The accuracy of helium ion CT based particle therapy range prediction: an experimental study comparing different particle and x-ray CT modalities

Abstract

This work provides a quantitative assessment of helium ion CT (HeCT) for particle therapy treatment planning. For the first time, HeCT based range prediction accuracy in a heterogeneous tissue phantom is presented and compared to single-energy x-ray CT (SECT), dual-energy x-ray CT (DECT) and proton CT (pCT). HeCT and pCT scans were acquired using the US pCT collaboration prototype particle CT scanner at the Heidelberg Ion-Beam Therapy Center. SECT and DECT scans were done with a Siemens Somatom Definition Flash and converted to RSP. A Catphan CTP404 module was used to study the RSP accuracy of HeCT. A custom phantom of 20 cm diameter containing several tissue equivalent plastic cubes was used to assess the spatial resolution of HeCT and compare it to DECT. A clinically realistic heterogeneous tissue phantom was constructed using cranial slices from a pig head placed inside a cylindrical phantom (ø150 mm). A proton beam (84.67 mm range) depth-dose measurement was acquired using a stack of GafchromicTM EBT-XD films in a central dosimetry insert in the phantom. CT scans of the phantom were acquired with each modality, and proton depth-dose estimates were simulated based on the reconstructions. The RSP accuracy of HeCT for the plastic phantom was found to be 0.3 ± 0.1%. The spatial resolution for HeCT of the cube phantom was 5.9 ± 0.4 lp cm−1 for central, and 7.6 ± 0.8 lp cm−1 for peripheral cubes, comparable to DECT spatial resolution (7.7 ± 0.3 lp cm−1 and 7.4 ± 0.2 lp cm−1, respectively). For the pig head, HeCT, SECT, DECT and pCT predicted range accuracy was 0.25%, −1.40%, −0.45% and 0.39%, respectively. In this study, HeCT acquired with a prototype system showed potential for particle therapy treatment planning, offering RSP accuracy, spatial resolution, and range prediction accuracy comparable to that achieved with a commercial DECT scanner. Still, technical improvements of HeCT are needed to enable clinical implementation.

Evaluation of the impact of a scanner prototype on proton CT and helium CT image quality and dose efficiency with Monte Carlo simulation

“…Detailed investigations on the biological dose can be found in Hansen et al (2014) Finally, pCT and HeCT show acceptable RSP accuracy with a relative RSP error below 1%. This agrees with the result of Volz et al (2021) who found a MAPE of 0.30% for helium ions (values without body material) for an experimental sensitometry phantom scan which is slightly better than our value of MAPE He = 0.67% for the realistic case. This might be attributed to the minor but acceptable offset in figure 9 which is not present in figure 3 of Volz et al (2021).…”

Considerations for Upright Particle Therapy Patient Positioning and Associated Image Guidance

“…By rotating the patient, full pCT scans may be acquired. pCT scans may be used directly for treatment planning, where the direct nature of assessing the relative stopping power provides high stopping power and range prediction accuracy (72)(73)(74)(75). pCT and upright treatment posture are inherently a well suited match, since pCT acquisition would be easier with a fixed beam and detector setup and rotating patient compared to acquisition with a rotating gantry.…”

Experimental helium‐beam radiography with a high‐energy beam: Water‐equivalent thickness calibration and first image‐quality results

“…The potential advantages of helium ions over protons for ion imaging in general were also shown in several other theoretical and simulation‐based studies 4,26–28 . However, further experimental works on helium‐ion imaging within the last 10 years are scarce 29–32 …”

“…Both Refs. 32 and 68 imaged cylindrical phantoms with different inserts using helium‐ion CT. Reference 68 used a phantom with a diameter of 100 mm and achieved an MAPD of 0.68% and an RMSD of 0.78% by comparing measured RSPs to ground‐truth RSPs in homogeneous regions voxel‐by‐voxel, while Ref. 32 used a phantom with a diameter of 150 mm and achieved an MAPD of 0.3%.…”

“…4,[26][27][28] However, further experimental works on helium-ion imaging within the last 10 years are scarce. [29][30][31][32] In the context of 𝛼RAD with the above-described system, this contribution focuses on two essential steps toward an envisaged clinical maturity of our method for quantitative imaging of cranial and later in particular abdominal and pelvic body regions.…”