Experimental investigations on carbon ion scanning radiography using a range telescope

Ion beams offer an excellent tumor-dose conformality due to their inverted depth-dose profile and finite range in tissue, the Bragg peak (BP). However, they introduce sensitivity to range uncertainties. Imaging techniques play an increasingly important role in ion beam therapy to support precise diagnosis and identification of the target volume at the planning stage as well as to ensure the correspondence between the planning and treatment situation at the actual irradiation. For the purpose of improved treatment quality, ion-based radiographic images could be acquired at the treatment site before or during treatment and be employed to monitor the patient positioning and to check the patient-specific ion range. This work presents the initial experimental investigations carried out to address the feasibility of carbon ion radiography at the Heidelberg ion therapy center using a prototype range telescope set-up and an active raster scanning ion beam delivery system. Bragg curves are measured with a stack of ionization chambers (IC) synchronously to the beam delivery. The position of the BP is extracted from the data by locating the channel of maximum current signal for each delivered beam. Each BP is associated to the lateral and vertical positions of the scanned raster point extrapolated from the beam monitor system to build up a radiography. The radiographic images are converted into water equivalent thickness (WET) based on two calibrations of the detector. Radiographies of two phantoms of different complexities are reconstructed and their image quality is analyzed. A novel method proposed to increase the nominal range resolution of the IC stack is applied to the carbon ion radiography of an Alderson head phantom. Moreover, an x-ray digitally reconstructed radiography of the same anthropomorphic head phantom is converted in WET through the clinically used ion range calibration curve and compared with the carbon ion radiography based on a γ-index approach, yielding a good correspondence in terms of absolute WET within ±3%, 3 mm distance-to-agreement and, 87% passing ratio. Imaging artifacts at interfaces within the irradiated phantom due to the finite size of the beam, resulting in multiple maxima, are addressed. Overall, this work demonstrates the feasibility of the prototype range telescope to acquire ion-based transmission imaging with a resolution of up to 0.8 mm WET.