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.