Room: Karl Dean Ballroom B1
Purpose: To quantitatively evaluate and compare proton, helium- and carbon-ion computed tomography (CT) for ion therapy treatment planning, in order to mitigate range uncertainty inherent in the conversion of Hounsfield units into relative stopping power (RSP).
Methods: Different ion-CTs were simulated at 2mGy physical dose for an ideal single-particle tracking detector with experimentally validated beam description for two clinical head&neck cases using the FLUKA Monte-Carlo code. RSP images were reconstructed by an iterative algorithm with integrated most-likely-path and total variation superiorization. Proton-beam treatment plans were optimized on the ground truth and recalculated on the ion-CTs. Variations of relative biological effectiveness (RBE) with ion type and energy for the simulated ion-CT scenarios were quantified by coupling the FLUKA-code to the repair-misrepair-fixation model and Monte-Carlo damage simulation algorithm, accounting for cell survival and DNA complex damage.
Results: Helium-CT offered superior image quality in terms of overall reduced RSP error, while carbon-CT showed the highest accuracy for bone and proton-CT for soft/brain tissue. All ion-CTs displayed comparable performances for dose calculation with minor variations in dose-volume histograms. For a 0.5%/0.5mm gamma-evaluation, carbon-CT exhibited 91% passing-rate, increasing to 98% for proton- and helium-CT. Using single field uniform dose, the average range variation was 0.34mm, 0.32mm and 0.54mm underestimation for proton-, helium- and carbon-CT, respectively. In more heterogeneous regions, proton-CT dose calculation resulted in over-range up to 0.8mm. Depending on the radiosensitivity parameters, the predicted mean RBE for cell survival was 0.82-0.85, 0.85-0.89 and 0.97-1.03 for proton-, helium- and carbon-CT, respectively, while only 0.82, 0.84 and 0.95 for double-strand break induction (using a diagnostic 130kVp X-ray spectrum as reference).
Conclusion: Low-dose ion-CT offers sub-millimeter range accuracy for ion therapy treatment planning and possibly reduced radiobiological implications compared to X-ray imaging. The comparison indicates that helium-CT offers the best trade-off between path estimation accuracy and statistics.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the DFG Cluster of Excellence Munich Centre for Advanced Photonics (MAP) and the DFG project Hybride-Bildgebung in Hadrontherapy fuer Adaptive Ionen-Strahlentherapie.