Helium Range Viability for Online Range Probing in Mixed Carbon-Helium Beams

TL;DR

This work tackles online range verification in carbon-ion therapy by leveraging mixed carbon-helium beams, whose helium component offers a longer range suitable for distal exit yet requires careful tracking of residual helium range. The authors extend the matRad planning toolkit to optimize mixed-beam plans, develop strategies (EW He, Const RaShi, EW RaShi) to maintain detectable helium signals within a detector's sensitive window, and validate these approaches across lung, prostate, and liver cases using pencil-beam calculations and TOPAS-based radiography simulations. They find that residual helium range is frequently limiting and detector sensitivity is crucial; range shifters increase detectable spots but introduce beam broadening and dose, with EW RaShi delivering the strongest gains overall. The results suggest promising potential for online motion management and beams-eye-view verification using mixed carbon-helium beams, albeit with a need for experimental validation and detector optimization to translate these planning insights into clinical practice.

Abstract

Background: Recently, mixed carbon-helium beams were proposed for range verification in carbon ion therapy: Helium, with three times the range of carbon, serves as an on-line range probe, and is mixed into a therapeutic carbon beam. Purpose: Treatment monitoring is of special interest for lung cancer therapy, however the helium range might not always be sufficient to exit the patient distally. Therefore mixed beam use cases of several patient sites are considered. Methods: An extension to the open-source planning toolkit, matRad allows for calculation and optimization of mixed beam treatment plans. The use of the mixed beam method in 15 patients with lung cancer, as well as in a prostate and liver case, for various potential beam configurations was investigated. Planning strategies to optimize the residual helium range considering the sensitive energy range of the imaging detector were developed. A strategy involves adding helium to energies whose range is sufficient. Another one is to use range shifters to increase the helium energy and thus range. Results: In most patient cases, the residual helium range of at least one spot is too low. All investigated planning strategies can be used to ensure a high enough helium range while still keeping a low helium dose and a satisfactory total mixed carbon-helium beam dose. The use of range shifters allows for the detection of more spots. Conclusion: The mixed beam method shows promising results for online motioning. The use of range shifters ensures a high enough helium range and more detectable spots, allowing for a wider-spread application.

Figures (9)

• Figure 1: Schematic set up of a mixed carbon-helium irradiation, in this example the helium energy is not high enough for the helium ions to exit the patient distally.
• Figure 2: Left: Example lung patient from the lung data set, outlined is the target volume () and the gantry angles for which the residual helium range was investigated. Middle: Prostate case, Right: Liver case
• Figure 3: Left: Box plot summarizing the minimum residual helium range of each lung patient for the different gantry angles. Right: Box plot summarizing the percentage of spots in each treatment plan with a residual helium range smaller than 1cm for each gantry angle.
• Figure 4: Axial CT slice for patient No.114 for different gantry angles. The overlay highlights the spots with sufficiently large residual helium range in green () and the ones with insufficient residual helium range in read ().
• Figure 5: Estimated () and fitted () Beam widening due to the use of a proximal range shifter of varying thickens for helium (left) and carbon (right).