A comparative study of two proton CT systems in terms of spatial resolution and RSP accuracy

Speaker: George Dedes
Institution: Ludwig-Maximilians-Universität München, Germany

Abstract

Purpose: Proton computed tomography (pCT) can provide sub 1% RSP accuracy, which may allow reducing range margins in proton therapy. Several scanner prototypes have been built or are in development phase. Different design approaches impact spatial resolution and RSP accuracy. This study aims at conducting the first direct comparison of two pCT prototype scanners installed at the same facility, by using the same image reconstruction algorithm and the same scanned object.
Methods: The phase-II pCT prototype of the U.S. pCT collaboration and the commercially oriented ProtonVDA scanner, both hosted at the Northwestern Medicine Chicago Proton Center, were used to scan a phantom containing cylindrical plastic inserts of known RSP. Images were reconstructed with a filtered backprojection algorithm accounting for curved proton paths. The RSP accuracy was expressed as the mean absolute percent error (MAPE) with respect to the reference RSP over all inserts. The spatial resolution was estimated from the radial edge spread function of high-density inserts.
Results: Both scanners achieved the same RSP MAPE (0.72%) when excluding the porous sinus insert. The overall MAPE of the ProtonVDA scanner was better (0.81% vs. 1.14%) when all inserts and the body of the phantom we accounted for. The phase-II scanner achieved a higher spatial resolution (0.61 lp/mm) than the ProtonVDA scanner (0.46 lp/mm), attributed to the lack of exit direction tracking.
Conclusion: The comparison between two prototype pCT scanners at the same clinical facility and with the same image reconstruction algorithm, showed that they both fulfill the requirement of RSP accuracy of about 1%. Their different design approaches were reflected in the spatial resolution, resulting in higher spatial resolution for a scanner with full tracking capabilities (phase-II) compared to a more compact tracker system which only provides the positions of protons but not their directions (ProtonVDA).

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