A comparison of relative stopping power derived by fast kV-switching dual-energy CT, single-energy CT, and proton CT in phantom and tissue.

BACKGROUND

The accuracy of relative stopping power (RSP) is a critical aspect for safe treatment planning in proton therapy. Dual-energy CT (DECT) has the ability to estimate RSP more accurately than conventional single-energy CT (SECT) due to its improved material separation. RSP accuracy is commonly determined in homogeneous phantom material as it can be more challenging to determine in heterogeneous biological tissues. Proton CT (pCT) is a modality that directly determines RSP and can be used as a comparative benchmark for evaluating the RSP accuracy of DECT and SECT derived values across homogeneous and heterogeneous material.

PURPOSE: In this study, DECT was compared to SECT in both homogeneous phantom inserts of known RSP and heterogeneous tissue samples of unknown RSP using pCT as a quantitative reference. The purpose of this study was to assess the ability of DECT to improve RSP estimation over SECT in unknown RSP of heterogeneous tissues.

METHODS

A variety of homogeneous phantom inserts were scanned using pCT, DECT, and SECT. The estimated RSP from each imaging modality was compared to the RSP of each insert directly measured using a multilayer ionization chamber (MLIC). Scans of heterogeneous tissue samples were also acquired using pCT, DECT, and SECT. The images from each modality were registered and a variety of tissue regions of interest were contoured. The percent difference of DECT and SECT derived RSP values compared to pCT was calculated across both homogeneous phantom and heterogeneous tissue regions and compared.

RESULTS

The mean absolute percentage error for the inserts compared to the known RSP was 1.6% ± 1.6% in pCT, 1.1% ± 1.6% in DECT, and 4.6% ± 6.2% in SECT. For inserts with relative electron density greater than 0.93 and excluding true water and aluminum, DECT average percent difference compared to pCT was -0.5% ± 0.02% while in SECT it was -1.5% ± 0.07%. In biological tissue, excluding enamel, the average percent difference for DECT compared to pCT was -0.3% ± 1.1%, while for SECT the percent difference compared to pCT was 0.6% ± 3.0%.

CONCLUSIONS

Fast kV-switching DECT obtained smaller RSP deviations from pCT compared to SECT in homogeneous tissue-equivalent phantom inserts. In heterogeneous tissue samples, DECT maintained improved RSP accuracy compared to SECT.