Abstract
Ion computed tomography (iCT) represents a potential replacement for x-ray CT (xCT) in ion therapy treatment planning to reduce range uncertainties, inherent in the semi-empirical conversion of xCT information into relative stopping power (RSP). In this work, we aim to quantify the increase in dosimetric accuracy associated with using proton-, helium- and carbon-CT compared to conventional xCT for clinical scenarios in proton therapy. Three cases imaged with active beam-delivery using an ideal single-particle-tracking detector were investigated using FLUKA Monte-Carlo (MC) simulations. The RSP accuracy of the iCTs was evaluated against the ground truth at similar physical dose. Next, the resulting dosimetric accuracy was investigated by using the RSP images as a patient model in proton therapy treatment planning, in comparison to common uncertainties associated with xCT. Finally, changes in relative biological effectiveness (RBE) with iCT particle type/spectrum were investigated by incorporating the repair-misrepair-fixation (RMF) model into FLUKA, to enable first insights on the associated biological imaging dose. Helium-CT provided the lowest overall RSP error, whereas carbon-CT offered the highest accuracy for bone and proton-CT for soft tissue. For a single field, the average relative proton beam-range variation was −1.00%, +0.09%, −0.08% and −0.35% for xCT, proton-, helium- and carbon-CT, respectively. Using a 0.5%/0.5mm gamma-evaluation, all iCTs offered comparable accuracy with a better than 99% passing rate, compared to 83% for xCT. The RMF model predictions for RBE for cell death relative to a diagnostic xCT spectrum were 0.82–0.85, 0.85–0.89 and 0.97–1.03 for proton-, helium-, and carbon-CT, respectively. The corresponding RBE for DNA double-strand break induction was generally below one. iCT offers great clinical potential for proton therapy treatment planning by providing superior dose calculation accuracy as well as lower physical and potentially biological dose exposure compared to xCT. For the investigated dose level and ideal detector, proton-CT and helium-CT yielded the best performance.
Highlights
Ion computed tomography represents a promising solution for direct assessment of the relative ion stopping power (RSP)
The resulting dosimetric accuracy was investigated by using the relative stopping power (RSP) images as a patient model in proton therapy treatment planning, in comparison to common uncertainties associated with x-ray CT (xCT)
The corresponding relative biological effectiveness (RBE) for DNA double-strand break induction was generally below one. Ion computed tomography (iCT) offers great clinical potential for proton therapy treatment planning by providing superior dose calculation accuracy as well as lower physical and potentially biological dose exposure compared to xCT
Summary
Ion computed tomography (iCT) represents a promising solution for direct assessment of the relative (to water) ion stopping power (RSP). In order to potentially replace x-ray imaging for proton therapy treatment planning, iCT must provide competitive image quality and dose calculation accuracy without enhancing the imaging dose exposure. While dual-energy CT demonstrates already clinically promising results (Wohlfahrt et al 2017) in terms of reduced range uncertainties, iCT is the intuitive solution providing better RSP accuracy (Hansen et al 2015) with the possibility of imaging directly at the beam isocenter. Heavier ions can yield improved image quality by benefiting from reduced multiple Coulomb scattering (MCS) (Hansen et al 2014, Volz et al 2017) They exhibit an increased physical dose per particle and suffer from an elevated loss of primaries due to fragmentation, affecting the noise of iCT (Gehrke et al 2018). The different scenarios were compared to the established practice using xCT both in terms of RSP uncertainties and imaging dose
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