Comparison of two dual-energy CT-based methods for proton stopping-power ratio estimation in brain cancer patients with metal implants.

Background and purpose: Accurate stopping-power ratio (SPR) estimation is crucial for proton therapy planning. In brain cancer patients with metal clips, SPR accuracy may be affected by high-density materials and imaging artefacts. Dual-energy CT (DECT)-based methods have been shown to improve SPR accuracy. This study evaluated the consistency between two SPR estimation methods in brain cancer patients: (1) a Hounsfield look-up table (HLUT) for DECT-generated virtual monoenergetic images (VMIs) and (2) the DirectSPR algorithm (Siemens Healthineers). Patient/material and methods: DECT scans were acquired for 11 brain cancer patients. Two SPR maps were generated: one using a 90 keV VMI with a HLUT and the other via the DirectSPR algorithm. The VMI HLUT was adjusted in high-density regions to align with the SPR of titanium. Clinically applied proton therapy plans were recalculated on both SPR maps and dose distributions were compared using dose-volume histograms. Furthermore, a voxel-wise SPR comparison and a separate titanium implant analysis were performed.

Results: Dose differences between the SPR methods were minimal for organs-at-risk. DirectSPR showed strong SPR agreement with the VMI HLUT approach for CT numbers up to 1500 HU (SPR~1.9). Beyond this, especially in regions with titanium implants, DirectSPR yielded higher SPR values than the VMI HLUT, suggesting an adjustment may also be needed for DirectSPR.

Interpretation: DirectSPR was consistent with the VMI HLUT up to 1500 HU but deviated at higher CT numbers. These deviations had limited impact on dose metrics, but they should be considered when choosing beam orientations.