Assessing quantitative material characteristics with low dose imaging in photon-counting and dual-energy computed tomography for radiotherapy

<i>Objective.</i>Photon-counting computed tomography (CT) is an advanced imaging technique that detects individual photons by energy, offering spectral imaging. While its application in radiotherapy remains under-explored, photon-counting CT (PCCT) could provide quantitative accuracy, particularly in proton stopping-power ratio (SPR) estimation, while it also potentially enables consistent low-dose imaging. This study evaluates estimation accuracy for relative electron density (RED), effective atomic number (<i>Z</i>_eff), and SPR under low-dose conditions. This could be beneficial for repeated imaging in radiotherapy.<i>Approach.</i>A Gammex Advanced Electron Density phantom (Sun Nuclear) was scanned on a PCCT at 120 kVp with varying imaging doses (5 mGy (low dose), 20 mGy, and 40 mGy). Virtual monoenergetic images (VMIs) were generated with energies of 40-70 and 150-190 keV (10 keV increments). Performance was compared to dual-energy CT (DECT) acquisition (80/140 kVp) using the SOMATOM Confidence scanner with similar dose levels, and identically VMIs were generated as for PCCT. Mean ± standard deviation (SD) of CT numbers was quantified for tissue-equivalent phantom inserts. Furthermore, the physical quantities, RED,<i>Z</i>_eff, and SPR accuracies were determined under high- and low-dose imaging conditions by quantifying the root-mean-squared-error (RMSE).<i>Main results.</i>PCCT generally showed lower CT number SD than DECT, while for both modalities, the SD increased at lower imaging dose. In addition, PCCT retained consistent mean CT numbers (differences up to 15 Hounsfield Unit (HU)), while DECT showed large CT number variations at 5 mGy (up to 1000 HU) in bone compared to higher dose. The accuracy of the physical quantities was independent of the dose for PCCT (0.7%-0.9% SPR RMSE), whereas DECT demonstrated larger differences with dose (1.1%-4.6% SPR RMSE).<i>Significance.</i>This study demonstrated that PCCT retains quantitative accuracy in low imaging dose conditions, outperforming DECT. This could be valuable for pediatric patients and in repeated imaging to reduce the cumulative dose.