Impact of beam hardening on proton stopping-power ratio estimation using photon-counting, dual-energy, and single-energy computed tomography.

Background and purpose: Accurate stopping-power ratio (SPR) estimation is essential for treatment planning of proton therapy. However, SPR accuracy is influenced by object size and position due to beam hardening. This study compared proton SPR estimation using photon-counting computed tomography (PCCT), dual-energy CT (DECT), and single-energy CT (SECT) across varying phantom sizes and insert positions. Materials and methods: Two phantoms with tissue-equivalent inserts were scanned using PCCT, DECT, and SECT scanners. Based on the PCCT and DECT scans, virtual monoenergetic images (VMIs) were created. Two SPR estimation methods were used: a DECT-based method and a SECT-based Hounsfield look-up table (HLUT). SPRs were calibrated on the Gammex Advanced Electron Density phantom and evaluated on a cylindrical phantom with four diameters. To assess size-dependency of the estimated SPR, tissue-equivalent bone inserts were placed centrally in the evaluation phantom, for each of the four diameters. To assess position-dependent SPR uncertainty, the inserts were placed at different distances from the centre of the evaluation phantom with fixed diameters. Results: For smaller phantom diameters, SPR estimation accuracy was comparable across PCCT, DECT, and SECT. For larger phantom diameters (30 and 40 cm), SPRs estimated based on SECT and DECT showed position-dependency. PCCT demonstrated the most stable SPR estimations across positions, with deviations between 0.9% and 2.1%. SPR estimations were more stable using a HLUT than the DECT-based SPR method. Conclusions: SPR estimated based on PCCT-VMI with a HLUT showed reduced impact of beam hardening effects related to size and position.