Da
d is the distance from the linac target to the far side of the barrier
Re-arranging:
B = Da.d²/ Dp x n x 250 x U x T x103 (2)
(1)
Primary Shielding Calculation
Annual Dose Example
Suppose:
Da = 0.3mSv
d = 5.0m
Dp = 2 Gy
n = 50
U= 0.25
T = 0.05
then: B = 2.4 x 10-5
and n = -log10B = 4.62 TVL
Primary Shielding Calculation
Instantaneous Doserate
IDR = D x 60 x B x 106/d²
(3)
where
IDR is the limiting dose rate in µSv/hour
D is the dose rate at the isocentre (1m from the target) in Gy/min
B is the barrier attenuation required to achieve the IDR
d is the distance from the linac target to the far side of the barrier
B = IDR.d²/D x 60 x 106 (4)
Re-arranging:
Suppose:
IDR = 7.5 µSv/h, d = 5.0m, D = 6Gy/min
Then:
B = 5.208 x 10E-7
n = -log10B = 6.28 TVL
Primary Barrier Thickness
in Concrete (2350kg/m³)
Energy (MV)
6
10
15
TVL (m)
0.343
0.389
0.432
Concrete
Required (m)
Annual
dose
0.3mSv
IDR
7.5µSv/h
Annual
dose
0.3mSv
IDR
7.5µSv/
h
Annual
dose
0.3mSv
IDR
7.5µSv/h
1.59
2.16
1.80
2.44
2.00
2.71
Note: In practice 1st and equilibrium TVL values can be used
Note that the longer the period of averaging of the radiation workload, the lower the
shielding thickness can become until the maximum permitted IDR is reached. A higher
IDR limit and averaging is helpful with high dose rate (FFF) linacs.
Secondary Shielding Calculation
Annual Dose
Da = Dp x n x 250 x 0.001 x U x IF x T x B x 103/d2
(5)
where
Da is the annual dose constraint (mSv)
Dp is the dose per patient (fraction) (Gy)
n is the number of patients/day
250 is the number of working days/year (5 days/week x 50 weeks/year)
0.001 is the maximum head leakage factor permitted (IEC)
U is the use factor
IF is the IMRT factor
T is the occupancy factor in the area adjacent to the barrier
B is the barrier attenuation required to achieve Da
d is the distance from the linac isocentre to the far side of the barrier
This is a similar equation to the primary shielding equation (1) for annual dose
but U always equals one to take account of the isotropy of the leakage and
scattered radiation and leakage and IMRT factors are added.
Re-arranging:
B = Da.d²/ Dp x n x 250 x 0.001 x U x IF x T x103 (6)
Secondary Shielding Calculation
Annual Dose Example
Suppose:
Da = 0.3mSv
d = 4.0m
Dp = 2 Gy
n = 50
U= 1
IF = 1
T = 0.05
then: B = 3.9 x 10-3
and n = -log10B = 2.42 TVL
Secondary Shielding Calculation
Instantaneous Doserate
Note that leakage radiation from the treatment head dominates over
scattered radiation from the patient
IDR = D x 60 x 0.001 x B/d² (7)
where
IDR is the limiting dose rate in Sv/hour
D is the dose rate at the isocentre (1m from the target) in Gy/min
B is the attenuation required to achieve IDR
d is the distance from the linac isocentre to the far side of the barrier
0.001 is the maximum leakage factor permitted
Re-arranging:
B = IDR.d²/D x 60 x 0.001
Suppose:
IDR = 7.5 µSv/h, d = 4.0m, D = 6Gy/min
Then:
B = 3.333 x 10E-4
n = -log10B = 3.48 TVL
(8)
Secondary Barrier Thickness
in Concrete (2350kg/m³)
Energy (MV)
6
10
15
TVL (m)
0.279
0.305
0.330
Concrete
Required (m)
Annual
dose
0.3mSv
IDR
7.5µSv/h
Annual
dose
0.3mSv
IDR
7.5µSv/
h
Annual
dose
0.3mSv
IDR
7.5µSv/h
0.67
0.97
0.74
1.06
0.80
1.15
Note: In practice 1st and equilibrium TVL values can be used
Note that the secondary TVLs are less than the corresponding primary TVLs due to the
lower energy spectrum of the leakage and scattered radiation
Calculation Factors in Annual Doses
• Use or Orientation Factor – U
• Occupancy Factor – T
• IMRT Factor - IF
Use (or Orientation) Factors
This is the fraction of beam-on time that the linac points in the direction of
the shielding calculation
Traditional Factors (except for TBI)
0º (down)
0.25
90º
0.25
180º (up)
0.25
270º
0.25
Max
0.30
More Complex Factors (Rodgers 2001), based on real information from
Radiotherapy Management Systems (Biggs,2009)
90º Intervals
Gantry Angle
Factor
0
0.31
90/270
0.213
180
0.263
45º Intervals
Gantry Angle
Factor
0
0.256
45/315
0.058
90/270
0.159
135/225
0.04
180
0.23
Actual Orientation Factors from Aria RMS
RSCH, Guildford
LinAc
Energy
MV
Beam Orientation
0º
90º
180º
270º
LA1
6
30.8
21.7
28.6
18.9
LA2
6
32.6
16.2
33.5
17.0
LA3
6
37.1
17.1
27.6
18.1
LA4
6
37.1
18.3
27.7
16.9
LA5
6
23.6
29.6
23.1
23.6
LA6
6
28.2
26.1
23.7
22.0
LA1
10
32.2
21.0
33.0
13.8
LA2
10
32.8
27.6
12.6
27.0
LA5
10
26.7
26.8
28.5
18.1
LA6
10
24.7
25.6
30.9
18.9
LA3
15
32.2
30.4
6.9
30.5
LA4
15
32.4
30.5
7.9
29.2
Occupancy Factors
Radiation Shielding for
Diagnostic Radiology, 2nd Edition
NCRP151 (2005)
Location
Location
Factor
Fully Occupied Areas
e.g. control rooms,
reception areas, nurses’
stations, offices, etc.
1
Partial Occupancy
e.g. staff rooms, adjacent
wards, clinic rooms,
reporting areas
0.2 - 0.5
Occasional Occupancy
e.g. corridors, store rooms,
stairways, changing rooms
wards and patient rooms,
unattended waiting rooms,
unattended car parks,
toilets, bathrooms
Factor
0.05 – 0.125
BIR 2012, DG Sutton, CJ Martin, JR Williams & DJ Peet
Fully occupied areas, e.g.
linac control rooms
1
Adjacent treatment room
0.5
Corridors, rest rooms, etc
0.2
Linac bunker entrances
0.125
Toilets, storage areas, etc
0.05
Outdoor areas with
transient pedestrian or
vehicular traffic, etc
0.025
IMRT Factor
.
The introduction of Intensity Modulated Radiotherapy (IMRT) means that the
beam is on longer for the same prescribed dose. This does not change the
dose in the primary beam, but increases the dose of leakage and scattered
radiation to the secondary barriers.
IMRT Factor =
MU (IMRT)
MU (Conventional)
Range of IMRT Factor: 2 – 10
A factor of 5 is often used in calculations of secondary shielding.
In calculating the annual dose external to secondary shielding, account needs
to be taken of the proportions of conventional and IMRT treatments at each xray treatment energy
For Volumetric Intensity Modulated Arc Therapy (VMAT): IF = 2.5 ?
Shielding Materials
Material
Density (kgm-3)
Comment
Breeze blocks
1100-1400
Up to 500kV with
supplementary lead or
steel shielding
Clay bricks
1600
Earth fill
1600
Useful if bunker below
ground level
Concrete
(poured or interlocking blocks)
2350
Density can vary with
mineral content
High density concrete
(poured or interlocking
blocks)
3800-4600
Density dependent on
iron ore content
Leadite®/Verishield®
3700-4000
Interlocking blocks
Steel
7900
Lead
11340
Normally used as
supplemental shielding
on existing rooms
New & Conventional Shielding Materials
Primary TVLs
Material
Density
kg/m³
6MV
mm
10MV
mm
15MV
mm
Concrete
2350
343
389
432
High Density Concrete
( MagnaDense®)
3800
213
241
268
High Density Concrete
(MagnaDense®)*
3800
184
219
253
High Density Concrete Blocks
(MegaShield®)
3840
210
239
265
4600
175
199
221
3700-4000
218
247
275
Steel
7870
98
105
108
Lead
11340
55
56
57
TVL based on density wrt concrete
TVL based on density wrt concrete
High Density Concrete Blocks
(MegaShield®)
TVL based on density wrt concrete
Ledite®/Verishield® Blocks
TVL based on density wrt concrete for
minimum density of 3700 kg/m³
*Jones et al, Health Physics 96 (1) 67-75; 2009
Comparison of Density Related TVLs wrt Concrete
and Actual Primary TVLs for MagnaDense® High
Density Concrete
Material
Density
kg/m³
6MV
mm
10MV
mm
15MV
mm
TVL based on
density wrt TVL in
concrete
(2350 kg/m3)
3800
213
241
268
Actual TVL*
3800
184
219
253
*Jones et al, Health Physics 96 (1) 67-75; 2009
Conclusion: the use of TVLs based on relative density to concrete leads to minor
over-shielding and safe installations
New & Conventional Shielding Materials
Secondary (90º) TVLs
Material
Density
kg/m³
6MV
mm
10MV
mm
15MV
mm
Concrete
2350
279
305
330
High Density Concrete
(MagnaDense®)
3800
173
189
205
High Density Concrete
(MagnaDense®)*
3800
160
181
-
High Density Concrete Blocks
(MegaShield®)
3840
171
187
202
3700-4000
178
194
210
Steel
7870
80
85
87
Lead
11340
45
46
47
TVL based on density wrt concrete
TVL based on density wrt concrete
Ledite®/Verishield® Blocks
TVL based on density wrt concrete for
minimum density of 3700 kg/m³
*Jones et al, Health Physics 96 (1) 67-75; 2009
Note on Flattening Filter Free (FFF) Linear
Accelerators
The flattening filter is not necessary for IMRT and VMAT. This increases the
dose rate and enables short treatment times for high dose hypofractionated
treatments, e.g. SBRT.
Elekta: 22 Gy/min @ isocentre
Varian: 24 Gy/min @ isocentre
Available at 6 and 10MV
only
Removing the flattening filter results in:
•
a softer x-ray beam with a lower depth dose and TVL
Elekta increase the electron beam energy in the accelerator so that the depth
dose is unchanged, but Varian do not (TVLe reduced from 370 to 360mm at
10MV*)
•
lower leakage radiation
FFF dose = 0.36 x FF dose
FFF doserate = 1.45 x FF doserate
* Kry et al, PMB, 53, 1933-46 (2008)
Scattered X-radiation down the
Maze
Radiation at the entrance of the maze arises from:
•
Scatter of the primary beam from the bunker wall
•
Scatter from the patient
•
Scatter of the head leakage radiation from the
bunker walls
•
Transmission of head leakage radiation through the
maze wall
Scatter of the Primary Beam from the Bunker Wall
Where:
S is the dose rate at the maze entrance
W is the radiation workload
U is the use factor toward the wall B
α is the reflection coefficient at the wall
A is the area filled by the beam at each reflection
d are distances as shown
Reflection Coefficients for Mono-energetic X-rays on
Concrete (NCRP51)
Scatter from the Patient
•
•
•
•
•
•
•
•
•
Where:
S is the dose rate at the maze entrance
a is the scatter fraction
W is the radiation workload
U is the use factor toward the wall B
F is the area of the beam in the plane of the patient
α is the reflection coefficient at the wall
A is the area filled by the beam at the reflection
d are distances as shown
Scatter of Head Leakage Radiation from the
Bunker Walls
•
•
•
•
•
•
•
•
Where:
L is the dose rate at the maze entrance
Lo is the leakage factor
W is the radiation workload
U is the use factor toward the wall B
α is the reflection coefficient at the wall
A is the area filled by the beam at the reflection
d are distances as shown
Transmission of Head Leakage Radiation
through the Maze Wall
Where:
L is the dose rate at the maze entrance
Lo is the leakage factor
W is the radiation workload
U is the use factor toward the wall B
t is the thickness of the maze wall
d is the distance from the target to the maze entrance
Maze Scatter – Identification of Reflective
Areas
Scatter from Patient
Scatter from Lateral Wall
A1 and A2 are the reflective areas in each case
Neutron Scatter down the Maze
• Neutrons produced in the head of the linac are first
moderated by the x-ray shielding.
• Neutrons are further moderated by scattering off the
walls of the bunker.
• The total neutron flux therefore consists of direct (fast)
neutrons, scattered neutrons and thermal neutrons.
• Fast neutrons obey the inverse square law, but scattered
and thermal neutrons are isotropically distributed, and
consequently the neutron flux does not drop as fast as
an inverse square law relationship.
Neutron Production
Neutron Dose as Percentage of Primary X-ray Dose at
Isocentre
%Sv/Gy @ 1m
Energy 10MV
15MV
18MV
20MV
25MV
Elekta
0.01
0.07
0.15
0.20
0.30
Varian
0.004
0.07
0.15
0.18
-
Note: A Quality Factor (Q) of 10 has been used to convert to Equivalent Dose
Barish 2009
Neutron Photoproduction Cross-Sections on
Lead
Calculation Methods
• Kersey’s Method
De = Di.So.exp(-d2/5)
S1.d1²
where De is the dose at the maze entrance
Di is the dose at the isocentre
So is the cross-sectional area where the maze
enters the treatment room
S1 is the cross-sectional area of the maze
d1 and d2 are as shown
Calculated Dose= 0.82 – 2.3 x Measured Dose
•Modified Kersey Method
McGinley & Huffman (2000)
Wu & McGinley (2003)
•Monte Carlo Modelling
Monte Carlo Simulations
A mesh tally plot showing ambient dose equivalents for: neutrons (left) and neutron induced photons (right) i.e. prompt
activation photons
180
14000
160
1by1 field size
10by10 field size
40by40 field size
12000
Ambient equivalent dose rate (Sv/hr)
Ambeint equivalent dose rate (Sv/hr)
140
120
100
80
60
40
1by1 field size
10by10 field size
40by40 field size
10000
8000
6000
4000
20
0
0
2000
5
10
15
20
25
30
Maze exit dose rate (mesh profile (7,1:48))
35
40
45
50
Neutron ambient dose equivalent rate
along the maze
Reproduced by kind permission of M Dunn, S Green & Z Ghani
0
0
5
10
15
20
25
30
Maze exit path (mesh profile (7,:))
35
40
45
50
Photon ambient dose equivalent rate
along the maze
Skyshine
Skyshine
Potential problem with
adjacent building
windows

dr
dc
source
DRsky 
where
Calculation
point C
2.5  10 2  DR0  Broof  1.3
(d r  2)  d c2
DRsky is the doserate due to skyshine at point C
DR0 is the doserate at the isocentre
B is the transmission factor for the roof
When access to the roof is prohibited and there are no adjacent buildings, an
IDR at the surface of the roof up to a maximum of 2mSv/h is acceptable
Proton Therapy
Monte Carlo simulation is essential for determining the
shielding for proton therapy facilities
Analogue calculations can overestimate the actual neutron dose rates by
a factor of 10 to 100, except at the maze entrance where they
underestimate the dose rates
MCPNX overestimates the actual dose rates by a factor of 1.2 to 2.4.
[Newhauser WD et al, NIMPR, 476, 80-84 (2002)]
Maze Design and Entrance Doors
To reduce the dose rate at the maze entrance, the maze
should
• be long (inverse square law) but this requires more space
• have several corners (more reflections)
To reduce the neutron dose
(largely capture gamma rays), a
simple door may be necessary
for x-ray energies > 10MV
If space is limited, a direct
door will be necessary and
laminated for higher
energies
General Design Recommendation
Maximum design energy of 15MV for linear
accelerator bunkers
[Cancer treatment facilities: Planning and design manual
(NHS, UK, 2011)]
Practical Considerations
•
•
•
•
•
•
•
•
•
•
Linac Orientation
Width of Primary Barriers
Joints and Shutter Bolt Positions
Nibs
Ducts
Lintels
Wall Height and Primary Ceiling Barriers
Laminated Walls
Direct Doors
Ground Shine
Linac Orientation
Dose rates at the maze entrance can be slightly higher if
the beam can point at the inner maze wall
Width of Primary Barriers
Internal Shielding
External Shielding
Joints and Shutter Bolt Positions
Avoid joints and shutter bolt positions in the shielding that
lie along primary ray paths when constructing the barrier
Forming Shutters
Tie Bolt
Position
Joint
Concrete
Isocentre
Primary Radiation
Nibs
Nibs in the shielding are very useful to reduce the amount of scattered
radiation reaching the maze entrance or door
Nib
Bunker with Maze
Bunker with Direct Door
Ducts in Barriers
Ensure that ducts do not align with radiation paths
Duct for services, e,g, electricity
cable, air conditioning, etc.
Duct for dosimetry cable between
the control room and the maze
Lintels in the Maze
Treatment rooms have a high ceiling for services such as air conditioning but
there is no need usually to maintain this height in the maze as long as it is
high enough for the delivery of the treatment unit. Lintels above the maze
reaching up to the treatment room ceiling height reduce the cross-sectional
area of the maze for the transmission of scattered radiation and help to
reduce the dose rate at the maze entrance. This is particularly useful for
reducing the neutron flux with higher energy linacs.
Wall Height and Primary Ceiling Barriers
It uses less material and is cheaper if the primary barrier in the
ceiling is exterior to the treatment room
Laminated Walls
To minimise the size of the bunker or when using an existing bunker for a higher energy linac,
steel or lead may be incorporated into the walls to increase the attenuation. At energies above
10MV, these metals will be a source of photoneutrons as shown (on the left) and there must be
sufficient concrete on each side of the metal to absorb the neutrons and capture gamma rays to
reduce the surface dose rates to acceptable levels. To minimise the amount of high Z material,
the metal can be shaped as shown on the right) to take account of the obliquity and extent of
the beam as it passes through the ceiling.
Direct Doors
For under door leakage, calculate
as a maze with three legs
Lead or steel
insert
Scattered
Radiation
Door
Scattered
Radiation
Door
Door
Overlap
Door trench
Floor
Plan View
Side Elevation
Ground Shine
With a physically thin wall (made of steel plate or lead blocks), it is
possible to get ground shine beneath the wall, adding to the dose
rate transmitted through the wall
Isocentre
Ground shine
Floor
Additional shielding on the floor at the foot of the wall may be
necessary to reduce the ground shine to an acceptable dose rate
General Bunker Arrangements
•
•
•
•
•
•
•
•
•
•
•
•
•
Relationship to control room
Access and entrance barrier
Modulator cabinet position
Storage for electron applicators and cut-outs
Position of alignment lasers, respiratory gating
camera and CCTV
Lighting level control
Chilled water supply
Services (water, power and IT)
Ventilation
Cable ducts and trunking
Decoration
Warning signs and lights
Engineering (safety) controls
Bunker for 6 and 10MV Linear Accelerator with
Magnetite Walls and No Door