Can a ToF-PET photon attenuation reconstruction test stopping-power estimations in proton therapy? A phantom study

Abstract

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.

Adaptive Proton Therapy of Pediatric Head and Neck Cases Using MRI-Based Synthetic CTs: Initial Experience of the Prospective KiAPT Study

“…A reduction of uncertainty of the so-obtained estimate of the electron density would be desired. In this regard, the facility of the authors recently presented a novel approach integrating time-of-flight PET into MRI-guided PT [ 63 ].…”

Innovations in dedicated PET instrumentation: from the operating room to specimen imaging

“…The in-beam PET scanner uses detectors, typically comprised of scintillator crystals and PMTs, for coincident detection of these 511 keV photons. The in-beam PET scanner operates by detecting positron-emitting isotopes generated during heavy ion therapy (Bäumer et al 2021).…”