The only clinical application of such approaches is PET imaging of annihilation photons – which is employed in just a few hadron therapy facilities. Currently, however, all these facilities perform PET after treatment delivery, and only detect positron emitters with half-lives (T1/2) of between 2 and 20 minutes (such as 15O, 11C, 30P and38gK). As a result, image acquisitions of at least a few minutes are needed and information is delayed with respect to dose delivery. This delay also increases biological washout, thereby reducing the PET activity.

In contrast, PET during treatment would enable imaging of short-lived (T1/2 below 19 s) nuclides. Such "beam-on PET" could provide real-time feedback with less biological washout. "The main benefit of using short-lived rather than long-lived nuclides is that potentially one obtains much faster feedback and can thus interrupt a 'wrong' irradiation before it is finished," explained Peter Dendooven, from the University of Groningen's KVI-Center for Advanced Radiation Technology.

To assess the feasibility of beam-on PET, Dendooven and colleagues measured the production of positron emitters during the stopping of 55 MeV protons. The aim: to identify which short-lived positron emitters are created in sufficient quantities to be relevant for in vivoverification of proton dose delivery (Phys. Med. Biol. 60 8923).