Clinical Implementation of Dual-energy CT for Proton Treatment Planning on Pseudo-monoenergetic CT scans

Purpose: To determine whether a standardized clinical application of dual-energy computed tomography (DECT) for proton treatment planning based on pseudomonoenergetic CT scans (MonoCTs) is feasible and increases the precision of proton therapy in comparison with single-energy CT (SECT). Methods and Materials: To define an optimized DECT protocol, CT scan settings were analyzed experimentally concerning beam hardening, image quality, and influence on the heuristic conversion of CT numbers into stopping-power ratios (SPRs) and were compared with SECT scans with identical CT dose. Differences in range prediction and dose distribution between SECT and MonoCT were quantified for phantoms and a patient. Results: Dose distributions planned on SECT and MonoCT datasets revealed mean range deviations of 0.3 mm, gamma passing rates (1%, 1 mm) greater than 99.9%, and no clinically relevant changes in dose-volume histograms. However, image noise and CT-related uncertainties could be reduced by MonoCT compared with SECT, which resulted in a slightly decreased dependence of SPR prediction on beam hardening. Consequently, DECT was clinically implemented at the University Proton Therapy Dresden in 2015. Until October 2016, 150 patients were treated based on MonoCTs, and more than 950 DECT scans of 351 patients were acquired during radiation therapy. Conclusions: A standardized clinical use of MonoCT for treatment planning is feasible, leads to improved image quality and SPR prediction, extends diagnostic variety, and enables a stepwise clinical implementation of DECT toward a physics-based, patient-specific, nonheuristic SPR determination. Further reductions of CT-related uncertainties, as expected from such SPR approaches, can be evaluated on the resulting DECT patient database. (C) 2016 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).