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

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

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

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