shielding measurements for a proton therapy facility s. avery, k. p. risolo, m. bartels, c. ainsley,...
TRANSCRIPT
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Shielding Measurements For A Proton Therapy Facility
S. Avery, K. P. Risolo, M. Bartels, C. Ainsley, J McDonough, R. L. Maughan
University of Pennsylvania
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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
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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
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Methods and Material I/V
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Methods and Materials II/V
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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
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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
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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
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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
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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
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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
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Results IV/V
LocationGantry Angle
γ-ray DE rate
Neutron DE rate
Total γ + neutron DE Rate
Ludlum Ion Chamber
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
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Results V/V Contributions to DE Dose Equivalent (µSv)
LocationContributing Source
Patients Treated
Radiat-ion Quality Deep Eye Shallow
5 Room 5 615 Photon 50 50 30
20 Room 5 615 Photon 150 150 120
25R Room 5 615 Photon 50 60 100
26A Room 1 453 Photon 130 130 140
26B Room 1 453 Photon 70 70 50
26C Room 5 615 Photon 10 10 20
34Cyclotron Vault 2316 Photon 20 20 20
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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
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Concluding Remarks
• Thanks to the authors Dr. Avery, Dr. Maughan, Dr. McDonough, Dr. Ainsley, and Mr. Bartels
• Thanks to the HPS