Shielding Measurements For A
Proton Therapy Facility
S. Avery, K. P. Risolo, M. Bartels, C.
Ainsley, J McDonough, R. L. Maughan
University of Pennsylvania
Introduction I/II
• Penn’s Roberts Proton Therapy Center
– 5 Treatment rooms
– Single Scattering, Double Scattering, Uniform
Scanning, Modulated Scanning
• Double scatting beam line components
– First and second scatters
– Modulator wheel
– Primary jaws
– Collimator (MLCs)
– Compensator
Introduction II/II
• Shielding calculations performed using analytical
methods including neutron spectra
• Calculations performed for IBA double scattering
mode
– Max beam current
– At any particular energy
– Usage factors
– Beam direction
– Occupancy factors
– Workload
Methods and Material I/V
Methods and Materials II/V
Methods and Materials III/V
•
Both photon sensitive and neutron sensitive
detectors required
– Total instantaneous dose equivalent
obtained from their sum
• Neutron detection
– Prescila detector for up to 100MeV
• Photon detection
– Ludum ion chamber
Methods and Materials IV/V
• Integrated dose equivalent readings
– Neutrak Dosimeters inside a Luxel plastic
blister pack
• Two types of detectors used:
–Those sensitive to 40 keV to 40 MeV
with a dose range of 0.2 mSv to 250
mSv
–Those sensitive to fast, intermediate,
and thermal neutrons with a dose range
of 0.1 mSv to 250 mSv
Methods and Materials V/V
• Detectors placed at various locations around
facility
– Primarily around treatment rooms 1 and 5
– Fast neutron detectors left in for one month
– Fast, intermediate, and thermal neutrons left
in place for an additional month
– Control detectors for each type
Results I/V
• A modified form of NCRP Report No. 51
equation used to calculate instantaneous dose
equivalent index rate
• Φp derived from Monte Carlo using GEANT4
• Np is proton beam intensity at the neutron
source, related to beam current
• Measurements normalized to a beam current of
6 nA emerging from the nozzle
Results II/V
• Points outside the cyclotron and ESS showed
shielding to be more than adequate with only
one reading above background
– Reading 4-5 times below the calculated value
• Points outside TR1 showed 2 readings below
background, with 6 other readings found below
their calculated values
• Measurements with the dose badges showed
only photon irrdatiation
Results III/V
• Neutron dose equivalent transmission data from
NCRP Report 144 used with the analytic method
• Measurements made under identical gantry
angle and proton energy conditions for both
neutron and gamma ray dose
• The instantaneous and integrated two month
dose equivalent data allows for calculate of
annual dose equivalents
Results IV/V
γ-ray DE
rate
Gantry Ludlum Ion
Location Angle
Chamber
Neutron DE Total γ + neutron
rate
DE Rate
Ludlum
Prescila
Meas.
Calc.
1
270
0.23
1.98
2.22
16.14
9
90
0.7
12.72
13.42
68.7
16
270
0.23
1.05
1.28
12.35
21
270
1.63
9.22
10.86
16.06
27
180
0
1.05
1.05
9.2
Results V/V
Contributions to DE
Dose Equivalent (µSv)
RadiatContributing Patients ion
Location Source
Treated Quality Deep
5
Room 5
615
Photon 50
20
Room 5
615
Photon 150
Eye
50
150
Shallow
30
120
25R
26A
26B
26C
60
130
70
10
100
140
50
20
20
20
34
Room 5
Room 1
Room 1
Room 5
Cyclotron
Vault
615
453
453
615
Photon
Photon
Photon
Photon
50
130
70
10
2316
Photon 20
Conclusions
• Shielding in place at Roberts Proton Center is
more than adequate
– Conservative shielding calculations
• Measured vs. Analytic
• Film dosimeters questionable
– Only saw photon component
Concluding Remarks
• Thanks to the authors Dr. Avery, Dr. Maughan,
Dr. McDonough, Dr. Ainsley, and Mr. Bartels
• Thanks to the HPS