Impact of transverse magnetic fields on water equivalent ratios in carbon-ion radiotherapy

The correct estimation of water equivalent ratio (WER) and the associated errors is highly important for the delivery of carbon-ion (C-ion) beams. In the case of MRI-guided C-ion radiotherapy (MRgCT) scenario, as a therapeutic approach for future potential MR-guided particle therapy, C-ion beam will be deflected in the presence of the magnetic field. The curvature of the track causes a retraction of the range. This range variation might be different in various materials, which leads to change the WER values. To evaluate the WER variations versus magnetic field strength, in this research work, WER of some materials, including bladder, brain, prostate, muscle, bone, polymethylmethacrylate (PMMA), polyoxymethylene (POM), polyethyleneterephthalate (PET), titanium (Ti), gold (Au), platinum (Pt); potentially encountered in C-ion radiotherapy dosimetry were assessed. A mono-energetic C-ion beam, incident on a volume containing materials as mentioned above, was simulated using the FLUKA Monte Carlo code. To validate the simulated C-ion beams, the ion ranges in the water at the calculated energies (100-400 MeV/n) were compared with experimental and analytical data reported in the literature. Moreover, the calculated WER results were compared with available experimental data in the absence of the magnetic field. The WER values were calculated for the materials mentioned above in the presence of 0.35, 1.5, and 3 T magnetic fields and compared to the case without a magnetic field. Good agreement with the experimental data regarding range prediction in water was achieved. No change in the WER value was observed at 100 MeV/n and 0.35 T for all the studied materials and energies. In the case of bladder, brain, and prostate, and muscle materials, no change in WER values was observed by changing the magnetic field strength up to 3 T at the range of the studied energies. The maximum change in WER values by applying the magnetic fields is relevant to the Ti material (-0.4%), which occurs at 400 MeV/n energy and 3 T magnetic field. Considering the potential encounter of the studied materials in the clinical practice of MRgCT, in the form of phantoms, dosimeters, detectors, fiducial markers, radio-opaque clips, patient anatomies, etc., the results of this study highly contribute to assess the variation of WER values of the studied dosimetric materials and the changes in C-ion ranges within these materials when the transverse magnetic fields are applied to the materials subject to study.