A simulation study on proton computed tomography (CT) stopping power accuracy using dual energy CT scans as benchmark

Abstract

Background. Accurate stopping power estimation is crucial for treatment planning in proton therapy, and the uncertainties in stopping power are currently the largest contributor to the employed dose margins. Dual energy x-ray computed tomography (CT) (clinically available) and proton CT (in development) have both been proposed as methods for obtaining patient stopping power maps. The purpose of this work was to assess the accuracy of proton CT using dual energy CT scans of phantoms to establish reference accuracy levels. Material and methods. A CT calibration phantom and an abdomen cross section phantom containing inserts were scanned with dual energy and single energy CT with a state-of-the-art dual energy CT scanner. Proton CT scans were simulated using Monte Carlo methods. The simulations followed the setup used in current prototype proton CT scanners and included realistic modeling of detectors and the corresponding noise characteristics. Stopping power maps were calculated for all three scans, and compared with the ground truth stopping power from the phantoms. Results. Proton CT gave slightly better stopping power estimates than the dual energy CT method, with root mean square errors of 0.2% and 0.5% (for each phantom) compared to 0.5% and 0.9%. Single energy CT root mean square errors were 2.7% and 1.6%. Maximal errors for proton, dual energy and single energy CT were 0.51%, 1.7% and 7.4%, respectively. Conclusion. Better stopping power estimates could signifi cantly reduce the range errors in proton therapy, but requires a large improvement in current methods which may be achievable with proton CT.

Comprehensive analysis of proton range uncertainties related to stopping-power-ratio estimation using dual-energy CT imaging

“…The estimated range uncertainty (2 σ ) was approximately 2.2% for three tumor sites, as compared with the 1% uncertainty reported in previous studies (Hünemohr et al , 2014; Bourque et al , 2014; Hansen et al , 2015). The major difference between our study and others is that CT number variation due to the beam hardening effect was previously avoided by using the same scanning condition, upon calibration and real measurements.…”

“…This result indicated that the calculation algorithm can be designed more robustly against CT number variation. New algorithms have recently been proposed (Hansen et al , 2015; Han et al , 2016; Taasti et al , 2016) and will be included for comparison purposes in our future work. Note that these findings might not be directly transferrable to real human tissues, due to the study being based on tissue surrogate phantoms and theoretical human tissues.…”

“…Other groups have experimentally evaluated the DECT approach on tissue equivalent materials (Hünemohr et al , 2014; Bourque et al , 2014; Hansen et al , 2015; Han et al , 2016; Taasti et al , 2016). These studies reported that the DECT approach can predict proton SPR values within 1% error for tissue equivalent materials.…”

A simple formulation for deriving effective atomic numbers via electron density calibration from dual-energy CT data in the human body

“…Those methods that use a calibration phantom with known ρ e and Z eff are of special practical interest because they do not require prior knowledge of the CT scanner spectra, which are not always readily available. Hansen et al . presented an SPR estimation scheme for proton therapy treatment planning by combining the calibration‐based methods of Saito for ρ e calculation and of Landry et al .…”

Current state and future applications of radiological image guidance for particle therapy

“…Finally, it is worth discussing the recent interest in dual energy CT (DECT) estimation of SPR, which has been reported to be superior to conventional, single energy CT conversion . Recently in‐room CT scanners with DECT capability have become available, and several centers are now equipped with the technology .…”