Investigations on novel imaging techniques for ion beam therapy: Carbon ion radiography and tomography

Ion beams exhibit a finite range and an inverted depth-dose profile (Bragg peak). These favourable physical properties allow superior tumour-dose conformality. However, they introduce sensitivity to range uncertainties. Although these uncertainties are typically taken into account in treatment planning, delivery of the intended treatment has to be ensured to prevent underdosage of the tumour or overdosage of surrounding critical structures. Thus, dedicated Quality Assurance procedures are desirable to provide in-vivo range verification before or during ion therapeutic irradiation. At the Heidelberg Ion Beam Therapy Center (HIT) experimental investigations are being carried out to address the feasibility of low dose radiographic or tomographic imaging of transmitted energetic ions to evaluate the correct patient positioning and to verify the ion range before or in between the treatment. These techniques rely on irradiation of the patient with a small fluence of ions at higher initial energy than for therapy, so that the exit range or residual beam energy can be measured. Therefore, the technical feasibility at HIT was first assessed by evaluating the requested energies for different clinical indications. Then, a first proof of principle phantom experiment using standard radiographic films was performed for both radiographic and tomographic acquisition, and the measurements were found in good agreement with corresponding Monte Carlo simulations. Following these promising results, a first prototype of a dedicated detector system, a stack of ionization chambers with newly acquired electronics, has been assembled and is being characterized for ongoing experimental investigations. Initial encouraging results for imaging phantoms with inhomogeneities of different complexity will be addressed. The further optimizations being evaluated and planned for an upgrade of the system towards an eventual clinical use will also be discussed.