Energy-painted ion radiography for precision radiotherapy

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The clinical potential of the highly precise ion radiotherapy is often compromised by uncertainties on the actual stopping power of patient's tissues and its distribution in space. We aim to develop a method to address these uncertainties by a high accuracy quantitative on-couch imaging based on ion radiography. While the majority of ion radiographic approaches uses protons, our method will be based on largely unexploited helium ions which have the potential to be the optimal radiation modality for ion imaging. The imaging device to be built will exploit a cutting-edge radiation detection technology developed at CERN called Timepix3. It enables to detect single ions and access their properties. The radiological thickness of the imaged object will be derived from the energy deposition measurement in a single thin detector behind the imaged object. In order to image the whole span of treated body part's thicknesses, a unique beam-energy painting method will be developed. Imaging times of a few seconds will be reached with a multi-energy operation of the synchrotron, which will enable to complete the image within a single accelerator spill. In contrast to the current ion imaging methods, the new method will feature a doubled dynamic range, allowing to image even the thickest treated body parts. For this the energy range of the accelerator has to be significantly extended beyond the currently maximal energy for helium. Furthermore, it has to be ensured that the beam diagnostics copes with the low beam intensities demanded to keep the imaging dose low. To guarantee high spatial resolution, an ion tracking system will be implemented in the imaging device. In order to reach a real impact on clinics, the imager will furthermore feature a compact size, easy manageability, high patient's safety and comfort. For a maximal exploitation of the large amounts of measured data, a dedicated data processing chain and advanced image reconstruction algorithms will be developed. The performance of the new imaging method will be evaluated quantitatively in clinical conditions at the Heidelberg Ion Beam Therapy Center, using in-house built body models. Moreover, the versatility of the method will allow a direct comparison of the accuracy of helium-based images with images based on other ion types like protons and carbon ions.This image guidance method, being a significant step ahead of the current status of medical ion beam imaging, will open the possibility for better tumor targeting in the clinics, leading to lower dose to healthy tissues. In addition to the expected decrease of the treatment complication rate, it will allow to apply higher dose to the tumor. In this way it might represent an important step towards treatments of radioresistant tumors, boosting of the full exploitation of the potential of the high-tech ion beam radiotherapy for the benefit of cancer patients.