Heavy ion radiotherapy is an accepted form of cancer therapy especially suitable for the precise and effective treatment of tumors close to organs at risk. The high precision of the treatment is currently achieved by using patient geometry data captured using X-ray computed tomography. With the application of new imaging methods directly measuring the density and stopping power of the patient tissue the present accuracy could be further improved.
One of the promising alternatives to X-ray computed tomography is high energy proton radiography capable of providing a precise density analysis of target materials. Within the scope of this work several experimental approaches towards the effective accuracy of this technique regarding density reconstruction were made using different types of targets (simple step wedges and head phantoms). The investigations especially focused on the applicability of radiographic images for treatment planning based on either conventional X-ray data being recalibrated with the help of single proton projections or native proton computed tomography. All of the presented experiments were conducted during parasitic beam times at the LANL pRAD facility in New Mexico, USA. Several issues regarding the stability of the accelerator as well as the optical quality of the available detector systems were disclosed. Nevertheless, high energy proton radiography proved to deliver a more accurate density reconstruction than conventional calibration approaches.
Further investigations with the GSI in-house treatment planning software TRiP98 revealed a significant difference in dose coverage of a virtual tumor volume when using different patient base data (X-ray computed tomography vs. recalibrated X-ray computed tomography vs. proton tomography). Although the current clinical method provided a good result in soft tissue regions, at higher densities, e.g. in the density range of bones, a significantly larger deviation was monitored. This could in specific cases lead to an ineffective treatment of tumors or even to an unwanted dose deposition in healthy organs with the conventional imaging approach.
High energy proton radiography promises to be a suitable technique for medical imaging purposes. Although current facilities are not yet designed for such applications, future treatment centers could be designed in a way to exploit the benefits of this technique. Beforehand, several improvements and modifications to those setups will be mandatory to advance the technique towards clinical implementations. Future experiments for medical applications are scheduled for the FAIR phase 0. Those will focus on including a suitable method for measuring the stopping power of the incident particles. A quality comparison to X-rays at equal dose deposition is planned as well
