title: Three dimensional reconstruction of therapeutic carbon ion beams in phantoms using single secondary ion tracks.
abstract: Carbon ion beam radiotherapy enables a very localised dose deposition. However, even small changes in the patient geometry or positioning errors can significantly distort the dose distribution. A live, non-invasive monitoring system of the beam delivery within the patient is therefore highly desirable, and could improve patient treatment. We present a novel three-dimensional method for imaging the beam in the irradiated object, exploiting the measured tracks of single secondary ions emerging under irradiation. The secondary particle tracks are detected with a TimePix stack-a set of parallel pixelated semiconductor detectors. We developed a three-dimensional reconstruction algorithm based on maximum likelihood expectation maximization. We demonstrate the applicability of the new method in the irradiation of a cylindrical PMMA phantom of human head size with a carbon ion pencil beam of [Formula: see text] MeV u-1. The beam image in the phantom is reconstructed from a set of nine discrete detector positions between [Formula: see text] and [Formula: see text] from the beam axis. Furthermore, we demonstrate the potential to visualize inhomogeneities by irradiating a PMMA phantom with an air gap as well as bone and adipose tissue surrogate inserts. We successfully reconstructed a three-dimensional image of the treatment beam in the phantom from single secondary ion tracks. The beam image corresponds well to the beam direction and energy. In addition, cylindrical inhomogeneities with a diameter of [Formula: see text] cm and density differences down to [Formula: see text] g cm-3to the surrounding material are clearly visualized. This novel three-dimensional method to image a therapeutic carbon ion beam in the irradiated object does not interfere with the treatment and requires knowledge only of single secondary ion tracks. Even with detectors with only a small angular coverage, the three-dimensional reconstruction of the fragmentation points presented in this work was found to be feasible.
authors.affiliation: Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120 Heidelberg, Germany.
authors.forename: Anna Merle
authors.initials: AM
authors.lastname: Reinhart
authors.forename: Claudia Katharina
authors.initials: CK
authors.lastname: Spindeldreier
authors.forename: Jan
authors.initials: J
authors.lastname: Jakubek
authors.forename: Mária
authors.initials: M
authors.lastname: Martišíková
mesh_terms: D000465:Algorithms; D063193:Heavy Ion Radiotherapy; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000295:instrumentation*; D016013:Likelihood Functions; D019047:Phantoms, Imaging; D011879:Radiotherapy Dosage