Beam-on imaging of short-lived positron emitters during proton therapy

In vivo dose delivery verification in proton therapy can be performed by positron emission tomography (PET) of the positron-emitting nuclei produced by the proton beam in the patient. A PET scanner installed in the treatment position of a proton therapy facility that takes data with the beam on will see very short-lived nuclides as well as longer-lived nuclides. The most important short-lived nuclide for proton therapy is ¹²N (Dendooven et al 2015 Phys. Med. Biol. 60 8923–47), which has a half-life of 11 ms. The results of a proof-of-principle experiment of beam-on PET imaging of short-lived ¹²N nuclei are presented. The Philips Digital Photon Counting Module TEK PET system was used, which is based on LYSO scintillators mounted on digital SiPM photosensors. A 90 MeV proton beam from the cyclotron at KVI-CART was used to investigate the energy and time spectra of PET coincidences during beam-on. Events coinciding with proton bunches, such as prompt gamma rays, were removed from the data via an anti-coincidence filter with the cyclotron RF. The resulting energy spectrum allowed good identification of the 511 keV PET counts during beam-on. A method was developed to subtract the long-lived background from the ¹²N image by introducing a beam-off period into the cyclotron beam time structure. We measured 2D images and 1D profiles of the ¹²N distribution. A range shift of 5 mm was measured as 6  ±  3 mm using the ¹²N profile. A larger, more efficient, PET system with a higher data throughput capability will allow beam-on ¹²N PET imaging of single spots in the distal layer of an irradiation with an increased signal-to-background ratio and thus better accuracy. A simulation shows that a large dual panel scanner, which images a single spot directly after it is delivered, can measure a 5 mm range shift with millimeter accuracy: 5.5  ±  1.1 mm for 1  ×  10⁸ protons and 5.2  ±  0.5 mm for 5  ×  10⁸ protons. This makes fast and accurate feedback on the dose delivery during treatment possible.