Patient CT-based simulation study of secondary-electron-bremsstrahlung imaging for range verification in proton therapy: comparison with prompt gamma and PET imaging for simplified proton pencil beam and SOBP irradiation scenarios

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Abstract Objective. Secondary electron bremsstrahlung (SEB) imaging, along with prompt gamma (PG) and positron emission tomography (PET) imaging, has been proposed as an in vivo range verification tool for proton therapy. This study presents the first simulation based on patient computed tomography (CT) data to investigate the feasibility of SEB imaging for range verification in proton therapy, while comparing the characteristics of SEB imaging with those of PG and PET imaging. Approach. A Monte Carlo simulation was performed using patient CT data for the irradiation of monoenergetic pencil beams and spread-out Bragg peak proton beams. The physical characteristics of SEB imaging were analyzed at three different anatomical sites and compared with those of PG and PET imaging. Main results. In all the treatment cases, SEB imaging exhibited higher production rates than PG and PET imaging, particularly in the regions with high CT values along the beam path. Although the SEB signal was more affected by scattering and absorption than the PET or PG signals, sufficient statistical counts for range verification (∼3 × 10–3 SEBs/proton) could potentially be detected outside the patient geometry. For pencil beam cases, the SEB and PET fall-offs were located 4–5 mm proximal to the dose fall-off, while the PG fall-off was located 0–1 mm distal to it. Significance. Results suggest that SEB imaging has the potential to offer a real-time range verification tool (by comparing measured and expected images), particularly for treating shallow-seated tumors using proton pencil-beam scanning delivery. Thus, this study represents a significant step towards the clinical application of range verification based on SEB imaging and promotes future efforts in this direction.

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