Abstract
Purpose: Recent studies demonstrate the feasibility of prompt‐gamma imaging for pencil beam proton therapy. In contrast to passive‐scatter proton radiotherapy, the existence of high background signal from secondary radiation produced by protons in the beam line components makes the detection of prompt‐gammas challenging. Our primary goal is to produce a more complete measurement of the prompt‐gammas produced in tissue for clinical treatment conditions. Methods: The proton treatment facility at iThemba Labs was studied in detail and all the nozzle components that interact with the proton beam were built and positioned at the locations specified by the manufacturer using the Geant4 Monte Carlo toolkit. The treatment nozzle was validated against depth dose and lateral profiles in a water phantom at therapeutic energies. The model included NaI detectors of different dimensions to detect the prompt‐gamma spectra, and standard gamma emitting sources in the energy range 0.661 to 4.438 MeV were used to determine the Gaussian broadening parameters of the detectors. Finally, prompt‐gamma spectra from the tissue phantom were simulated over a range of energies in order to make comparisons with the experimental spectra currently being measured. Results: The prompt‐gamma energy spectra show prominent peaks caused by collisions of proton with nuclei in the tissue phantom. Single and double escape peaks of 4.44 MeV were clearly seen, while the 6.129 MeV peak was unresolved due to the Doppler broadening effect, peaks at 6.916 and 7.115 MeV were not observed, but the single and double escape peaks of 6.129 MeV were visible. Conclusion: Our study confirms the feasibility of prompt‐gamma imaging during passive proton radiotherapy to assist for on‐line treatment verification. The energy range from 3.0 MeV to 5.0 MeV is a suitable candidate for further investigation and the presence of background radiation can be minimized by introducing proper shielding and collimation.
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