Time-of-flight based neutron background reduction in secondary-electron-bremsstrahlung imaging for in-vivo range verification of proton therapy: a Monte Carlo study

In proton therapy, range verification is important to monitor the location of the Bragg peak within the patient's body. Secondary-electron-bremsstrahlung (SEB) imaging is a promising method of proton range verification. Unfortunately, the SEB images measured by the dedicated gamma camera contain neutron-induced backgrounds as well as the SEB counts. To improve the accuracy of range verification in SEB imaging, we propose to apply a time-of-flight (TOF) method to reduce neutron background in measured images of SEB. The SEB and neutron generation and transport were calculated by a Monte Carlo simulation. Proton pencil beams were irradiated to a water phantom and then the time spectra and profiles of SEB and neutron were obtained by scoring on a detector. The total count of neutrons was approximately 10 times higher than that of SEB and increased with higher proton energy. The TOF method, selecting a time window for the TOF spectrum of SEB, reduced the neutron background by more than 97%. We concluded that the TOF methods can dramatically reduce neutron background and improve the accuracy of proton range verification in SEB imaging.