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 Location Gantry Angle γ-ray DE rate Neutron DE rate Total γ + neutron DE Rate Ludlum Ion Chamber Ludlum PrescilaMeas.Calc

Results V/V Contributions to DE Dose Equivalent (µSv) Location Contributing Source Patients Treated Radiat- ion QualityDeepEyeShallow 5Room 5615Photon Room 5615Photon RRoom 5615Photon ARoom 1453Photon BRoom 1453Photon CRoom 5615Photon Cyclotron Vault2316Photon20

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