Dosimetric comparison of proton therapy plans calculated using fast kV-switching dual-energy CT versus single-energy CT.

Fast kV-switching dual-energy CT (DECT) has been demonstrated to more accurately predict relative stopping power (RSP) for proton therapy planning over the conventional method of HU-RSP conversion using single-energy CT (SECT). It is essential to evaluate the improvement in tissue characterization, reduction in range uncertainties, and accuracy in the dose calculation. In this study, treatment plans are calculated in a phantom on SECT and DECT. A dosimetric comparison to a ground truth reference model is evaluated. SECT and DECT scans were taken of a cylindrical head phantom, which contained 10 different inserts. Eight separate single-field plans were optimized to target a central planning target volume (PTV), each ranging through a different insert. A ground truth reference scan was created based on the known geometry, density, and composition of the phantom and inserts. The plans were recalculated without re-optimization onto the reference scan. The difference in dose and range is calculated from the nominal plan to the reference plan to evaluate the accuracy of RSP calculation using DECT compared to SECT. The D95 for the PTV resulted in an average percent relative error of -0.56% ± 0.19% in DECT and -0.79% ± 0.39% in SECT when calculated on the reference CT. The DECT average dose to the organs at risks 1 mm distal and proximal to the PTV was more accurate than SECT for all plans except for the plan that ranged through breast and adipose. The average difference in range when calculated on the reference image is 0.20 mm ± 0.75 mm compared to an average range change of 1.01 mm ± 1.10 mm planned with SECT. The positive value indicates over-ranging in the reference scan into distal structures. Plans calculated with DECT compared to the reference plan showed decreased range error in proximal and distal structures than the plans calculated using SECT. The average dose difference from DECT to the reference plan was less than in SECT. This suggests DECT potential as a more accurate means of calculating RSP for proton therapy treatment planning over conventional SECT.