Helium ion beam imaging for image guided ion radiotherapy

Abstract

BackgroundIon beam radiotherapy provides potential for increased dose conformation to the target volume. To translate it into a clinical advantage, it is necessary to guarantee a precise alignment of the actual internal patient geometry with the treatment beam. This is in particular challenging for inter- and intrafractional variations, including movement. Ion beams have the potential for a high sensitivity imaging of the patient geometry. However, the research on suitable imaging methods is not conclusive yet. Here we summarize the research activities within the “Clinical research group heavy ion therapy” funded by the DFG (KFO214). Our aim was to develop a method for the visualization of a 1 mm thickness difference with a spatial resolution of about 1 mm at clinically applicable doses.MethodsWe designed and built a dedicated system prototype for ion radiography using exclusively the pixelated semiconductor technology Timepix developed at CERN. Helium ions were chosen as imaging radiation due to their decreased scattering in comparison to protons, and lower damaging potential compared to carbon ions. The data acquisition procedure and a dedicated information processing algorithm were established. The performance of the method was evaluated at the ion beam therapy facility HIT in Germany with geometrical phantoms. The quality of the images was quantified by contrast-to-noise ratio (CNR) and spatial resolution (SR) considering the imaging dose.ResultsUsing the unique method for single ion identification, degradation of the images due to the inherent contamination of the outgoing beam with light secondary fragments (hydrogen) was avoided. We demonstrated experimentally that the developed data processing increases the CNR by 350%. Consideration of the measured ion track directions improved the SR by 150%. Compared to proton radiographs at the same dose, helium radiographs exhibited 50% higher SR (0.56 ± 0.04lp/mm vs. 0.37 ± 0.02lp/mm) at a comparable CNR in the middle of the phantom. The clear visualization of the aimed inhomogeneity at a diagnostic dose level demonstrates a resolution of 0.1 g/cm2 or 0.6% in terms of water-equivalent thickness.ConclusionsWe developed a dedicated method for helium ion radiography, based exclusively on pixelated semiconductor detectors. The achievement of a clinically desired image quality in simple phantoms at diagnostic dose levels was demonstrated experimentally.

Evaluation of water equivalent ratio (WER) values for polyethylene, polymethyl methacrylate, polystyrene, lead, tungsten and aluminum at helium ion energies ranging from 25-250 MeV/u through Monte Carlo simulation

“…Some plastic materials, containing PMMA, PE, PS can be applied instead of water for dosimetry purposes. Moreover, some of the metallic materials, including Al, W, and Pb are used as the components of the beam line or dosimeters (23,(27)(28) . WER values can be applied as a conversion factors to convert dose in mentioned above solid dosimetric materials to dose in water.…”

A High-Granularity Digital Tracking Calorimeter Optimized for Proton CT

“…In recent years, along with protons, heavier ions (mostly helium or carbon) have been considered for imaging given their smaller deviation due to multiple Coulomb scattering. The expected effect of an improved spatial resolution was observed, once the ion fragmentation was taken into account [25][26][27][28][29][30][31].…”

Roadmap: helium ion therapy

“…Furthermore, a precise knowledge of the creation of fragments and secondary particle yields is a necessity for treatment verification techniques (see section 6 on imaging and treatment verification), including positron emission tomography (PET) or experimental online monitoring techniques using prompt gammas or charged fragments. Besides their direct application in therapy, 4 He ions are also a prime candidate for radiography applications (Martisikova et al 2018) and are part of novel beam delivery techniques like the simultaneous mixing of carbon and 4 He beams (Volz et al 2020).…”