4D CT
Just kidding!
Daniel A. Low
Room Shielding for IMRT
Mallinckrodt Institute of Radiology
Washington University School of Medicine
St. Louis, Missouri 63110
Daniel A. Low
Washington
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Outline
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Mallinckrodt Institute of Radiology
Washington University School of Medicine
St. Louis, Missouri 63110
General facility planning
General facility planning
Shielding formalism
Conventional shielding example
IMRT changes in formalism
Neutrons
• IMRT versus Conventional
– Tomotherapy
– Conventional Linac – SMLC
– Conventional Linac – DMLC
• Tomotherapy:
– Power, cooling, data transfer similar to
conventional RT
• Conventional Linac
– Power, cooling, data transfer is same as
conventional RT
• Shielding?
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Conventional Shielding Formalism
• Start with the conventional formalism
• Concept – determine the amount of shielding
to limit dose/dose rate beyond a shielding
barrier or barriers
• Geometry (distances, angles)
• Multiple barriers
• Different shielding materials
– reflection, transmission coefficients
• Different particles (photons, neutrons)
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Conventional Radiation Therapy
Room Shielding Formalism
• NCRP Reports No. 49 (≤
(≤10 MeV),
MeV), 51, and 79
• Primary and secondary (leakage and scatter) barriers
• Barrier thickness based on:
Where:
B = barrier transmission
P = weekly design exposure rate
W = workload
U = use factor
T = occupancy
d = distance from radiation source
m = correction factors for
secondary barriers
Pd 2
B=
•m
WUT
McGinley, Shielding Technologies for Radiation Oncology
Facilities, second edition, Med. Phys. Pub. 2002
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Typical High Energy Installation
6'-7'concrete
7'-8'
concrete
7'-8'
concrete
• The fraction of transmission that the barrier
requires to reduce the radiation intensity to
an acceptable value
• Unitless
1.3 m
secondary
barrier
primary
barrier
isocenter
primary
barrier
linac rotation plane
Barrier Transmission B
Pd2
B=
•m
WUT
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Design Exposure Rate P
Workload
• The dose rate allowed by regulatory agencies
• Typically weekly
• The amount the linear accelerator is
operating, defined at 1 m (typically isocenter)
• Nominally equivalent to the total weekly dose
delivered
• Units: Gy week-1
– Some regulations have “instantaneous” dose rates
as well
• Units = Sv week-1
B=
Pd2
•m
WUT
B=
Pd2
•m
WUT
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Use Factor U
Occupancy Factor T
• The fraction of time that a linear accelerator
is pointing towards the barrier
• Relevant only for “primary” barriers (take
as 1 for secondary barriers)
• Unitless
• Fraction of time a location is occupied by a
human
• Typically only used for “uncontrolled areas”
• Most relevant to locations such as hallways,
staircases, etc.
• Unitless
Pd2
B=
•m
WUT
Pd2
B=
•m
WUT
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Distance d
Secondary Barriers
• Typically distance measured from isocenter
to point of calculation
• Divided into leakage and scatter
• Leakage = head leakage
– Location from which distance is measured
depends on source of radiation
– Typical specification is that leakage is 0.1% of
primary beam intensity
– Distances measured from linac head (use closest
approach for d)
• Very important component (squared)
• When outside barrier, distance overestimates
dose falloff
– barrier acts as closer source
– dose falloff is faster than using d
– conservative estimate
• Units: meters
B=
2
Pd
•m
WUT
• Scatter
– Patient
– Wall/barrier
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Leakage
POI
secondary
barrier
Scatter – Patient
B=
2
1000 Pd sec
B=
WT
reflection
coefficient
isocenter
primary
barrier
field size (cm2)
at scatterer
scatterer-POI target-scattering
distance
surface (1m)
secondary
barrier
dsec
primary
barrier
P
2
2 400
d sec
d sca
aWT
F
dsec
isocenter
primary
barrier
linac rotation plane
primary
barrier
linac rotation plane
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Scatter - Patient
Scatter - Barrier
Scattering barrier-POI
distance
target-scattering
barrier distance
• a = ratio of scattered radiation at 1m from
the patient to the primary radiation at 1m
from the target
• F = field size at patient
• Scattered beam has different energy than
primary
B=
reflection
coefficient
2
2
Pd sec
d sca
αAWTU
field size (cm2)
at scattering surface
secondary
barrier
dsec
– Compton scattering
primary
barrier
isocenter
dsca
linac rotation plane
primary
barrier
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Scatter - Barrier
Parameters
• T Occupancy factor
– occupational = 1
– uncontrolled
» 1.0 = full occupancy, offices, shops, labs
» ¼ = partial occupancy, corridors, rest rooms
» 1/16 = occasional occupancy, waiting rooms,
toilets, stairways, elevators
• α = reflection coefficient of scattering barrier
– barrier material
– beam energy
– scattering angle
• A = field size at scattering barrier (cm2)
• Scattered beam has different energy than
primary
– Compton scattering
B=
• U Use factor (primary)
– 0.25 walls, ceiling
– 1.0 floor
• W Workload (photon treatments only)
2
2
Pd sec
d sca
αAWTU
– LowLow-energy 350 Gy week-1
– HighHigh-energy 250 Gy week-1
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Parameters
Parameters
• a (scatter fractions)
• P (occupational)
– Scattering angle and beam energy
– Taylor and Rodgers (1999)
– Maximum permissible dose rate
– 0.05 Sv per year
– ALARA = 0.1 mSv per year
• P (uncontrolled)
– 1.0 mSv per year
– 0.02 mSv in any one hour (cannot irradiate annual
limit too quickly!)
– This is satisfied if either:
» Dose to an occupyable space (T=1) at max
exposure <0.5 mSv in one year and 0.02 mSv
in one hour (T = 1)
» Dose to most irradiated person limited to 1.0
mSv in one year (implies occupancy factor)
– Other statestate-defined limitations!
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Parameters
6 MV
10 MV
18 MV
24 MV
10
1.04 x10-2
1.66 x10-2
1.42 x10-2
1.78 x10-2
20
6.73 x10-3
5.79 x10-3
5.39 x10-3
6.32 x10-3
30
2.77 x10-3
3.18 x10-3
2.53 x10-3
2.74 x10-3
45
1.39 x10-3
1.35 x10-3
8.64 x10-4
8.30 x10-4
60
8.24 x10-4
7.46 x10-4
4.24 x10-4
3.86 x10-4
90
4.26 x10-4
3.81 x10-4
1.89 x10-4
1.74 x10-4
135
3.00 x10-4
3.02 x10-4
1.24 x10-4
1.20 x10-4
150
2.87 x10-4
2.74 x10-4
1.20 x10-4
1.13 x10-4
Scattering fraction
decreases with
increasing beam
energy
Scattering fraction decreases
with increasing scatter angle
Table II: Scatter fractions (a) at 1 m from a human-sized phantom, target-to-phantom distance of 1 m and
field size of 400 cm2. Data from McGinley(McGinley 2002) and Taylor and Rogers(Taylor and Rodgers
Washington
1999).
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Barrier Transmission to Thickness
• NCRP 49 and 51
• Can use TVLs from NCRP 51
• Scattered mean beam energy (MeV
(MeV))
Beam
E
(MV)
Angle (deg)
Scatter Angle (deg)
0
10
20
30
40
50
70
90
6
1.6
1.4
1.2
0.9
0.7
0.5
0.4
0.2
10
2.7
2.0
1.3
1.0
0.7
0.5
0.4
0.2
18
5.0
3.2
2.1
1.3
0.9
0.6
0.4
0.3
24
5.6
3.9
2.7
1.7
1.1
0.8
0.5
0.3
Scattered beam
energy decreases
with increasing
scattering angle
Incident
Electron
Energy
(MeV)
MeV)
6
18
24
Table III: Mean energy of patient-scattered radiation as a function of scattering angle and
megavoltage beam energy. Data from McGinley(McGinley 2002) and Taylor and Rogers(Taylor
and Rodgers 1999).
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Material
TVL1 (m)
first tvl
TVLe (m)
subsequent
tvls
concrete
steel
lead
concrete
steel
lead
concrete
steel
lead
0.35
0.099
0.055
0.47
0.108
0.51
0.109
-
0.035
0.099
0.057
0.43
0.108
0.46
0.109
-
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Example Calculations
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S
d
IMRT Modifications
Primary shield, occupational (T=1), ordinary concrete
20 MV, 4.4 m away, lateral barrier (U=1/4)
W = 500 Gy week-1 at isocenter
P = 0.0001 Sv week-1 (ALARA)
B = 1.55 x 10-5
Three ways to calculate (S = shield thickness):
– NCRP 51 curve E.8 S = 2.21 m
– NCRP 51 TVL table
» T1 = 0.48m, Te = 0.44 m
» S = 2.16m
– Varian has TVL data (0.457 m)
» S = 2.20m
B=
Pd 2
WUT
• All methods have very similar results!
– Total TD
– Total MUs
W = TD = Ec • MU ≅ MU
E ≡ TD MU
• Ec = conventional radiation therapy delivery efficiency
factor, assumed to be equal to 1
• The workload value is the same for primary and
secondary barrier calculations
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IMRT
Room Shielding Formalism
IMRT
Room Shielding Formalism
• Depending on the complexity of the target volume
and the intensity modulation device
Ec >> E IMRT MU c << MU IMRT
Conventional
efficiency
• Workload is either
IMRT
efficiency
• Because EIMRT<< 1
– decouple workloads for primary, leakage, and scatter
shielding calculations
• Occupancy factor (T) remains unchanged
• Weekly design dose rate (P) remains unchanged
• Primary, leakage, and scatter components still
separated
• For workload estimates, the
t total delivered weekly
monitor units and the total delivered weekly dose are
decoupled
• Look at
– MLC IMRT
– sequential tomotherapy delivery
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Primary Barrier FormalismFormalism-MLC
• W = TD
• U approximately same as conventional
Primary Barrier Formalism
-Sequential Tomotherapy
• W = TD
• Average number of indexes (I) used to treat patients
• U has well defined, broad distribution in angle
– Define as product of
– fraction of gantry angle (κ) that the beam is incident on the
primary barrier
– λ is the primary barrier leakage correction factor
κ = θ i θ t WtUt =TD Iκλ
• The product Iκλ may be very close to conventional
therapy use factor
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Secondary Barrier Formalism - Leakage
• W = MUs
• Workload increases by >2 relative to
conventional
• There may be an increase in secondary
barrier thickness due to the increased
leakage component
Secondary Barrier Formalism - Scatter
• W = TD
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Experimental Verification
Experimental Verification
• DMLC delivery 14 cm wide, 40 cm long portal was used,
with 2, 4, 6, 8, 10, 12, and 14 cm wide sliding windows
• For tomotherapy a 3.7 x 20 cm2 open field (at isocenter)
was used
• 300 cm3, 6 atmosphere air-filled pressurized ionization
chamber located at 30 cm beyond the primary barrier
Center of the Field
Center of the Field
Center of the Field
Center of the Field
Dose Fraction = 0.000
Dose Fraction = 0.222
Dose Fraction = 0.778
Dose Fraction = 1.000
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Varian Clinac 2300C/D
Measurement Point
30cm from the wall
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Fraction Dose of Open Field
4 cm Window Example
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Results - DMLC
0.7
Window
Width
14x40 cm2
2
4
6
8
10
12
14
4, 10x10 cm2
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0
0.2
0.4
0.6
Fraction MUs
0.8
1.0
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EIMRT
Beyond
Shield
0.194
0.264
0.343
0.394
0.448
0.492
0.521
0.326
E = efficiency
EIMRT
Isocenter
Ratio
±0.04
0.169
0.259
0.332
0.393
0.444
0.486
0.524
0.323
1.15
1.02
1.03
1.00
1.01
1.01
0.99
1.01
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Results - Tomotherapy
Lamda
1.2
1.7
1.0
1.6
1.5
0.8
1.4
0.6
λ
Relative Dose Rate
Results - Tomotherapy
1.3
0.4
1.2
0.2
1.1
0.0
-15
-10
-5
0
5
10
1.0
15
Incidence Gantry Angle (deg)
Normalized Dose Rate
Conservative
estimate!
50
100 150 200 250 300 350 400
Arc Length (deg)
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Efficiencies (1/E)
Normalized Output Factors Inside and Outside the Room
Isocenter and Inside Room
1.2
0
Conventional
Beam
Energy
1.0
Beam Intensity Modulated
unwedged
(MU/cGy)
wedged
(MU/cGy)
MLC
(MU/cGy)
6 MV
1.2
2.4
3.4
Serial
Tomothera
py
(MU/cGy)
9.7
18 MV
1.0
1.5
2.8
8.1
25 MV
1.0
1.5
2.8
8.1
0.8
Outside Room
0.6
Outside Room
Inside Room
Isocenter
0.4
0.2
0.0
0
5
10
15
20
25
30
35
Ratio of MUs to tumor dose (1/E) for typical conventional and IMRT
deliveries. Data from Followill et al.(Followill, Geis and Boyer 1997)
40
Field Size (cm)
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Results - Shielding
Neutrons
• Neutron generation is
function of MUs
• IMRT
• Data from Rodgers (2001)
• r = percent IMRT
r (%)
1/E
0
– MLC: efficiency
Additional
TVLs
0 TVL
50
4
0.40 TVL
50
10
0.74 TVL
100
4
0.60 TVL
100
10
1.0 TVL
Field Size
(cm x cm)
Gantry
angle (deg)
Neutron
Rem
(nSv/MU)
Photon Dose
(nGy/MU)
0x0
0
1.12
0.77
20x20
0
0.98
0.70
40x40
0
0.78
0.73
90
0.60
0.80
270
0.83
0.80
(7 angles)
0.85
0.54
Abdomen
IMRT
0
1.14
0.48
Breast
IMRT
0
1.02
Neutron equivalent dose and photon dose 40x40
outside the treatment room door for some
tested beam configurations. Data courtesy of 40x40
Boyer (private communication).
Abdomen
IMRT
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0.58
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Conclusions
Conclusions
• MLC
– Significant Field Size Factor (0.35 - 0.80 relative to
40x40 cm2)
– DMLC
» 40x40 cm2 overestimates IMRT conformal therapy
shielding requirements
» Using W=TD - accurate within 10% to 2 cm window
– Primary Barriers - Comparable to conventional shielding
– Secondary Barriers - Possible increase due to leakage
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• Sequential Tomotherapy
» MU component (λ
(λ) from 15% to 60% compared to geometric
estimate TD * κ of single index workload.
» 6 indexes, I κ λ ranges from 0.52 for 180°
180° to 0.30 for 360 °
» Very similar to Use factors of conventional!
– Primary Barrier
» Comparable to conventional shielding due to reduced
number of treated patients and I κ λ product
–
Secondary Barriers
» Increased due to leakage
» MIR experience ≅ 3,000 MU/fraction
» 300,000 MU/week for 20 patients
» Similar to high energy W
WtUt = TDIκλ
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