Objective. The aim of the phantom study was to validate and to improve the computed tomography (CT) images used for the dose computation in proton therapy. It was tested, if the joint reconstruction of activity and attenuation images of time-of-flight PET (ToF-PET) scans could improve the estimation of the proton stopping-power. Approach. The attenuation images, i.e. CT images with 511 keV gamma-rays (γCTs), were jointly reconstructed with activity maps from ToF-PET scans. The β + activity was produced with FDG and in a separate experiment with proton-induced radioactivation. The phantoms contained slabs of tissue substitutes. The use of the γCTs for the prediction of the beam stopping in proton therapy was based on a linear relationship between the γ-ray attenuation, the electron density, and the stopping-power of fast protons. Main results. The FDG based experiment showed sufficient linearity to detect a bias of bony tissue in the heuristic look-up table, which maps between x-ray CT images and proton stopping-power. γCTs can be used for dose computation, if the electron density of one type of tissue is provided as a scaling factor. A possible limitation is imposed by the spatial resolution, which is inferior by a factor of 2.5 compared to the one of the x-ray CT. γCTs can also be derived from off-line, ToF-PET scans subsequent to the application of a proton field with a hypofractionated dose level. Significance. γCTs are a viable tool to support the estimation of proton stopping with radiotracer-based ToF-PET data from diagnosis or staging. This could be of higher potential relevance in MRI-guided proton therapy. γCTs could form an alternative approach to make use of in-beam or off-line PET scans of proton-induced β + activity with possible clinical limitations due to the low number of coincidence counts.

The $$^\text {nat}$$ nat C(p,x)$$^{11}$$ 11 C reaction has been discussed in detail in the past [EXFOR database, Otuka et al. (Nuclear Data Sheets 120:272–276, 2014)]. However, measured activation cross sections by independent experiments are up to 15% apart. The aim of this study is to investigate underlying reasons for these observed discrepancies between different experiments and to determine a new consensus reference cross section at 100 MeV. Therefore, the experimental methods described in the two recent publications [Horst et al. (Phys Med Biol https://doi.org/10.1088/1361-6560/ab4511, 2019) and Bäcker et al. (Nuclear Instrum Methods Phys Res B 454:50–55, 2019)] are compared in detail and all experimental parameters are investigated for their impact on the results. For this purpose, a series of new experiments is performed. With the results of the experiments a new reference cross section of (68±3) mb is derived at (97±3) MeV proton energy. This value combined with the reliably measured excitation function could provide accurate cross section values for the energy region of proton therapy. Because of the well-known gamma-ray spectrometer used and the well-defined beam characteristics of the treatment machine at the proton therapy center, the experimental uncertainties on the absolute cross section could be reduced to 3%. Additionally, this setup is compared to the in-beam measurement setup from the second study presented in the literature (Horst et al. 2019). Another independent validation of the measurements is performed with a PET scanner.

ZusammenfassungDieser Übersichtsartikel präsentiert die Entwicklung und den technischen Fortschritt der Positronenemissionstomografie (PET) hin zum digital arbeitenden PET-System (dPET). Der Fokus liegt hierbei auf den PET-Hardwarekomponenten zur Detektierung sowie Verarbeitung und Ortung des Signals zur klinischen Bildgebung. Es werden technische Unterscheidungen und Vorteile der dPET-Systeme gegenüber konventionellen PET-Systemen aufgezeigt. Dazu zählen zum Beispiel größere Detektorflächen mit sehr empfindlichen und kompakten Photodetektorsystemen in Verbindung mit einer verbesserten Elektronik zur schnellen Berechnung der Orts-, Zeit- und Energieauflösungen der Signale. Die daraus neu erschlossenen Anwendungsbereiche und Perspektiven in der dPET-Bildgebung werden zusätzlich thematisiert.