Shielding Measurements For A Proton Therapy Facility

1. Shielding Measurements For A Proton Therapy Facility S. Avery, K. P. Risolo, M. Bartels, C. Ainsley, J McDonough, R. L. Maughan University of Pennsylvania

2. 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

3. 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

4. Methods and Material I/V

5. Methods and Materials II/V

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. Results IV/V

13. Results V/V

14. 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

15. Concluding Remarks • Thanks to the authors Dr. Avery, Dr. Maughan, Dr. McDonough, Dr. Ainsley, and Mr. Bartels • Thanks to the HPS