SU-E-T-168: Characterization of Neutrons From the TrueBeam Treatment Head




Calculate neutron production and transport in the TrueBeam treatment head, as input for vault design and phantom dose calculations.


A detailed model of the treatment head, including shielding components off the beam axis, was created from manufacturer's engineering drawings. Simulations were done with Geant4 for the 18X, 15X, 10X and 10FFF beams, tuned to match measured dose distributions inside the treatment field. Particles were recorded on a 70 cm radius sphere surrounding the treatment head enabling input into simulations of vaults.


For the 18X beam, 11×10^9 neutrons/MU were observed. The energy spectrum was a broad peak with average energy 0.37 MeV. With jaws closed, 48% of the neutrons were generated in the primary collimator, 18% in the jaws, 12% in the target, and 10% in the flattening filter. With wide open jaws, few neutrons were produced in the jaws and consequently total neutron production dropped to 8.5×10^9 neutrons/MU. Angular distributions were greatest along the beam axis (12×10^9 neutrons/MU/sr, within 2 deg of the beam axis) and antiparallel to the beam axis (7×10^9 neutrons/MU/sr). Peaks were observed in the neutron energy spectrum, corresponding to elastic scattering resonances in the shielding materials. Neutron production was lower for the other beams studied: 4.1×10^9 neutrons/MU for 15X, 0.38×10^9 neutrons/MU for 10X, and 0.22×10^9 neutrons/MU for 10FFF. Despite dissimilar treatment head geometries and materials, the neutron production and energy spectrum were similar to those reported for Clinac accelerators.


Detailed neutron production and leakage calculations for the TrueBeam treatment head were done. Unlike other studies, results are independent of the surrounding vault, enabling vault design calculations.